Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.
THE ECOLOGICAL LIFE ZONES OF PUERTO RICO
AND THE U.
S. VIRGIN ISLANDS
BY J. J. EWEL AND J. L. WHITMORE
INSTITUTE OF TROPICAL FORESTRY
RIO PIEDRAS, PUERTO RICO
FOREST SERVICE
U. S. DEPARTMENT OF AGRICULTURE
RESUMEN
La mayoria de los paises neotropicales, excepto Mexico y Brasil, han sido delineados en mapas
por el sistema Holdridge de zonas de vida natural. Para poder comparar con estas areas, se hizo un
mapa de Puerto Rico y las Islas Virgenes, el que incluye una descripcion de cada una de sus zonas
de
vida.
El texto en
Ingles contiene balances hidricos, una descripcion de
la teoria Holdridge
sobre biotemperatura, y muchos mas detalles en general. La corta version en Espanol se ofrece
como manual de campo para ser usada con el mapa.
En
estas
islas hay
seis zonas de
vida,
las cuales existen tambien en Centro y Sur America,
representando
alli areas extensas. Con este mapa sera posible aplicar directamente en estas areas
investigaciones hechas en Puerto Rico y
las
Islas Virgenes y, a la inversa, practicas satisfactorias
llevadas
a cabo en dichas areas seran aplicables en
las zonas de vida correspondientes en Puerto
Rico y las Islas Virgenes.
SUMMARY
Most
of
the
neotropical
nations
except Mexico
and
Brazil
are mapped
according
to
the
Holdridge system of ecological life zones. In order to have comparability with these areas, Puerto
Rico and the U.S. Virgin Islands were mapped and a discussion and a description of each life zone
were
written. The manuscript
in
English
offers water balances,
a description of the Holdridge
theory
of biotemperature,
and
greater
detail
in
general. The shortened version
in Spanish
is
intended as a field manual to be used with the map.
Six
life zones are present on these islands and are present also in South and Central America,
representing
large
areas there. Research done on the islands will be directly applicable to those
areas and conversely, successful practices done there will be applicable in corresponding life zones
on the islands.
THE ECOLOGICAL LIFE ZONES OF PUERTO RICO AND THE U. S. VIRGIN ISLANDS
2/
V
John J. Ewel
and Jacob L. Whitmore
n
INTRODUCTION
The
ecosystems of
the Commonwealth
of Puerto
Rico and
the
U.
S.
Virgin
Islands
are
extremely
important
for
two
reasons.
First,
they
form
a
substantial
component
of
the
life-support system for nearly three million people. Second, environmental changes due to man’s
impact show up quickly on densely populated islands; this region can, therefore, indicate to other
tropical and subtropical countries the kinds of changes that they might expect in the near future.
During the past 100 years Puerto Rico and the U.
S. Virgin Islands have gone through a cycle
consisting of massive
degradation
of
their
natural ecosystems
(see,
for example, Wadsworth,
1950) followed by an increasing general awareness, within the past five years, of the importance
and
unique
and
fragile
nature
of
the
plant-animal communities
of
this
region.
Ecosystem
modification
in the past was primarily due to agricultural activities, while today the issues are
highways, transmission
lines, ports, refineries, mines, power plants, and urbanization. Although
ecological
considerations now
constitute
an
integral
part
of most
land
use
planning, much
remains to be done.
This life zone map, tied to a world-wide system, was prepared to facilitate comparisons and the
flow
of related
ecological information
into
Puerto
Rico
and
the
U.
S.
Virgin
Islands from
countries which encompass similar environments. Likewise, these countries, most of which have
not yet undergone the degree of ecological modification that Puerto Rico and the U.
S. Virgin
Islands have, may benefit from the experiences of these islands.
In Puerto Rico and the Virgin Islands the ecosystems have been better studied than in most
tropical regions.
Ecological
studies of the
area prior
to
1900, most of which were qualitative
observations, have been summarized by Wadsworth (1950). Murphy (1916) was one of the first
to provide detailed descriptions of Puerto Rico’s forests after the turn of the century. His work
was followed by the classical study (involving less than four months of field work!) by Gleason
and Cook (1926)
(see
also. Cook and Gleason,
1928), which
is
still
a standard reference
for
descriptions of Puerto Rican plant communities. About 20 years
after the Gleason and Cook
study,
J.
S. Beard began a series of investigations throughout the Caribbean, summarized in his
monograph on the vegetation of the Windward and Leeward Islands (Beard,
1949). Dansereau
(1966) published physiognomic and floristic descriptions of more than 100 vegetation types in
Puerto Rico, but did not include a detailed map of his units. A companion article by Buell and
Dansereau (1966) did, however, contain maps of the vegetation of the Roosevelt Roads military
base. Williams (1967) described several vegetation types in Puerto Rico from a physiognomic and
In cooperation with the University of Puerto Rico
2 / Department of Botany, University of Florida, Gainesville
3 /
U. S. Forest Service, Institute of Tropical Forestry, Rio Piedras, Puerto Rico
1
floristic viewpoint and compared them to forests in Southeast Asia and Texas. He used the map
of forest types and
forest regions prepared by
Little and Wadsworth (1966). Several in-depth
studies of specific forests have recently been carried out in Puerto Rico, most notably the studies
by Odum and co-workers (Odum, 1970a) and by Howard and co-workers (Howard, 1968, plus
sequential
articles
in same journal)
in two vegetation types within
the Luquillo Experimental
Forest.
The
basis of classification used
for
this map was the
life zone system devised by Holdridge
(1947,
1967).
Its desirable features include:
-1.)
it is simple enough to permit classification based
on data likely to be available or easily obtained; 2.) it encompasses the variation inherent over the
earth’s surface, yet without creating a huge number of units which differ only slightly among
themselves;
3.)
it
is reproducible, such
that individuals working independently
classify similar
landscape units in the same way; 4.) it is hierarchical, permitting the subdivision of primary units
into locally important sub-units; and
5.)
its units seem to bear a close relationship to natural and
biologically
meaningful
landscapes,
i.e.
the
units
produced
a
recognizable
order
from
the
apparent chaos of nature.
The
life zones mapped are broad bioclimatic units, each of which may encompass a variety of
soils,
vegetation,
microclimates,
and
land
use
patterns. The
Holdridge
system
of
life zone
classification has now been applied to enough areas (Table
1) so that it has become a useful tool
for comparing widely separated
locations. Some of the maps
(e.g.
Peru, Colombia, Venezuela,
Ecuador,
Dominican
Republic, Panama)
have been accompanied by monographs
containing
descriptions of many of the life zones, their vegetation, and the prevalent land use patterns.
THE HOLDRIDGE MODEL
Holdridge
first
published
his
classification of world plant formations (now more correctly
termed
“life
zones”)
in
1947.
In
the
ensuing
26
years
various
minor
refinements
and
modifications were made
in that original scheme— primarily changes in nomenclature—and these
have been incorporated into the presently used diagram. Figure
1.
Life Zones
Each
life zone
lies within:
1.) a Latitudinal Region
(left side of Figure
1); 2.) an Altitudinal
Belt
(right
side of Figure
1); and
3.) a Humidity Province (bottom of Figure
1). The variables
used
to
delineate
any
given
life
zone
are
mean
annual
precipitation
and mean
annual
biotemperature.
Mean biotemperature
is mean air temperature modified by substituting zero for values outside
the range of 0 to 30 C, as discussed in Appendix A. The mean biotemperature scale appears along
the
sides of Figure
1. The values decrease geometrically from 30 at the bottom to
1.5 C at the
top.
The
Latitudinal
Region
at
any
given
location
is
determined
by
increasing
the mean
biotemperature to the value
it would have at sea level. For making this conversion a lapse rate of
6 C per 1000 m
is commonly used; most areas of the world
fall in the range of 5.5 to 6.5 C per
1000 m. The mean
biotemperature not
corrected
for
elevation
at
any
place determines the
Altitudinal Belt. In naming life zones the name of the Altitudinal Belt is dropped from the lowest
Altitudinal
Belt within
a given
Latitudinal
Region. Thus, we
refer to Subtropical Wet Forest,
rather than Subtropical Premontane Wet Forest.
1
Table
1. Countries which have been mapped according to Holdridge’s life zone system.
COUNTRY
MAP
SCALE
REFERENCE*
Colombia
1:1,000,000
Espinal et al. (1963)
Costa Rica
1:750,000
Tosi(1969)
Dominican Republic
1:250,000
Tasaico (1967)
Ecuador
1:1,000,000
Vivanco de
la Torre, et al.
(no date)
El Salvador
1:1,000,000
Holdridge (1959a)
Guatemala
1:1,000,000
Holdridge (1959b)
Haiti
1:500,000
OAS (1972)
Honduras
1:1,000,000
Holdridge (1962b)
Jamaica**
1:500,000
Gray and Symes
( 1969)
Nicaragua
1:1,000,000
Holdridge
( 1962c)
Nigeria**
1:3,000,000
Tosi (1968)
Panama
1:500,000
FAO (1971)
Peru
1:1,000,000
Tosi (1960)
Thailand (seven sites)
1:500,000
Holdridge et al. (1971)
United States, East of
the Rockies**
1:26,800,000
Sawyer and Lindsey (1964)
Venezuela
1:2,000,000
Ewel et al. (1968)
* For countries which have been mapped more than once only the most recent work is cited.
** From climatological and topographic data.
3
Figure
1
Diagram
for
the
classification
of
world
life
zones
according
to
the
Holdridge
system.
(Diagram
courtesy
of
Tropical
Science
Center,
San
Jose,
Costa
Rica).
Mean annual precipitation
is the second variable used
to define
life zones. Precipitation lines
traverse Figure
1
from
the bottom upward and
to
the
right
at
a 60° angle. The values shown
increase from a low of 62.5 mm per year at the left-hand side of Figure
1
to values in excess of
8000 mm per year at the lower right-hand corner of the chart. As with biotemperature, the scale
of mean annual precipitation
is geometric.
Mean
annual
biotemperature
and mean
annual
precipitation bound
in two
directions the
hexagons which circumscribe
life zones on
the
chart. The
final boundaries are formed by the
potential
evapotranspiration
ratio,
which
is
the
ratio
between
mean
annual
potential
evapotranspiration and mean annual precipitation.
It
is, therefore, a general statement of wetness
or dryness of the environment as indicated by the names of the humidity provinces at the base of
Figure
1. The potential evapotranspiration
ratio
line of
1.00, where mean annual precipitation
equals mean annual potential evapotranspiration,
is drawn diagonally across the chart. Life zones
to the left of this line tend to have an annual water deficit, while
life zones located to the right of
this unity line tend to have an annual water surplus.
The hexagonal boundaries which delineate the
life zones connect the midpoints of the series of
triangles
resulting
from
the
intersection
of
the
precipitation,
biotemperature, and
potential
evapotranspiration lines. The corners of each hexagon constitute transitional portions of the life
zone and may contain many features common to adjacent
life zones. These transitional areas are
sometimes distinguished as separate mapping units.
Subunits
The
life zones
are broad,
first-order units of classification, each of which may contain any
number of subunits defined by such factors as soils, rainfall distribution, or drainage. Holdridge
(1967)
calls the second-order units defined by such factors associations, but describes them
in
terms of the physiognomy of mature vegetation, rather than
floristics. The use of physiognomy
rather
than taxonomy
as
a
basis
for
distinguishing associations permits
the
identification of
similar or identical associations in biogeographically distinct regions. Within one region, however,
there
are often
floristically
derived names which
arc
helpful
in
describing associations,
as
for
example
the
cativo
{Prioria
copaifera
Gris.)
forests
of lower
Middle America
and
northern
Colombia, or the tabonuco {Dacryodes excelsa Vahl)
forests on
Puerto Rico and some of the
Lesser Antilles.
Definition of a site as corresponding to a particular association characterizes only the mature
vegetation
to
be
expected
there.
However,
the
site
might
well
have
been
modified
and
consequently now occupied by successional vegetation: perhaps crops, pasture, secondary forest,
or
a
fire-arrested subclimax. Life zones and the associations of which they are composed then,
define only
the potential vegetation, or range of vegetation types which might be found
in an
area, but do not define what might actually be there at any given time.
It may well be, however,
that
successional
processes
are
well
correlated
with
life
zones
and
associations.
Through
knowledge of the larger units
it might be possible
to predict the physiognomy, floristics, rate of
change, and net primary productivity of serai vegetation. Such information will surely be of value
in
the
near
future
as we
become
more
and
more
concerned
with
man’s
impact
on
his
environment.
5
Most applications of the Holdridge system
so
far have been concerned as
is this work, with
classification
at
the
life zone
level. More
detailed maps have been prepared
for some
areas,
however, and
the recent book by Holdridge
et al. (1971) includes detailed descriptions of the
soils, climate, and vegetation of 46 associations. A logical next step in Puerto Rico and the Virgin
Islands would be classification, at the association level, of areas of particular interest: perhaps the
National and Commonwealth
Forests,
areas undergoing agricultural or urban development, or
critical watersheds.
Soils maps are available for both Puerto Rico (Roberts, 1942) and the U. S.
Virgin
Islands
(Rivera
et
al.,
1970);
superposition
of
these upon
the
life zone map would
delineate, at the
soil series
level, many of the second-order units, or associations. Another work
which should be very useful in the subdivision of life zones is the hydrogeologic map prepared by
Briggs and Akers (1965). The
life zone map
is intended to be a first step, providing a baseline for
more detailed studies in the future.
MAPPING PROCEDURES
Previous Efforts
Holdridge’s system was
first applied to Puerto Rico by Tosi (1959), but his work covered only
the western part of the island and was done prior to the elimination of high temperatures in the
calculation of biotemperature. Kumme and Briscoe
( 1963) mapped the island using climatic data,
but they modified the climatic limits of the
life zone boundaries, so their work is of limited value
for comparing Puerto Rico with regions where the usual boundaries have been applied.
Climatic Data
Puerto
Rico and
the
Virgin
Islands have
typical
insular
climates,’ with
the
influence of the
surrounding
ocean
much
in
evidence.
The
northeast
trades,
plus
land-sea
breeze
systems,
ameliorate
coastal
temperatures particularly on the north
side. Most precipitation
is orographic
and there
is a definite rainshadow effect evident
in southern Puerto Rico and, to a lesser extent,
in some interior valleys. The
rainfall distribution pattern on islands surrounding the Caribbean is
much more even than on most land areas within the
tropics. An informative and very readable
description of local climate has been prepared by Calvesbert (1970).
Prior
to starting the
field work the literature was searched for precipitation and temperature
data. Sources consulted
for long-term means were
the
U.
S. Weather Bureau supplements for
1931
through
1952
and
1951
through
1960
(U.
S. Weather Bureau,
1959?
; U.
S. Weather
Bureau,
1965), and
the
1969 annual summary
for Puerto Rico and the Virgin Islands (ESSA,
1970).
For
stations which
had
been
discontinued
prior
to
1960,
the paper by Kumme and
Briscoe (1963) was used, as
it includes data summarized back to
1900. The result was a
list of
143 stations for Puerto Rico alone (plus 40 for the Virgin Islands), each with one or more years
of precipitation data. Of those
in Puerto Rico,
122. had
at least
five years, 104 had
at least ten
years, and 88 had
at
least
fifteen years of data recorded. The unusually high density of one
pluviometer per 56 km- provided
excellent ground
control for the mapping. Also,
1 19 of the
stations had
at
least one year of temperature data recorded. Most of the mean temperature data
were based on daily averages of the maximum and minimum.
6
Base Maps
Excellent base maps, both with and without topography, are available for Puerto Rico and the
U. S. Virgin Islands. Those relied on most heavily were:
U. S. Dept, of Interior, Geological Survey. 1952. Puerto Rico e Islas Limitrofres. 1:240,000.
Estado Libre Asociado de Puerto Rico, Autoridad de Carreteras.
1969. Mapa de Carreteras
Estatales de Puerto Rico.
1 :222,222.
Rand McNally and Co. 1968. Road Map of Puerto Rico. 1:300,000.
U.
S. Dept, of Interior, Geological Survey.
1954-1958. Virgin Islands of the United States.
(8 sheets).
1 :24,000.
The
final
life zone boundaries were drawn on the U.
S. G.
S. Virgin Islands sheets at 1:24,000
cited above, and on the 1:120,000 version (1951) of Puerto Rico e
Islas Limitrofes, U. S. G. S.
All information was then transferred to the following two sheets, which served as the principal
base for preparation of the final published life zone map:
Dept, of Defense, Aeronautical Chart and Information Center, U.
S. Air Force.
1969. San
Juan.
1:250,000,
and:
1967.
Fajardo,
Puerto
Rico;
British
Virgin
Islands; Virgin
Islands of the United States.
1 :250,000.
Field Reconnaissance
The mapping procedure consisted of driving over roads where changes
in topography or the
available climatic data indicated that two or more
life zones would be encountered. Puerto Rico
has about
1 1,000 km of paved roads, and these, plus many secondary roads, made almost every
area on
the island
readily
accessible.
Life zone boundary
locations were recorded using road
maps, vehicle mileage, roadside distance markers, and altimeter-determined elevations. Interpola-
tions between boundary intersections were based on observations in the field and on topography.
The
life zone boundaries in the Luquillo Forest were based,
in part, on the Forest Service type
map.
Given a vast number of meteorological stations, plus level terrain,
it would be entirely possible
to map
life zones directly from
climatological data. Most areas of the world, however, possess
neither
of
these
two
characteristics,
so
considerable
field
reconnaissance
is
necessary. The
identification
of
life
zones
in
the
field
is
accomplished
by
synthesizing
and
evaluating
information
flowing
into
the mapper
as he
observes
a
landscape. Some of
this information
consists of fairly fine detail, such as soil texture and parent material, abundance of epiphytes, leaf
sizes and
shapes,
species composition, and
tree
heights. Other information, however,
is more
macroscopic: crown forms, land use practices, vegetation color, layering in forest canopies, and
drainage
patterns. The mapper
integrates
this
information
and
tries
to separate observations
caused by
soils, topography, or winds, for example, from observations which reflect long-term
average moisture and temperature conditions. Next, this integrated assessment of the landscape is
mentally
correlated with previous observations made at areas where mean annual precipitation
and temperature were known. This process of repeated observation and correlation enables one
to
place any particular landscape into perspective with
regard
to numerous other landscapes.
Patterns among these emerge, such that the mapper soon has a feel for the amount and kinds of
variation which can be expected to be encountered within any life zone in a particular geographic
region. The key
to
life zone mapping
is continual, astute observation, coupled as frequently as
possible to checkpoints where rainfall and temperature data are available.
7
In Puerto Rico and the Virgin Islands time did not permit the compilation of species
lists or
structural measurements of vegetation. In the description of each life zone, however, are listed a
few
readily recognized
species which, when taken together, might be useful locally as life-zone
indicators. Any
of
these
species might be found outside
the
life zone
for which
it
is
listed,
depending on soil type, drainage and other local environmental factors. When found together and
in abundance, however,
these species
are usually
indicative of the
gross
climatic
limits which
define
the
life zone
for which
they
are
listed. With few exceptions, botanical nomenclature
follows Little and Wadsworth (1964), which was also the source of the common names cited in
the text.
The Map
The color scheme employed
is the same as that which has been used on most other life zone
maps. The yellow-to-green-to-blue-to-purple color sequence indicates increasing moisture, from
the subhumid to the superhumid humidity provinces. Solid colors indicate the basal, or sea level,
belt, and
successively
lighter tones indicate
increasingly cooler altitudinal
belts. Broken black
lines separate
life zones in the same altitudinal belt, while solid black lines separate life zones in
different
altitudinal
belts.
Locations which
are
transitional among two or more
life zones are
common in Puerto Rico and the Virgin
Islands, but have not been delineated here as they have
been on the
life zone maps of Colombia, Costa Rica, the Dominican Republic, and Panama. The
lines
separating
life
zones
are
not
contours,
isotherms,
or
isohyets, nor should
the map be
construed
as an attempt to indicate either actual or potential vegetation. The units are ones of
broad
bioclimatic
similarity and each may encompass several forest associations, land uses, and
successional stages.
THE LIFE ZONES
In spite of
its latitude (approximately
18° N), Puerto Rico, according to Holdridge’s system of
classification,
is
in
the Subtropical
Latitudinal
Region. (On
life zone maps published prior to
1966
the
term
“Subtropical”
referred
to
the Premontane
Altitudinal
Belt
in
the
Tropical
Latitudinal
Region,
as
well
as
to
the
Subtropical
Latitudinal
Region.)
Sea
level
mean
biotemperatures
for Puerto Rico and
St. Croix (see Appendix A) are lower than 24°, the lower
limit for the Tropical Latitudinal Region. This distinction within the* geographic tropics reflects
marked
differences
in
the physiognomy and
species composition of sea-level
forests north or
south of about
12 to
15 degrees latitude. The selection of the word Subtropical to refer to these
differences in vegetation unfortunately may lead to confusion because of the generally accepted
astronomical use of the word
tropics
to apply
to
all areas between the Tropics of Cancer and
Capricorn (23°27’). Nevertheless, there are major differences in the vegetation within this range
of latitude and
it
is necessary
to distinguish them. Holdridge’s use of the word Subtropical does
refer to latitudinal differences, and not altitudinal differences, as interpreted by Odum (1970b),
Six
life zones are found
in Puerto Rico and the Virgin Islands, ranging from dry through rain
forest
in the basal, or sea
level,
belt, and wet plus rain forest in the Lower Montane Altitudinal
Belt. The area occupied by each
life zone
is shown in Table 2; Subtropical Lower Montane Rain
Forest
occupies
the
smallest
area,
accounting
for
only
0.1%
of
the
study
region,
while
Subtropical Moist Forest
is the dominant life zone, covering more than 58% of the area.
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9
Areas
were
determined
by
planimetering
the
life
zone
map.
Areas
should
be
within
two
percent
of
their
true
value;
differences
are
due
to
inclusion
or
exclusion
of
associated
small
islands
and
estuaries,
plus
instrument,
operator,
and
map-scale
errors.
The following brief descriptions of each of the
life zones include bar graphs showing rainfall,
soil moisture, and runoff for one or more
sites in each
life zone. The
soil moisture and runoff
were
calculated using the method devised by Tosi (Ewel et
al.,
1968; Holdridge et al., 1971;
FAO, 1971) which assumes mature vegetation and zonal soils, neither of which predominate at
the
sites for which water balances were
calculated. The bar graphs should, therefore, be used
primarily
for comparison with
water balances
calculated
for other
sites, rather than for the
absolute values shown
for
soil moisture and
runoff. The water balance calculations assume an
effective
soil
depth
of one
meter
and
available
moisture
was
calculated from
the
data
in
Lugo-Lopez (1953), except for the Glynn series, in St. Croix, data for which were provided by L.
H.
Rivera from
the
files of the U.
S.
D.
A.,
Soil Conservation
Service. Appendix B includes
additional discussion of Tosi’s method of water balance calculations, as well as complete water
balance tables for the ten sites illustrated in bargraph form.
The Subtropical Dry Forest Zone
This
life zone
is the driest of the six found
in the study region and covers sizable areas in the
Virgin
Islands,
Vieques,
southwestern
Puerto
Rico,
plus
all
of Mona
Island,
Culebra,
and
Desecheo. Mean annual rainfall ranges from a minimum of about 600 to a maximum of 1000 or
1100 mm
(see
Figure
1). Mean monthly
rainfall, and
calculated
soil moisture and runoff are
shown for two sites in this life zone: Santa Isabel, Puerto Rico (Figure 2), and Kingshill, St. Croix
(Figure
3). The annual patterns
at
these two
sites
are very
similar and
the differences in
soil
moisture between them arc due primarily
to soil moisture-holding capacities. At both locations
the
soil
is wettest during
the
latter four or
five months of the year and the small amount of
runoff which occurs
at
these
sites (61 mm per year
at Santa
Isabel and
166 mm per year
at
Kingshill)
is produced in September through November.
The vegetation
in
this
life zone (Figures 4-10) tends to form a complete ground cover, and is
almost
entirely deciduous on most
soils. Palms
are usually absent from the canopy, leaves are
often small and succulent or coriaceous, and species with thorns and spines are common. Tree
heights usually do not exceed
15 m and the crowns are typically broad, spreading, and flattened,
with sparse foliage. Fire
is common on the better soils, where the successional vegetation includes
many grasses, and
large amounts of organic debris accumulate on the soil surface during'the dry
season (Figure
5). On the poorer soils, however, the vegetation is more water-limited and organic
debris does not accumulate
to the extent that surface
fires are possible. Coppicing is a common
means of regeneration of many of the woody
species found
in
this
life zone and successional
forests,
therefore, often consist of an almost-impenetrable maze of tangled, close-growing small
stems. Plants here are low
in moisture content and the wood of most species is hard and durable
(Figure 6).
The forests of this life zone, at least on Puerto Rico, are richer in birds than are the wetter life
zones.
Kepler
and
Kepler (1970)
reported
31
species
per
1000
individuals
in
the
Guanica
Commonwealth
Forest
(Subtropical
Dry
Forest),
compared
to
only
20
species
per
1000
individuals in the much wetter Fuquillo Experimental Forest. The number of individuals reported
by them for the drier forest was higher also: 190 birds per linear kilometer in the Guanica Forest
compared with only 57 per kilometer in the Fuquillo Forest. In discussing the greater number of
insect-eating
birds
at
Guanica,
they
pointed
out
that
the
wetter
Fuquillo
Forest
has
well-developed lizard and frog populations, which might be performing the same function in the
10
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11
Figure
2
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
Santa
Isabel,
Puerto
Rico,
in
Subtropical
Dry
Forest.
Teresa
series,
with
available
moisture
of
222
mm
per
meter.
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available
moisture
of
130
mm
per
meter.
13
Figure
4
Subtropical
Dry
Forest.
Xeric
hillside
with
cacti,
thorny
legumes,
and
grasses.
Southeast
coast
of
St.
Croix.
14
Figure
5
Subtropical
Dry
Forest.
One
of
the
many
burned
areas
seen
on
St.
Croix
during
the
dry
months
of
February
through
April.
15
Figure
6
Subtropical
Dry
Forest.
Fence-post
quality
coppice
of
the
Guanica,
Puerto
Rico
landscape.
Coppicing
is
a
common
means
of
regeneration
for
many
dry
forest
tree
species.
(Photo
by
N.
Kirby).
Luquillo
Forest
as insectivorous birds
in
the
drier Guanica Forest.
In looking
at the broader
question of why
the
structurally simple
xeric
forest should have more
birds, numerically and
taxonomically,
than
the complex wetter
forest, they
state: “The answer may
well
lie
in
the
greater relative ability of xeric land birds from potential source areas to colonize oceanic islands,
the
larger number of xeric, compared
to mesic,
islands that could serve as stepping stones for
dispersion
in the Caribbean, and the proximity of the xeric forest to island coasts where potential
propagules would first land.”
Much of the Subtropical Dry Forest zone
in Puerto Rico and
the Virgin Islands
lies in the
warm
and
wet
transitional
parts of the
life zone, thus has characteristics of the dry part of
Subtropical Moist Forest and the cool portion of Tropical Dry Forest. Examples of such areas are
the
Lajas
Valley
in
Puerto
Rico and most of the
central part of
St. Croix, including Estate
Thomas, one of the U.
S. Forest Service’s experimental areas. These transitional portions of the
Subtropical Dry
Forest zone support
a man-modified vegetation which approaches a savanna-
woodland aspect (Figure
7). The grass cover
is often complete, evergreen trees are rare, and tall
trees
with
flattened
spreading crowns
are common.
Fire
is
often
a common and important
component of this transitional environment.
Extensive
areas
of the Subtropical Dry
Eorest zone on
the Virgin
Islands and
in southern
Puerto
Rico
overlie
limestone, and
the vegetation on
these
soils
{e.g.
the
Aguilita, Ensenada,
Fredensborg, and Sion
series)
is more
xerophyllous than
that which would be expected on a
zonal
soil. The
soil surface
in these associations often lacks any grass or herb cover, and many of
the
trees and shrubs are evergreen. At
East End,
St. Croix, a shallow, limestone-derived
soil
is
coupled
with
a
strong,
salt-laden
sea
breeze
to
produce
an
extreme
edapho-atmospheric
association
in which the vegetation
is open and the trees, with wind-pruned crowns, are less than
five meters tall, as seen in Figure 8.
Some of the low alluvial areas on the south coast of Puerto Rico contain saline
soils, such as
the Santa
Isabel
series, and
the vegetation on these
sites
is dominated hy Prosopis juliflora (Sw.)
DC. (bayahonda, mesquite), an introduced
tree, shown in Figure 9
. Parkinsonia aculeata L. (palo
de rayo, Jerusalem-thorn)
is another exotic leguminous tree which often forms nearly pure stands
on
coastal
alluvial
soils with
imperfect
drainage. Mangrove
forests, including red {Rhizophora
mangle
L.,
mangle
Colorado),
black
{Avicennia
nitida
Jacq.,
mangle
prieto),
and
white
{Laguncularia racemosa (L.) Gaertn.
f., mangle bianco) form parts of the coastal associations in
this
life zone, but the development of tall, luxuriant mangrove forests may, in some locations, be
limited
by
the
scarce
surface
runoff, which can
result
in higher
salinities and lower nutrient
inputs than would be the case along coasts with more rainfall. The stand of red mangrove which
was used
in one of the
first metabolic
studies of a whole mangrove ecosystem (Golley et al.,
1962) was located in the Subtropical Dry Forest zone on the south coast of Puerto Rico.
Species which are easily recognized and which, when taken as a group, are useful indicators of
this
life zone
in Puerto Rico and
the
U.
S.
Virgin
Islands include Bursera simaruba
(L.) Sarg.
(almacigo,
turpentine-tree), Prosopis juliflora (Sw.) DC. (bayahonda, mesquite), Ceplialocereus
royenii (L.) Britton and Rose (sebucan, dildo), Pictetia aculeata (Vahl) Urban (tachuelo, fustic),
Bucida
buceras
L.
(near, oxhorn
bucida), Guaiacum
officinale
L. (guayacan, lignumvitae),
G.
sanctum
L.
(guayacan
bianco, holywood
lignumvitae), Leucaena glauca
(L.) Benth.
(zarcilla,
16
17
Figure
7
Subtropical
Dry
Forest.
Drought-and
fire-resistant
Bursem
simaruba
near
Salinas,
Puerto
Rico.
The
grasses
are
burned
annually.
18
Figure
8
Subtropical
Dry
Forest.
Eastern-most
part
of
St.
Croix.
Dry
atmpspheric
association
caused
by
continual
exposure
to
19
tantan),
Tamarindus
indica
L.
(tamarindo,
tamarind), Acacia macracantha Humb. &
Bonpl.
(tamarindo
silvestre,
steel
acacia), A. farnesiana
(L.) Willd. (aroma, sweet
acacia), Melicoccus
bijugatus Jacq. (quenepa, Spanish-lime), and five species of Capparis (burro, caper).
Agriculture
in
the Subtropical Dry Forest zone
is, at best, a marginal business, except under
irrigation. The amounts of water which must be added to maintain crops and pastures are low
compared with irrigation demands in many drier parts of the world, however, and the better soils
in this
life zone are under cultivation. Potential evapotranspiration
is greater than rainfall in the
Subtropical Dry Forest zone, so the repeated use of irrigation water can sometimes result in salt
accumulation
in
the upper
soil horizons.
In
the
Lajas Valley, for example, the use of ground
water for irrigation resulted
in the loss of agricultural productivity on some
soils, until the mid
1950’s when water from another area was made available for irrigation (Bogart et al., 1964).
Goats are pastured on the poorer soils but to a much lesser extent than in years past. Charcoal
production, a common land use in many high-stress tropical environments, was once common in
this
life zone in Puerto Rico, but
this practice
is now almost extinct here. The Guanica Forest,
which has been protected from charcoal cutting, goat grazing, and subsistence farming for more
than 40
years,
is
perhaps
the
best example
of natural vegetation
in Subtropical Dry Forest
anywhere
in
the world.
Forestry
has
little commercial prospect in this
life zone, except in the
limited production of exceedingly durable
fence posts and a few high-quality specialty woods.
Swietenia mahagoni
Jacq.
(caoba
dominicana. West
Indies mahogany),
a prime example
(see
Figure
10),
is native
to Subtropical Dry Forest, though not to Puerto Rico and the U. S. Virgin
Islands (Little and Wadsworth,
1964). Plantations of this species have been very successful in St.
Croix and some, more limited, success has been achieved with this species in the Guanica Forest.
Guaiacum
officinale
L.
(guayacan,
lignumvitae), another species which produces
a high-value
specialty
wood,
does
occur
naturally
in
the
Subtropical Dry
Forest zone
of these
islands.
Urbanization of the Subtropical Dry Forest zone
is often a water-limited process; water must be
supplied
by
subterranean
aquifers,
brought
in from sources such
as reservoirs maintained by
runoff from
other, wetter
life zones, or trapped
in
artificial catchments, as
is common in the
Virgin Islands.
The Subtropical Moist Forest Zone
This
life zone, which covers more area (almost 5500 km2, or 58% of the total) in Puerto Rico
and the U.
S. Virgin Islands
tlian any of the other five
life zones found there, is delineated by a
mean
annual
rainfall
of
1000
or
1100 mm
to
about
2000
or
2200 mm
and
a
mean
biotemperature between about
18 and 24 C. Monthly rainfall, soil moisture storage, and runoff
are shown for three stations
in the Subtropical Moist Forest zone in Figures 11, 12, and
13. San
Juan (Figure
11),
at
sea
level, has
a water regime which
is surprisingly similar to those of the
higher elevation
sites:
Barranquitas,
at 671 m (Figure
12) and Aibonito, at 640 m (Figure
13).
Part of this similarity
is due to the fact that the many temperatures at San Juan which exceed
30 C (annual mean of 25.9 C) are substituted by zero when calculating biotemperature (annual
mean of 23.0 C); water balance calculations are based on the latter. Another similarity of these
three stations
is their rainfall distribution patterns. The major differences in Figures 11, 12, and
13 result from the different
soil moisture-holding capacities assumed for each of the three sites,
which ranged from a low of
1 19 mm per meter for the Cialitos series at Barranquitas to a high of
245 mm per meter for the Mucara
series
at Aibonito. At
all three sites moisture deficits are a
20
Figure
1 0 Subtropical Dry
Forest. Moist
cove,
Estate
Bellevue,
St. Croix
with
unusually straight small-leaf
mahogany, Swietenia mahagoni, about 40 years old.
21
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=i<03
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CO
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8
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om
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Figure
11
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
San
Juan,
Puerto
Rico,
in
Subtropical
Moist
Forest.
Ri'o
Lajas
s
series,
with
available
moisture
of
141
mm
per
meter.
o
c/2
23
Figure
12
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
Barranquitas,
Puerto
Rico,
in
Subtropical
Moist
Forest.
Cialitos
series,
with
available
moisture
of
1
19
mm
per
meter.
c
c/5
OJ
C\J
—
^
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uuuu ‘d3iVM
24
Figure
13
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
Aibonito,
Puerto
Rico,
in
Subtropical
Moist
Forest.
Mucara
series,
with
available
moisture
of
245
mm
per
meter.
serious problem only during March and April and considerable runoff occurs during the last three
to six months of each year. In general, the moisture regimes in this
life zone are nearly ideal on
an annual basis, with sufficient water to support agriculture, yet no excessive wetness.
Most of the Subtropical Moist Forest zone in the study region has been deforested at one time
or another (as in Figure 14, for example), primarily because the climatic conditions which define
its boundaries also encompass good conditions for a wide variety of crops. With the exception of
regions of serpentine- or limestone-derived
soils, most of the
land
in
this
life zone remains in
some form
of non-forest
use. A
sizable
tract
of cutover
forest
in
the
zonal
association
of
Subtropical Moist Forest remains
in
the Luquillo Experimental Forest, occupying the disjunct
section
located
off of
the
southwest corner of the main
forest boundary, southwest of Rio
Cubuy. This and other scattered remnants of the zonal association vegetation are characterized
by trees up to 20 m
tall, with rounded crowns, like those of the mango tree. Many of the woody
species are deciduous during the dry season and epiphytes are common, but seldom completely
cover branches and trunks. Grasses, in both natural and improved pastures, form the dominant
landscape
in the Subtropical Moist Forest zone in Puerto Rico today (Figure
15). They form a
complete ground cover, except on poor soils, and are fire-maintained on the southern slopes of
the Cordillera Central, as, for example, on the Mucara soil series around Cayey.
The moist limestone
hill region of northern Puerto
Rico produces an
interesting catena of
associations
in the Subtropical Moist Forest zone. These
hills (Figure
16), which are oriented
northeast to southwest, are quite moist on the gentle northern slopes, are even more humid on
the
extremely
steep southwest
slopes, and
are quite
xeric on
top. Data from
an
Institute of
Tropical Forestry Annual Report (U. S. Forest Service, 1950) show that diameter growth rates of
trees on the west-facing (leeward) slopes averaged almost twice as great as those on the east-facing
(windward) slopes; further, growth rates at the tops of the slopes were significantly slower than
those reported for the bottoms. These differences in vegetation and
tree growth are apparently
due
to differences
in microclimate and
soils resulting from aspect and slope. Guassia attenuata
(O.
F. Cook) Beccari (palma de
Iluvia)
is
a palm endemic
to Puerto Rico and
a conspicuous
component of the limestone
hill
forests
in Subtropical Moist
Forest.
It often grows near the
summits
of the
hills
and,
because
it
is
usually
taller than
the
surrounding
trees,
is
readily
identified from
a distance. During the mapping this species was not observed on the limestone
hills
in Subtropical Wet
Forest, but
it might be found
there on some of the more xeric
sites.
Little and Wadsworth (1964)
list 21
tree species as common members of these moist limestone
hill forests.
A second edaphic condition which results in an interesting association in the Subtropical Moist
Forest zone
is the serpentine-derived
soil (Nipe and Rosario series) in southwestern Puerto Rico,
where good
examples
can
be
seen
in
the Susua Commonwealth
Forest. This
soil supports
a
unique vegetation, which contains a number of endemics, but does not support any significant
agriculture or forestry. The trees are slender, open-crowned, and usually less than 12 m
tall. The
forest floor
is open, for the excessively drained
soil supports
little herbaceous growth. Most of
the species are sclerophyllous and the vegetation
is almost completely evergreen. Tschirley
<2 /.
(1970) found 105 species of woody plants larger than 2.5 cm dbh on 0.9 ha in this association,
compared
to
only
85
species on
1.4 ha
in
the wetter, more
luxuriant
Luquillo
Forest. The
number of woody stems was the same at both sites: 2.4 per 10 m2. This surprising richness of the
25
Figure 14 Subtropical
Moist
Forest.
Steep
deforested
slopes
of the
central mountain
range, Puerto
Rico,
where
intensive farming and multiple
species crops commonly produce high yields (Photo by D.
Pool).
26
27
Figure
15
Subtropical
Moist
Forest.
Large
areas
have
been
put
into
pasture
grasses.
Pastures
in
this
life
zone
can
be
highly
productive.
28
1
igurc
16
Subtropical
Moist
Forest.
Limestone
hills
association.
Described
also
as
haystack
hills
or
“mogotes,”
this
dogtooth
pattern
of
marine
origin
is
typical
of
nearly
20%
of
Puerto
Rico.
woody
flora on
the serpentine
soil, which contradicts most current ecological dogma, awaits a
satisfactory explanation.
Mangroves are found along the coast in the Subtropical Moist Forest zone where they appear
to grow taller than in the Subtropical Dry Forest zone, the only other sea-level life zone found in
Puerto Rico and the Virgin Islands. Just inland from the mangrove forests are alluvial soils where
the
ground
water may
be
slightly
brackish. Most
of these
soils were
cleared
long
ago
for
agriculture, but
they probably
supported
impressive
forests
at one
time. Gleason and Cook
(1926) (also in Cook and Gleason, 1928) described a stand of Ptewcarpus officinalis Jacq. (palo
de polio, swamp bloodwood), with their impressive ribbon buttresses (Figure
17), located near
Humacao
in
southeastern
Puerto
Rico.
Other
remnants
of
alluvial swamp
forest
in
the
Subtropical Moist Forest zone remain near Dorado, on the north coast of Puerto Rico, and at La
Boquilla, north of Mayaguez.
Roystonea borinquena O.
F. Cook (palma
real, Puerto Rico royalpalm), endemic to Puerto
Rico, Vieques, and St. Croix (Little and Wadsworth, 1964) and shown in the center of Figure
18,
is a common and readily
identified
species found
in many
areas within the Subtropical Moist
Forest zone. Tabebuia heterophylla (DC.) Britton (roble bianco, “white cedar”) sometimes forms
nearly pure stands in abandoned fields, particularly
in those portions of the
life zone where the
mean annual precipitation exceeds 1600 mm. Species of Nectandra and Ocotea, commonly called
“laureles,” are prominent in many of the older secondary forests. Spathodea campanulata Beauv.
(tulipan africano, African tulip tree) plus the common coffee shade
Erythrina poeppigiana
(Walp.)
O.
F. Cook (bucayo
gigante, mountain immortelle), Inga vera
Willd. (guaba), and
I.
laurina
(Sw.)
Willd.
(guama,
“sweetpea”),
are
also common
and
readily
recognized from
a
distance. Cedrela odorata L. (cedro hembra, Spanish-cedar), Hymenaea courbaril L. (algarrobo,
West-Indian-locust), Delonix regia (Bojer) Raf. (flamboyan, flamboyant-tree), and Ficus laevigata
Vahl (jaguey bianco, shortleaf fig) are common roadside
trees. Members of the Melastomaceae,
Piperaceae,
and
Rubiaceae
are
common
shrubs.
Cecropia
peltata
L.
(yagrumo
hembra,
trumpet-tree)
and Didymopanax morototoni
(Aubl.)
Decne.
and
Planche. (yagrumo macho,
matchwood) are found in successional forests in all moist, wet, and rain
life zones of Puerto Rico
and the Virgin Islands, but rarely extend into dry forest areas.
Throughout
the
world.
Subtropical Moist
Forest
(and
its Premontane
counterpart
in
the
Tropical Latitudinal Region)
is among the most intensively used
life zones. Pasture seems to be
the predominant land use in this zone in Puerto Rico today, but sugarcane, which requires little
or no irrigation water in this life zone, and coffee still occupy large areas. Pineapples are planted
in the drier portions of the Subtropical Moist Forest zone along Puerto Rico’s north-central and
northwest
coast,
while
tobacco
is
still
grown
extensively
in
certain
areas,
notably
in
the
Comerio-Cidra-Barranquitas region.
The
Subtropical
Moist
Forest
zone,
with
its
intermediate
moisture
conditions— neither
excessive rainfall nor high irrigation requirements— is the
life zone in Puerto Rico and the Virgin
Islands which
is
best
adapted
for
a wide
variety
of land use systems, many of which offer
potentially
high
yields.
Intensive
food
production
on
the
flats,
forage
production on
the
moderate
slopes, and managed
tree crops on the steep slopes are
all possible here. In addition,
this
life zone provides a much-needed supply of water on some of the drier islands, such as the
Virgin Islands and Vieques.
29
Figure 17 Subtropical Moist Forest. There are few remnants of the
alluvial swamp forest left in Puerto Rico,
all under heavy development pressures. This wet-season scene includes buttresses of Pterocarpus
officinalis, with ferns of the gQvms Acrostichum in the background.
30
Figure 18 Subtropical Moist Forest. Roystonea borinquena (center), tobacco shed (right side) and dwellings
on slope (left) in the central mountain range, Puerto Rico (Photo by D. Pool).
31
The Subtropical Wet Forest Zone
Subtropical Wet Forest occupies much of the higher parts of Puerto Rico’s mountains and, like
the wet- and
rain-forest
life zones yet
to be described,
is not found in any other island in the
study region. This
is a high rainfall
life zone, encompassing areas with mean annual precipitation
within the approximate range of 2000 to 4000 mm per year.
Rainfall, soil moisture, and runoff
graphs
are
included
for three
locations (Figure
19, 20, and 21) which range
in mean annual
rainfall from 2150 mm
(Carite Dam,
Figure
19)
to 2900 mm (Rio Blanco, Figure
21).
Soil
moisture drops below field capacity
at these three stations only for three months, at most, and
the water deficit is very small. Significant amounts of runoff occur at each of these sites during at
least seven months and, in some cases, all year. The annual runoff at these three sites (827 mm to
more
than
1600 mm)
is greater than
the
rainfall input
in most
areas of the Subtropical Dry
Forest zone.
The abundant moisture of this life zone
is evident in the character of the vegetation. Epiphytic
ferns, bromeliads, and
orchids
are common,
the
forests
are
relatively
rich
in
species, and the
growth
rates of successional
trees are rapid. A mat-forming fern, Gleichenia bifida Spreng. and
the common
tree-fern, Cyathea arborea
(L.)
J.
E.
Smith
(helecho gigante) form
a common
roadside combination here, as shown
in Figure 22. Mature forest remnants in this life zone exist
in the Carite and Toro Negro Commonwealth Forests and the Luquillo Experimental Forest. The
zonal association of Subtropical Wet Forest in Puerto Rico corresponds to what is locally referred
to as the tabonuco type, named for the dominant
Dacryodes excelsa Vahl (Figure 23). Two
other very prominent
species
are Sloanea berteriana Choisy
(motillo) and Manilkara bidentata
(A. DC.) Chev.
(ausubo,
bulletwood).
This
is an
impressive
forest, containing more than
150
species
of
trees, and forming
a dark, complete canopy
at about 20 m. One of the best brief
descriptions, both physiognomic and dendrological, of this association can be found in the article
by Wadsworth (1951). A multitude of studies on tropical forestry and ecology have been carried
out
in
this association over the
last 40 years, primarily
in the Luquillo Experimental Forest. By
far the most complete and broad-based of these was the series of investigations reported on in the
massive volume edited by Odum (1970a), which makes this one of the most intensively studied
forest types in the world.
Serpentine
areas,
described
for
the
Subtropical Moist
Forest
zone,
are
also found
in the
Subtropical Wet Forest zone. The vegetation on serpentine
is similar in the two life zones, except
that
it
is
greener,
denser, more
lush,
and,
most
strikingly,
contains more
epiphytes
in
the
Subtropical Wet
Forest. The
species
are
almost
all
evergreen
and
sclerophyllous,
giving
the
impression of an anomalous wet desert or dry rain forest. Excellent examples of this association
are available in the Maricao Commonwealth Forest.
As
in
the
case
in
the Subtropical Moist
Forest zone of Puerto
Rico, described
previously,
limestone
is the parent material for many of the soils in the Subtropical Wet Forest zone as well.
These limestone-derived
soils characterize several important azonal associations in
this
life zone
and include most of the Rio Abajo Commonwealth' Forest. Both Swietenia macrophylla King
(caoba hondureha, Honduras mahogany) and Tectona grandis
L.
f. (teca, teak), introduced to
Puerto Rico from climates much drier than that of Subtropical Wet Forest {e.g. Subtropical Dry
Forest,
Tropical
Dry
Forest,
Tropical Moist
Forest),
have done
well
in
plantations
on. the
limestone-derived
soils
in
the Rio Abajo
Forest. The natural vegetation on these soils
is much
32
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33
Figure
19
Mean
rainfall,
soil
nioislure,
and
runoll
calculated
for
Carite
Dam,
Puerto
Rico,
in
Subtropical
Wet
Forest.
Los
Guineos
soil
series,
with
available
moisture
of
320
mm
per
meter.
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34
series,
with
available
moisture
of
320
mm
per
meter.
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35
Figure
21
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
Rio
Blanco,
Puerto
Rico,
in
Subtropical
Wet
Forest.
Pandura
series,
with
available
moisture
of
441
mm
per
meter.
Figure 22 Subtropical Wet
Forest.
Cyathea arborea spreading over mat of Gleichenia
bifida
in
the higher
central mountains of Puerto Rico. These two plants commonly invade roadsides in this life zone.
36
37
rigiirc
23
Subtropical
Wet
b\)rcst.
Two
examples
Dacryodes
excelsa
in
the
climatic
association
of
the
Luquillo
Mountains.
Both
trees
are
about
a
meter
in
diameter.
more xerophytic than would be expected under the high rainfall of Subtropical Wet Forest, as is
true of most limestone-derived
soils. The dry edaphic condition, therefore, has an effect similar
to
that
of reduced
rainfall,
a
consideration which must
be
taken
into
account
in land use
planning.
Much of this life zone in Puerto Rico
is covered by successional vegetation as a result of the
emigration of much of the former rural population, due in part at least, to the inappropriateness
of most of the Subtropical Wet Forest zone for agriculture, as described by Tosi (1959). On these
unoccupied lands Piper aduncum L. (higuillo) often forms nearly pure stands. Cecropia peltata L.
(yagrumo
hembra,
trumpet-tree),
Didymopanax
morototoni
(Aubl.)
Decne.
and
Planch,
(yagrumo macho, matchwood), and Ochroma lagopus Sw. (guano, balsa) are abundant in many
successional
forests
(see
Figure
24). Prestoea montana
(R. Grab.) Nichols {-Euterpe globosa
Gaertn.
in
Little and Wadsworth,
1964) (palma de
sierra,
sierra palm)
is very common in the
wetter portions of the Subtropical Wet Forest zone. The species listed by Little and Wadsworth
(1964)
as components
of
their Lower
Luquillo
Province
pertain
chiefly
to Subtropical Wet
Forest, as do the species enumerated by Smith (1970 a & b).
Much of this
life zone, particularly in the western portion of the island,
is covered by coffee
plantations, but to a lesser extent than formerly. A coffee plantation, consisting of an overstory
of trees, a middle layer of coffee bushes, and a ground cover, is an agricultural ecosystem which
physiognomically resembles the
forest
it replaced. Shade-grown coffee has historically been
a
stable and successful land use in many wet, mountainous tropical areas. Some sugarcane is grown
in this
life zone also, as no irrigation water
is required here, though
it is possible that insolation
may sometimes be
limiting. The dominant trend in land use however, as on the rest of Puerto
Rico, seems to be an increase in grazing, but pasture management in the Subtropical Wet Forest
can be
difficult due
to both the vigorous growth of weeds and problems of soil compaction.
Perhaps the greatest value of the Subtropical Wet Forest zone in Puerto Rico
is as a source of
runoff which
supplies water
to
the
drier coastal
areas where the population
is concentrated.
Future land use planning of areas within the Subtropical Wet Forest
life zone should certainly
recognize the fact that any manipulations here may have very important downstream effects.
The Subtropical Rain Forest Zone
This,
the
wettest
of
the
sea-level
belt of Subtropical
life zones (lower
rainfall limit about
3800 mm), occupies very
little area
in Puerto
Rico, occurring only in a single crescent-shaped
band on
the windward
faces of
the
Luquillo
Mountains.
It
lies wholly within the Luquillo
Experimental
Forest
and encompasses,
appropriately
enough, much of the
area traversed by
visitors going to
the recreation area
in “the
rain
forest.” This
life zone
is characterized by
a
superabundance of precipitation,* as shown in Figure 25. The water regime at La Mina (Figure 25,
based on only eight years of rainfall data) indicates that the
soil is at field capacity all year, and
abundant
runoff
is produced
every month,
with
six months each year yielding more
than
300 mm. The annual total of 3400 mm of runoff is more than twice as much as most areas of the
world
receive
as annual rainfall input! The constantly wet soil eliminates water as a potentially
limiting growth factor in this environment, but oxygen stress, which can inhibit root respiration,
may exert an important influence on plant growth.
The species found here
are
the same,
for the most
part,
as those found in the surrounding
Subtropical Wet Forest. This may be due, in part, to the fact that water is not a limiting factor in
38
a) Cecropia peltata
c) Ochroma lagopus
b) Didymopanax morototoni
Figure 24 Subtropical Wet Forest. Three common,
fast-growing,
large-leaved,
successional
tree
species with similar physiognomies:
a)
Cecropia
peltata,
b) Didymopanax
morototoni, and c) Ochroma lagopus.
39
40
Figure
25
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
La
Mina,
Puerto
Rico,
in
Subtropical
Rain
Forest.
Los
Guineos
soil
series,
with
available
moisture
of
320
mm
per
meter.
either of these two
life zones, or, in part, to the fact that in Puerto Rico, as is often the case on
islands,
a few species perform many functions and spread over a wide variety of environments,
much more
so than would be the case on a continent.
If the
life zone occupied a large area or
were adjacent to a source of potential colonizers from the same life zone, one might well expect
the flora to be distinct from that of Subtropical Wet Forest, but that
is not the case on Puerto
Rico. The main features of the Subtropical Rain Forest are the high frequency of palms, Prestoea
montana (R. Grab.) Nichols (palma de
sierra, sierra palm) in this case (Figures 26 and 27), and a
superabundance
of
epiphytes;
no
stem,
building,
highway
marker,
or
fence
post
escapes
colonization and most leaves, even those in the upper canopy, are covered with epiphyllae. The
spiny
tree-fern, Nephelea
portoricensis
(Kuhn)
Tryon,
is more abundant
here
than
in
the
Subtropical Wet Forest.
Because of the small area
it occupies, the Subtropical Rain Forest in Puerto Rico is primarily
of academic
interest
and
recreational
value.
Last
year
nearly
a
million
people
visited
the
recreation area at La Mina. The Bano de Oro Natural Area, much of which lies in this life zone,
may be the only place in the world where an example of the mature vegetation of Subtropical
Rain Forest is likely to receive long-term protection, while still being readily accessible.
The Subtropical Lower Montane Wet Forest Zone
There
are two Lower Montane
life zones
in Puerto
Rico, of which
this
is by
far the most
extensive, occurring in both the eastern and central parts of the island up to the summits of most
mountains above 1000 meters and occasionally extending down to almost 700 meters. No peaks
in Puerto Rico are high enough to encounter the low temperatures necessary for the occurrence
of the next highest, or Montane, Altitudinal Belt. Rainfall, soil moisture, and runoff are shown in
Figure 28
for Guineo
Reservoir, located
in the Lower Montane Wet Forest
life zone. Like the
Subtropical Rain Forest station (La Mina,
Figure
25),
the
soil
at Guineo
Reservoir
is at field
capacity
all year and runoff occurs each month, but the total (1700 mm annually) is only about
half of that at La Mina.
The Colorado forest type, named for the common Cyrilla racemiflora L. (palo Colorado, swamp
cyrilla)
corresponds
to
the mature
vegetation
of
the zonal
association
in Subtropical Lower
Montane Wet Forest. The Cyrilla
is the same species which grows as a shrub or small tree in the
titi swamps
in
the
southeastern
U.
S.,
but
in
the mountains of Puerto
Rico
it
is
a
large,
reddish-barked canopy
tree. The hollow trunks of the older individuals are the main nesting sites
for the nearly extinct Puerto Rican parrot, Amazona
vittata
vittata (Boddaert). This forest
is
characterized by open-crowned
trees, many of which have their leaves grouped toward the ends
of the branches, and whose crowns, when seen in profile, resemble somewhat-flattened bouquets.
The leaves tend
to be coriaceous and dark, giving the canopy a brownish or even reddish cast.
This
forest
is poorer in
species than
the adjacent Subtropical Wet
Forest; Wadsworth (1951)
reported
that only 53
tree
species have been found
in
this type, compared to more than 150
species
in
the adjacent tabonuco type in Subtropical Wet Forest. In addition to the Cyrilla, he
lists Ocotea spathulata Mez. (nemoca), Micropholis chrysophylloides Pierre (caimitillo), and M.
garciniaefolia
Pierre
(caimitillo verde)
as abundant components of this association,
at
least
in
eastern Puerto Rico.
Two
additional associations
are commonly found
in Lower Montane Wet Forest
in Puerto
Rico. The
first of these
is
a cloud-forest
type, variously
referred
to as elfin woodland, mossy
41
42
Figure
26
Subtropical
Rain
Forest.
Frcstoea
montana,
dominant
species
on
cove
sites,
scattered
individuals
on
other
sites.
Epiphyte
are
extremely
abundant
(Photo
by
N.
Kirby).
43
Figure
27
Subtropical
Rain
Forest.
Prestoea
montana
roots,
apparently
an
adaptation
to
saturated
soil.
X
I-
44
Figure
28
Mean
rainfall,
soil
moisture,
and
runoff
calculated
for
Guineo
Reservoir,
Puerto
Rico,
in
Subtropical
Lower
Montane
Wet
Forest.
Los
Guineos
soil
series,
with
available
moisture
of
320
mm
per
meter.
forest, montane
thicket, or dwarf forest. A detailed study of this association,
in a site which
straddles both the Lower Montane Wet Forest and Lower Montane Rain Forest
life zones, was
recently
carried
out by Howard
and
coworkers
in
the
Pico
del
Oeste
area of the Luquillo
Mountains. Their results were reported in a series of papers (16 published through 1971) in the
Journal of the Arnold Arboretum. The introductory article in the series (Howard, 1968) included
physiognomic
and
floristic
descriptions of the
association,
as
well
as an excellent review of
previous work. The vegetation
is characterized by gnarled trees less than seven meters
tall, high
basal
area, small diameters,
a
large number of stems per unit area, and extremely slow growth
rates. All trees are evergreen and sclerophyllous here, and the leaves tend to be grouped near the
ends
of
the
branches. Common
trees
include
Ocotea
spathulata Mez
(nemoca),
Eugenia
borinquensis Britton (guayabota), Tabebuia rigida Urban (roble de sierra), Weinmannia pinnata L.
(oreganillo), and Calycogonium squamulosum Cogn. (jusillo), all of which (except W. pinnata) are
also found
in the zonal association, or Colorado
type. Interestingly enough, the palo Colorado
{Cyrilla
racemiflora
L.), which
is
so common
in
the
zonal
association,
is not found
in
this
atmospheric association.
Decomposition
is apparently slow
in
this association, and the upper soil seems to be almost
pure organic matter, while the subsurface soils are clayey and oxygen-deficient. Tree roots form a
tight, complete mat on the surface and do not appear to penetrate the soil very deeply. Another
impressive feature of this association
is the epiphyte load, which covers almost
all stems, even
those which are perfectly vertical and smooth-barked (see Figures 29 and 30).
In an event entirely unexpected for an area as well known biologically as Puerto Rico, Kepler
and Parkes (1972) recently discovered a new bird species, Dendroica angelae Kepler and Parkes
(elfin woods warbler), which is apparently endemic to this association in Puerto Rico.
Several
theories
have
been
proposed
to
explain
the
dwarf
stature
of
the
trees
in
this
association. Murphy (1916) felt that exposure to strong winds affected the physiognomy of this
vegetation, which he referred
to
as the “hurricane hardwood” type. Gleason and Cook (1926)
and
Beard
(1942)
also
attributed
it
to the high winds which continually buffet
these peaks,
which, when coupled with the wet soils, might produce physiological drought. Wadsworth and
Bonnet (1951) discussed water-saturated soils, with their impeded drainage, as factors influencing
tree growth in the adjacent Colorado type. Odum (1967) considered the dwarf cloud forest to be
transpiration-limited,
in
that the continual cloud cover and saturated
air impede transpiration,
resulting in limited flow of essential mineral nutrients from the
soil into the leaves. This theory
was recently bolstered by
the
findings of Weaver et
al.
(1973), who measured extremely low
transpiration rates for several tree species in the dwarf cloud forest association.
A
second
azonal
association
in
the Lower Montane Wet
Forest zone
is
the palm
brake,
consisting
of nearly
pure
strands
of Prestoea montana
(R.
Grab.) Nichols; examples of this
association are readily accessible in both the Luquillo and Toro Negro Forests. The palm brake,
seen
in Figure 31, has
several
characteristics of a successional ecosystem:
it
is dominated by a
single
species,
the borders between
it and adjacent associations
are very sharp, and
it extends
across
life zone boundaries, reaching down
into Subtropical Wet Forest and Subtropical Rain
Forest. Beard (cited in Wadsworth, 1951) felt that the occurrence of palm brake was related to
45
Figure 29 Lower Montane Wet
Forest. Liverworts account for most of the epiphyte biomass in this dwarf
cloud forest atmospheric association (Photo by D. Pool).
46
Figure 30 Lower Montane Wet Forest. Dwarf cloud forest liverworts capture horizontally moving water while
bromeliads such as this one collect throughfall and rainfall (Photo by D. Pool).
47
Figure 31 Lower Montane Wet
Forest. Palm brake
association.
Prestoea montana,
is still in doubt (Photo by D. Pool).
Cause of these communities, dominated by
48
rates of erosion and
that some of the areas where the palm formed pure stands were probably
former
landslides. Destruction of all trees by hurricanes, except the sierra palm which sheds its
fronds
in high winds and
therefore does not uproot (Murphy,
1916), might be another factor
which may
account
for
the pure stands
in Puerto
Rico.
In addition
to forming nearly pure,
possibly
successional,
stands on
steep
soils, Prestoea
is
also an important component of the
mature, mixed forest. The details of its life cycle there were worked out by Bannister (1970).
Because of the high rainfall here, the Lower Montane Wet Forest zone
is too
fragile for any
commercial forestry or agriculture, although
in some areas dairy cattle are pastured, mostly on
molasses grass {Melinus minutiflora Beauv.). Here again, as in other
life zones with exceedingly
high rainfall, the primary value lies in watershed yield, and managers of lands within this life zone
should probably hold that as their foremost goal.
The Subtropical Lower Montane Rain Forest Zone
This life zone occupies less area than any other in Puerto Rico and the U. S. Virgin Islands, and
is found only in a narrow band on the windward slopes of the Luquillo Mountains, immediately
above
the
Subtropical
Rain
Forest. No weather stations have operated
in
this
life zone long
enough
to justify
the
calculation of a complete water balance, but Baynton (1968) measured
several standard weather variables
for one
year, from March 1966 through February
1967. He
reported a mean annual temperature (based on the mean of average daily maximum and average
daily minimum) of 18.6 C, an annual rainfall of 4533 mm, and a mean realtive humidity of 98.5
percent!
If these conditions can be considered representative of the long-term mean values for
this
site, they would indicate an average, year-round runoff of almost 300 mm per month, and
some months could yield almost twice that amount.
The vegetation of this
life zone
in Puerto Rico
is very similar to that of Lower Montane Wet
Forest;
the
characteristic which
distinguishes
the two
is
the
greater abundance of epiphytes,
epiphyllae, palms, and
tree
ferns
in
the Lower Montane Rain Forest. Most of this
life zone
in
Puerto
Rico
is
in
the dwarf cloud
forest
association,
where much of the vegetation on
the
exposed
ridges
has
a windswept
appearance
(see Figure 32).
Howard
(1969)
described
the
morphology and structure of many of the species found
in
this environment, while Gill (1969)
documented the formation of aerial roots, which are extremely abundant on many species in this
association. The water-saturated
soil
is covered with a soil-free root mat, and the root-soil-earth-
worm relationships here were investigated by Lyford
( 1969). Epiphytes, most of which are leafy
hepatics, cover everything (Figure 33); this component of the flora was described by Fulford et
al. (1970). In a recent study. Weaver (1972) removed
all of che epiphytes from three plots and
compared these to plots with the epiphytes
left intact. He found that although the total amount
of water
reaching
the ground
was
little
affected,
the
distribution
pattern of throughfall and
stemflow was significantly altered.
Lower Montane
Rain
Forest
in
Puerto
Rico
is
primarily
a
biological
curiosity,
but
an
invaluable one since
it represents an environmental extreme and, as such,
is an excellent tool for
investigating the response of natural ecosystems to environmental
stress.
It
is indeed fortunate
that the limited amount of this life zone in Puerto Rico
is located in a publicly-controlled forest,
including
the upper parts of the Baho de Oro Natural Area, where long-term protection
is the
goal.
49
Figure 32 Lower Montane Rain Forest. Dwarf cloud forest association, East Peak Road, Luquillo
Mountains. The windward slope vegetation
is affected by constant breezes and cloud
cover (Photo by D. Pool).
50
Figure 33 Lower Montane
Rain
Forest.
Dwarf cloud
forest
association. An
eerie
habitat
dominated by
perpetual wetness and semi-darkness. The fern
is Cyathea pubescens, common on the higher peaks
and ridges of Puerto Rico (Photo by D. Pool ).
51
APPENDIX A: BIOTEMPERATURE
Many
factors,
including
light
quality,
temperature
range,
seasonality,
and
prevalence
of
hurricanes, vary with latitude and thus might influence the changes in vegetation one observes as
one moves north or south of the equator. Holdridge (1967) hypothesized that one factor which
might
be
closely
related
to
these
changes
is
biotemperature,
since
the
presence
of
high
temperatures, with
their effects on increased respiration and water stress, might reduce the net
assimilation of vegetation in these areas. He therefore suggested that biotemperature calculations
exclude not only temperatures below 0 C, but also temperatures above some upper limit, beyond
which photosynthesis might be exceeded by respiration. He proposed 30 C as a tentative value to
be used
as an upper cutoff. The
best
that can be expected
at
the
level of whole ecosystems,
however,
is
that
there
may
be
some
good,
albeit
empirical,
correlation
between
high
temperatures, as recorded
in standard weather stations, and the observed changes in vegetation.
Holdridge (1967) suggested that one method of calculating a biotemperature value which might
correlate well with these vegetation changes would be to substitute zero for all air temperatures
greater than 30 C. Such calculations were made for ten weather stations in Puerto Rico and the
U. S. Virgin Islands.
The temperature data came from four sources: data for two of the stations, San Juan Airport
and Santa Isabel, were obtained from the 1966 hourly data summaries published by ESSA (U. S.
Weather
Bureau);
the
data
from
Kingshill,
St.
Croix
were
obtained
from
the
U.
S.
D.
A.
Agricultural Research Service; the data for the El Verde Experiment Station were supplied by the
Puerto Rico Nuclear Center; the data from the remaining six stations came from Briscoe (1966).
The
ten
stations,
their elevations, and
the percent of temperatures which were greater than
30 C for each month are shown
in Table A.l. On an annual basis, these percentages ranged from
0 at
El Verde (510 m above sea level) and El Yunque (1060 m) to a high of 14.8 at Kingshill, St.
Croix (67 m). Several stations— Santa Isabel, Rio Blanco, Cape San Juan, Gurabo, Kingshill—had
individual months where more than 20% of the recorded temperatures exceeded 30 C.
Calculating biotemperatures by substitution of zero for temperatures greater than 30 C has the
effect
of
depressing
the
mean
temperature
over
any
period
which
includes
such
high
temperatures. The
results of the biotemperature calculations are plotted, by months, in Figure
A.l. The shaded portion of each plot represents
the amount by which
the biotemperature
is
decreased below the mean air temperature. All stations are plotted in Figure A.l except El Verde
and
El Yunque, where
there was no difference between
air temperature and biotemperature.
Three of the stations—Santa
Isabel, San Juan Airport, and Kingshill—show a greater difference
between
air
temperature
and
biotemperature
during
the
latter
part of the year, about June
through November. Three other stations—Cape San Juan, Fajardo, and Catalina—show greater
differences during the early part of the year, February through May. The other stations show no
apparent seasonal patterns. The difference between biotemperature and air temperature decreases
with increasing altitude. Catalina, at 150 m above sea level, has an annual biotemperature regime
only
slightly lower than
its air temperature (Figure A.l). At the El Verde Experiment Station,
located
at 510 m,
air temperatures above 30 C occurred
for only
five hours during a one-year
period.
52
Table
A.l.
Frequency
of
temperatures
greater
than
30
C.
Values
shown
are
percent
of
hours
when
temperatures
exceeded
30
C;
number
in
parenthesis
under
each
percent
is
the
number
of
days
record
upon
which
the
percent
is
based.
Only
days
with
complete
24-hour
records
were
used.
Value
of
“0”
indicates
that
no
temperatures
exceeded
30
C,
while
“0.0”
indicates
that
some
hourly
temperatures
exceeded
30C
but
that
these
accounted
for
less
than
0.1
percent
of
the
record
examined.
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52
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—I
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O
wo
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wo
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vOO
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53
*
This
station
is
located
at
the
Fajardo
water
filtration
plant;
not
at
the
city
of
Fajardo
(see
Briscoe,
1966).
54
Figure
A.l
Monthly
mean
air
temperatures
and
biotemperatures
at
eight
stations
in
Puerto
Rico
and
the
U.
S.
Virgin
Islands.
b\)r
Kingshill,
line
A
is
temperature
calculated
by
substitution
of
30
for
all
temperatures
greater
than
30
C\
line
B
resulted
from
the
exclusion
of-all
temperatures
greater
than
30C
from
the
calculations.
Table
A. 2
shows
the
mean
annual
air
temperature
compared
with
the
mean
annual
biotemperature
at each of the ten stations. The greatest difference was at Gurabo, where mean
annual air temperature was 5.7 C higher than the mean annual biotemperature, while at Catalina
the mean annual biotemperature was only 0.7 C lower than the mean annual air temperature.
The biotemperature calculations clearly suggest that, assuming there is some biological validity
to
the upper
limit of 30 C, Puerto Rico and
the U.
S. Virgin
Islands should be
classified
as
belonging
to the Subtropical Latitudinal Region, according to Holdridge’s nomenclature. All of
the ten mean annual biotemperatures are
less than 24 C, Holdridge’s lower limit for the Tropical
Latitudinal Region.
In Costa
Rica, Holdridge et
al. (1971) estimated biotemperatures from
limited hourly data
coupled with mean maximum and mean minimum temperatures. They estimated the differences
between mean annual air temperature and mean annual biotemperature to be, in most cases, less
than 2 C. The stations they dealt with ranged from about 8° 30’ N to about
1 1° 00’ N latitude; all
of
their
study
region was
classified
as belonging
to
the Tropical Latitudinal Region. Shibata
(1970), who also worked with data from Costa Rica, found that, even at 1500 meters above sea.
level,
there was a slight (less than 0.5 C) depression of the biotemperature. She used data from
only four hours per day, however, rather than the
full 24 hour record. At Managua, Nicaragua
(12° 8’ N) Shibata (1970), again using four data points per day, found that the mean annual air
temperature of 27.4 C reduced to a biotemperature of 19.0 C; a drop of 8.4 C, or 30.7 percent.
She classified Managua as part of the Subtropical Latitudinal Region. She concluded, on the basis
of published vegetation maps and temperature
calculations, that 31 C might be a better upper
cutoff for biotemperature calculations than 30 C.
Using
data
provided
by
S.
C.
Snedaker
(School
of Forest
Resources and
Conservation,
University of Florida) for Finca Murcielagos (15 meters above sea
level, located on Lake Izabal,
Guatemala,
at
15°
30’
N),
the mean
annual
biotemperature
was
calculated
to
be
17.6 C,
compared to a mean annual air temperature of 25.1 C. Slightly more than 15 percent of the total
record
(1962-1967)
of
almost
30,000
hourly
temperatures
were
greater
than
30 C.
Finca
Murcielagos, located about three degrees lower in latitude than Puerto Rico, has a biotemperature
which
is depressed even more below
air temperature than are the biotemperatures of sea-level
stations in Puerto Rico.
There are, of course, many ways to manipulate temperature data
in hopes of obtaining some
sort of empirical correlation with natural vegetation. Figure A.l, for example, shows the results
of calculating biotemperature
at
Kingshill,
St. Croix
three different ways:
1.) by excluding
all
hours with temperatures greater than 30 C from the calculations; 2.) by substituting 30 C for all
hourly temperatures greater than 30 C; and
3.) by substituting zero for
all hourly temperatures
greater than 30 C. The result
is three very different temperature regimes; which of these, if any,
however,
is functionally related to the structure of natural ecosystems remains to be tested in the
field.
A second example utilizes the extensive weather data published by Briscoe (1966). Using his
tables, which provided mean hourly temperatures by months, the data were scanned for values
greater than 30 C. Zero was substituted for these high temperatures, and biotemperatures were
55
Table A. 2. Mean annual values of air temperature and biotemperature for ten stations in Puerto
Rico and the U. S. Virgin Islands. These calculations were based on hourly values.
Station
Elevation
Mean Annual
Air Temperature*
Mean Annual
Biotemperature
Difference
Decrease
(meters)
(°C)
CC)
CO
(%)
Santa Isabel
1
1
25.2
21.3
3.9
15.5
San Juan Airport
(Weather Bureau)
19
25.9
23.0
2.9
11.2
Rio Blanco
31
25.6
21.6
4.0
15.6
Cape San Juan
39
27.0
22.2
4.8
17.8
Gurabo
50
24.7
19.0
5.7
23.1
Kingshill, St. Croix
67
26.8
22.1
4.7
17.5
Fajardo**
71
25.4
23.8
1.6
6.3
Catalina
150
24.1
23.4
0.7
2.9
El Verde
(Experiment Station)
510
22.
1
^2
1
0.0
0
El Yunque
1050
18.7
18.7
0.0
0
* May
differ from
value shown
in Table A. 3 because
the two means are based on different
periods of record.
** This station
is located
at
the
Fajardo water filtration plant; not at the city of Fajardo (see
Briscoe, 1966).
56
calculated in the usual way. The result of this method of biotemperature calculation is shown in
Table A. 3 for those seven stations for which mean hourly temperatures by months were available.
Since the data were expressed as hourly means, by months, rather than individual hourly values,
the biotemperature
is much closer
to
the
air temperature when calculated
this way. That
is, a
mean of 30 or 31
hourly values
is much less likely to be greater than 30 C than
is any of the
individual hourly values which go into making up that mean.
A major research need
is a series of studies at two
levels:
1.) to determine what quantitative
aspects of natural ecosystems are well correlated with various weather-station temperature data,
and
2.) to determine which environmental factors that are well correlated with air temperatures
directly
affect
the structure and function of whole ecosystems. Only such studies can properly
test the various hypotheses concerning the relationships between weather station data and natural
vegetation.
57
Table A. 3. Mean
annual
values
for
air temperature
and
biotemperature
for
seven stations in
Puerto Rico. These calculations were based on the mean monthly values, by hours, of
Briscoe (1966).
Station
Elevation
(meters)
Mean Annual
Air Temperature*
(°C)
Mean Annual
Biotemperature
(°C)
Difference
CC)
Decrease
(%)
San Juan Airport
(Weather Bureau)
19
25.6
25.4
0.2
0.8
Rio Blanco
31
25.5
24.8
0.7
2.7
Cape San Juan
39
26.6
25.9
0.7
2.6
Gurabo
50
24.5
21.5
3.0
12.2
Fajardo**
71
25.0
25.0
0.0
0
Catalina
150
24.3
24.3
0.0
0
El Yunque
1050
18.4
18.4
0.0
0
* May
differ from value shown
in Table A. 2 because
the two means are based on different
periods of record.
** This station
is located
at
the Fajardo water filtration plant; not at the city of Fajardo (see
Briscoe, 1966).
58
APPENDIX B: WATER BALANCES
The following tables (B.l
through B.IO)
are the complete water balance for ten stations in
Puerto Rico and the U.
S. Virgin Islands. The method used was that of Tosi (Ewelefa/., 1968;
Holdridge et al., 1971; FAO, 1971), which combines features of Holdridge’s life zone system and
its relation
to water budgets
(see,
for example, Holdridge, 1962a) with the tabular accounting
developed by Thornthwaite (1948; Thornthwaite and Mather,
1957). One of Tosi’s innovative
features
is the correction applied to potential evapotranspiration in climates where the potential
evapotranspiration
is
greater
than
the
annual
precipitation
(i.e.
where
P.
ET/R^*!).
Tosi
proposed that special adaptations of the vegetation in dry areas result in a decrease of potential
evapotranspiration;
this
corrected
potential
evapotranspiration
is .shown
for
the two
driest
stations. Tables B.l and B.2.
The calculated water balances, in keeping with Tosi’s criteria, assume the presence of mature,
natural vegetation. This
is not the case at most of the sites included, so the water-use rates shown
in the
tables should be taken as maximum probable values; actual evapotranspiration
is usually
greater from mature terrestrial ecosystems than from those which are successional.
Holdridge assumes that mean annual potential evapotranspiration
is a linear function of mean
annual biotemperature, with the intercept
at 0 and
a slope of 58.93 mm °C~1. Smith (1967)
compared
potential
evapotranspiration
calculated
according
to
Holdridge’s assumption
with
values obtained during an eleven-year period from four Thornthwaite-type potential evapotran-
spirometers in Jamaica and found that the annual values differed by only about four percent, but
monthly deviations were often greater.
Moisture relations of the soils shown in the tables are from data in Lugo-L6pez (1953), except
for the Glynn series, data for which were provided by L. H. Rivera from the files of the U. S. D.
A., Soil Conservation Service. An effective rooting depth of one meter was assumed for all soils;
soil moisture changes in millimeters therefore refer to millimeters per meter of soil depth.
One further caveat with regard to the water balances:
the calculations are based on long-term
means, so the actual values shown might be combinations which never occur in nature. The water
balance calculations should, therefore, be used primarily for the trends they indicate, rather than
for the prediction of actual conditions at a specific point at a particular time.
Water balances such as these are perhaps most useful when used in conjunction with additional
empirical data from
specific
sites. For example, streamflow data are available for a number of
drainages
in Puerto Rico and
this information, coupled with
the water balances, can result in
useful guides to land utilization planning in various kinds of environments. Antonini et al. (1973)
recently applied water balance calculations such as these to a watershed encompassing four life
zones
in
the Dominican
Republic. The
water budgets were coupled
to
a model of land use
changes to predict the effects of various land use policies on siltation in a downstream reservoir.
Bogart
et al
(1964)
compiled much
of the watershed data
available
for Puerto Rico and
calculated
water
budgets,
including
rainfall' inputs,
evapotranspiration,
runoff, ground-water
discharge, and
storage,
for the whole island. Similar calculations, using rainfall and streamflow
59
data combined with Thomthwaite’s water balances have been reported for Puerto Rico by Giusti
and Lopez (1967) and for the U.
S. Virgin Islands by Bowden (1968) and Bowden ef a/. (1970).
The
life zone map, together with the Tosi water balances, might be one useful way to look at
important subdivisions of the area’s water budget.
Another useful application might be the simulation of water budgets assuming that rainfall can
vary
considerably
with
respect
to
its
longterm mean; such
calculations might be
useful
if
combined with predictive variables, such as Palmer’s drought index, which Calvesbert (1967) has
apphed to Puerto Rico.
60
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
°C
23.1
23.1
23.5
24.7
25.8
26.3
27.1
26.9
26.4
25.8
25.4
24.5
25.2
Biotemperature
“C
23.1
23.1
23.5
24.7
24.9
22.9
17.6
16.4
17.7
17.8
21.1
23.2
21.3
Potential
evapotranspiration
mm
116
105
118
120
125
111
88
82
86
89
102'
116
1258
P. ET. adjusted for
dry climate
mm
83
75
84
86
90
79
63
59
62
64
73
83
901
Precipitation
mm
22
24
13
43
112
100
70
113
150
131
79
44
901
Actual
evapotranspiration
mm
83
75
48
61
90
79
63
59
62
64
73
83
840
Water surplus
mm
0
0
0
0
22
21
7
54
88
67
6
0
Soil moisture change
mm
-61
-51
-35
-18
+22
+21
+7
+54
+88
+12
0
-39
Moisture available in soil at
end of the month
mm
122
71
36
18
40
61
68
122
210
222
222
183
All runoff
mm
0
0
0
0
0
0
0
0
0
55
6
0
61
Soil moisture deficit
mm
100
151
186
204
182
161
154
100
12
0
0
39
Precipitation deficit
mm
61
51
71
43
0
0
0
0
0
0
0
39
Total moisture deficit
mm
161
202
257
247
182
161
154
100
12
0
0
78
Table B.l Water balance for Santa Isabel, Subtropical Dry Forest, assuming mature vegetation on
Teresa soil series (available moisture = 222 mm). Elevation = 8 m, P. ET./R =
1 .40.
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
°C
25.7
24.9
25.4
26.9
26.9
28.3
28.4
28.1
28.0
27.3
26.6
25.3
26.8
Biotemperature
°C
23.9
23.7
22.8
20.1
20.2
14.3
13.3
14.8
16.2
17.7
20.3
23.0
19.2
Potential
evapotranspiration
mm
120
108
114
97
101
69
67
74
78
89
98
115
1130
P. ET. adjusted for
dry climate
mm
117
105
111
95
98
67
65
72
76
87
96
112
1102
Precipitation
mm
67
39
32
46
119
96
84
104
145
150
154
66
1102
Actual
evapotranspiration
mm
109
60
43
51
98
67
65
72
76
87
96
112
936
Water surplus
mm
0
0
0
0
21
29
19
32
69
63
58
0
Soil moisture change
mm
-42
-21
-11
-5
+21
+29
+19
+32
+24
•
0
0
-46
Moisture available in soil at
end of the month
mm
42
21
10
5
26
55
74
106
130
1
130
130
84
All runoff
mm
0
0
0
0
0
0
0
0
45
63
58
0
166
Soil moisture deficit
mm
88
109
120
125
104
75.
56
24
0
0
0
46
Precipitation deficit
mm
50
69
79
49
0
0
0
0
0
0
0
46
Total moisture deficit
mm
138
178
199
174
104
75
56
24
0
0
0
92
Table B.2 Water
balance
for
Kingshill,
St.
/Croix,
Subtropical Dry
Forest,
assuming mature
vegetation on Glynn
soil
series
(available moisture = 130 mm). Elevation = 61 m, P.
ET./R =
1.03
61
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
°C
24.2
24.3
25.4
25.1
25.5
26.9
27.2
27.5
26.9
27.1
25.7
25.9
25.9
Biotemperature
°C
24.2
24.3
24.3
24.8
25.5
21.8
22.2
20.2
20.2
19.8
24.4
23.9
23.0
Potential
evapotranspiration
mm
121
111
122
120
128
106
111
101
98
99
118
120
1356
Precipitation
mm
119
74
56
95
181
144
159
181
172
148
165
138
1632
Actual
evapotranspiration
mm
121
111
107
120
128
106
111
101
98
99
118
120
1341
Water surplus
mm
0
0
0
0
51
38
48
80
74
49
47
18
Soil moisture change
mm
“2
-37
-51
-25
+51
+38
+24
0
0
0
0
0
Moisture available in soil at
end of the month
mm
139
102
51
26
79
117
141
141
141
141
141
141
All runoff
mm
0
0
0
0
0
0
24
80
74
49
47
18
291
Soil moisture deficit
mm
2
39
90
115
64
24
0
0
0
0
0
0
Precipitation deficit
mm
2
37
66
25
0
0
0
0
0
0
0
0
Total moisture deficit
mm
4
76
156
140
64
24
0
0
0
0
0
0
Table B.3 Water
balance
for San Juan
(Airport, Weather Bureau),
Subtropical Moist
Forest,
assuming mature vegetation on Rio Lajas
soil
series (available moisture =
141 mm).
Elevation = 4 m; P. ET./R = 0.83.
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
(=Biotemperature)
°C
19.6
19.9
21.4
22.9
23.3
23.9
23.7
23.4
23.4
23.6
21.7
21.0
22.3
Potential
evapotranspiration
mm
98
91
107
111
117
116
119
117
113
118
105
105
1317
Precipitation
mm
89
77
70
101
149
108
121
155
169
184
150
104
1478
Actual
evapotranspiration
mm
98
91
107
111
117
116
119
117
113
118
105
105
1317
Water surplus
mm
0
0
0
0
32
0
2
38
56
66
45
0
Soil moisture change
mm
-9
-14
-37
-10
+32
-8
+2
+38
+7
0
0
-1
Moisture available in soil at
end of the month
mm
235
221
184
174
206
198
200
238
245
245
245
244
All runoff
mm
0
0
0
0
0
0
0
0
49
66
45
0
160
Soil moisture deficit
mm
10
24
61
71
39
47
45
7
0
0
0
1
Precipitation deficit
mm
9
14
37
10
0
8
0
0
0
0
0
1
Total moisture deficit
mm
19
38
98
81
39
55
45
7
0
0
0
2
Table B.4 Water balance for Aibonito, Subtropical Moist Forest, assuming mature vegetation on
Mucara soil series (available moisture = 245 mm). Elevation = 640 m; P. ET./R = 0.89.
62
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
(=Biotemperature)
°C
19.7
19.7
20.5
21.3
22.1
22.8
22.7
23.1
22.9
22.7
21.8
20.3
21.7
Potential
evapotranspiration
mm
99
90
103
103
111
110
114
116
111
114
106
102
1279
Precipitation
mm
93
77
54
105
140
123
112
148
222
187
142
136
1539
Actual
evapotranspiration
mm
99
90
103
103
111
no
114
116
111
114
106
102
1279
Water surplus
mm
0
0
0
2
29
13
0
32
111
73
36
34
Soil moisture change
mm
-6
-13
-49
+2
+29
+13
-2
+23
.0
0
0
0
Moisture available in soil at
end of the month
mm
113
100
51
53
82
98
96
119
119
119
119
119
All runoff
mm
0
0
0
0
0
0
0
9
111
73
36
34
263
Soil moisture deficit
mm
6
19
68
66
37
21
23
0
0
0
0
0
Precipitation deficit
mm
6
13
49
0
0
0
2
0
0
0
0
0
Total moisture deficit
mm
12
32
117
66
37
21
25
0
0
0
0
0
Table B.5 Water balance for Barranquitas,
on Cialitos
soil series (available
0.83.
Subtropical Moist Forest, assuming mature vegetation
moisture = 119 mm). Elevation
= 671 m; P. ET./R =
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
(=Biotemperature)
°C
20.9
20.8
21.3
21.8
22.3
22 9
23.2
23.7
23.7
23.4
22.8
21.8
22.4
Potential
evapotranspiration
mm
105
95
107
106
112
111
116
119
115
117
no
109
1322
Precipitation
mm
107
86
65
134
245
223
227
220
276
247
182
137
2149
Actual
evapotranspiration
mm
105
95
107
106
112
111
116
119
115
117
no
109
1322
Water surplus
mm
2
0
0
28
133
112
111
101
161
130
72
28
Soil moisture change
mm
0
-9
-42
+28
+23
0
0
0
0
0
0
0
Moisture available in soil at
end of the month
mm
320
311
269
297
320
320
320
320
320
320
320
320
All runoff
mm
2
0
0
0
no
112
111
101
161
130
72
28
827
Soil moisture deficit
mm
0
9
51
23
0
0
0
0
0
0
0
0
Precipitation deficit
mm
0
9
42
0
0
0
0
0
0
0
0
0
Total moisture deficit
mm
0
18
93
23
0
0
0
0
0
0
,
0
0
Table B.6 Water balance for Carite Dam, Subtropical Wet Forest,
Los Guineos soil series (available moisture = 320 mm).
0.62.
assuming mature vegetation on
Elevation = 610 m; P. ET./R =
63
Long-temi mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
(=Biotemperature)
"C
18.9
19.1
19.1
20.0
21.1
22.1
222
223
22.1
21.7
20.8
20.0
20.8
Potential
evapotranspiration
mm
95
87
96
97
106
107
111
112
107
109
101
100
1228
Precipitation
mm
74
75
86
156
279
186
208
302
327
346
195
101
2335
Actual
evapotranspiration
mm
95
87
96
97
106
107
111
112
107
109
101
100
1228
Water surpltis
mm
0
0
0
59
173
79
97
190
220
237
94
1
Soil moisture change
mm
-21
-12
-10
+43
0
0
0
0
0
0
0
0
Moisture available in soil at
end of the month
mm
229
287
277
320
320
320
320
320
320
320
320
320
All runoff
mm
0
0
0
16
173
79
97
190
220
237
94
1
1107
Soil moisture deficit
mm
21
33
43
0
0
0
0
0
0
0
0
0
Precipitation deficit
mm
21
12
10
0
0
0
0
0
0
0
0
0
Total moisttire deficit
mm
42
45
53
0
0
0
0
0
0
0
0
0
Table B.7 Water balance for Garzas, Subtropical Wet Forest, assuming mature vegetation on Los
Guineos soil series (available moisture = 320 mm). Elevation = 829 m; P. ET./R = 0.53.
Long-term mean
.Month
Year
Jan.
Feb.
Mar.
.Apr.
May
June
July
.Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
X
23.4
24.1
26.1
25.3
26.6
26.9
26."
26.2
26.3
25.2
23.6
25.6
Biotemperature
X
23.3
23.9
-
19.2
24.3
17.1
22.8
20.5
19.6
20.0
21.2
24.0
23.6
21.6
!
1
'
Potential
evapotranspiration
mm
116
109
96
118
86
110
103
98
97
106
116
118
f
12-3
!
Precipitation
1
mm
159
126
97
\~5
322
289
268
30"
359
325
251
218
2896
!
1
i
.\ctual
evapotranspiration
mm
116
109
96
118
86
110
103
98
97
106
116
118
1273
1
1
Water surplus
mm
43
17
1
5
~
236
179
165
209
262
219
135
100
1
Soil moisture change
mm
0
0
0
0
0
0
0
0
0
0
0
0
1
_
.
-
-
Moisture available in soil at
end of the month
mm
441
441
441
441
441
441
441
441
441
441
441
441
1
i
.All runoff
mm
43
r
1
57
236
179
165
209
262
219
135
100
1623
!
!
Soil moisture deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
i
1
!
1
Precipitation deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
i
!
Total moisture deficit
1
mm
1
0
0
0
0
0
0
1
0
0
0
0
0
Table B.8 Water
balance
for
Rio
Blanco
Lower,
Subtropical Wet
Forest,
assuming mature
vegetation on Pandura soil series (available soil moisture = 441 mm). Elevation = 40 m,
P. ET./R = 0.44.
64
Long-term mean
Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
Air temperature
(=Biotemperature)
“C
19.8
19.3
19.9
20.8
21.3
22.1
22.3
22.7
22.7
22.3
21.6
20.5
21.3
Potential
evapotranspiration
mm
99
88
100
101
107
107
112
114
110
112
106
103
1259
Precipitation
mm
364
283
183
236
515
478
357
472
385
437
511
439
4660
Actual
evapotranspiration
mm
99
88
100
101
107
107
112
114
110
112
106
103
1259
Water surplus
mm
265
195
83
135
408
371
245
358
275
325
405
336
Soil moisture change
mm
0
0
0
0
0
0
0
0
0
0
0
0
Moisture available in soil at
end of the month
mm
320
320
320
320
320
320
320
320
320
320
320
320
All runoff
mm
265
195
83
135
408
371
245
358
275
325
405
336
3401
Soil moisture deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Precipitation deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Total moisture deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Table B.9 Water balance
for La Mina, Subtropical Rain Forest, assuming mature vegetation on
Los Guineos soil series (available moisture = 320 mm). Elevation = 625 m; P. ET./R =
0.27.
Long-term mean
•Month
Year
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
(mean or total)
.Air temperature
(=Biotemperature)
Of
17.6
17.4
17.8
18.6
19.7
20.7
21.1
21.1
20.7
20.3
19.6
18.4
19.4
Potential
evapotranspiration
mm
88
79
89
90
99
100
106
100
104
98
98
89
1 140
Precipitation
mm
104
109
111
212
418
231
196
321
400
411
223
127
2863
Actual
evapotranspiration
mm
88
79
89
90
99
100
106
100
104
98
98
89
1140
Water surplus
mm
16
30
122
319
131
90
221
296
313
125
38
Soil moisture change
mm
0
0
0
0
0
0
0
0
0
0
0
0
Moisture available in soil at
end of the month
mm
320
320
320
320
320
320
320
320
320
320
320
320
All runoff
mm
16
30
122
319
131
90
221
296
313
125
38
1723
Soil moisture deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Precipitation deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Total moisture deficit
mm
0
0
0
0
0
0
0
0
0
0
0
0
Table B.
1 0 Water
balance
for
Guineo
Reservoir,
Subtropical
Lower Montane Wet
Forest,
assuming mature vegetation on Los Guineos soil series (available moisture = 320 mm).
Elevation = 914 m; P. ET./R = 0.40.
65
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ACKNOWLEDGEMENTS
Dr. Frank Wadsworth (U.
S. D. A., Forest Service) reviewed the manuscript very thoroughly.
He made many helpful improvements, corrections, and clarifications.
Professor Roy Woodbury and
Dr. Modesto Capiel (both of the University of Puerto Rico)
reviewed
the paper, with special attention to
floristics and climatology, respectively. Each of
them made many useful suggestions and
corrections.
Dr. Thomas Geary (U.
S. D. A., Forest
Service) suggested that we start this project, and supported
it throughout; he also made helpful
suggestions on the manuscript.
Mr. Ernesto
Striker, University of Florida, planimetered the area occupied by each life zone
and wrote a computer program for biotemperature analysis. Mr. Juan Bartolome, of the Puerto
Rico Department of Natural Resources, provided many of the biotemperature calculations. Mr.
Steve Gladden, Department of Geography, University of Florida, wrote the computer program
used for the calculation of water balances. Computer time for water balance and biotemperature
calculations
was
provided
by
the
Northeast
Regional
Data
Center,
University
of
Florida,
Gainesville.
72
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UNITED
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J.J. EWEL
and
J.L. WHITMORE
.
UHtTEO
STATES
DEPARTMENT
Of
AGRICULTURE
ER
FOR
LATIN
AMERICAN
STUDIES
CARTOGRAPHIC
LABORATORY,
INSTITUTE
OF
TROPICAL
FORESTRY
RIO
PIEORAS.
PUERTO
RICO,
1973.
UNIVERSITY
or
FLORIDA
GLENN
M
FISH,
CARTOGRAPHER
CLASSIFICATION OF WORLD LIFE ZONES OR PLANT FORMATIONS
by L.R. Holdridge
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