BULLETIN OF MARlNE SCIENCE, 60(3): 643-647, 1997
LAI AND LEAF SIZE DIFFERENCES IN TWO
RED MANGROVE FOREST TYPES IN
SOUTH FLORIDA
Rafael J. Araujo, Juan C. Jaramillo and Samuel C. Snedaker
ABSTRACT
Leaf area indices (LAI) were evaluated for two different red mangrove (Rhizophora mangle
L.) forest types (basin and dwarf) in southeast Florida using the plumb line and canopy light
interception methods, LAI and leaf size (length, width and area) were found to be significantly
different in the two forest types, The basin forest type had the highest LAI (5.7), lowest light
transmissions (7.2 percent of ambient) and the largest leaf dimensions in contrast to the dwarf
type (LAI 3.0, percent light 17.2). The results reflect different environmental forces acting
upon each location and reinforce the practical ecological value of the classification of mangrove forest types.
Individuals of Rhizophora mangle L. (Rhizophoraceae) exhibit a remarkable
plasticity in their above-ground and canopy architecture. The extreme forms range
from tall excurrent trees in closed canopy forests to small scattered decurrent
individuals that rarely exceed 1.5 m in height in what is called a dwarf forest
(Lugo and Snedaker, 1974). Although leaf shape is relatively uniform with an
ovate or elliptic outline, a pointed apex, and an entire margin (Tomlinson, 1986;
Araujo and Polania, 1988), leaf sizes are highly variable. In general, mean leaf
size in R. mangle tends to be largest (10-15 cm long and 5-7 cm wide) in
optimum habitats and become progressively smaller in increasingly suboptimum
habitats (Canoy, 1975; Snedaker and Brown, 1981). Thus, leaf size in Rhizophora
is considered a reliable indicator of chronic stress and conversely, the relative
vigor of the vegetation (Cintr6n and Schaeffer-Novelli, 1983; Camilleri and Ribi
1983). In general, crown shape and leaf arrangement in individual tree species
are responses to controlling factors which include competitiveness and the ability
to respond to stress and environmental change (Tomlinson, 1986; Oldeman, 1978).
The leaf area index (LAI) is defined as the total leaf surface area per unit
ground-surface area measured in equivalent units (e.g., m2 m-2), and is one parameter used to estimate the photosynthetic capacity of plants and their role in
gas, water, carbon and energy exchange (Pool, 1973; Russell et al., 1989). Since
the LAI integrates leaf number and leaf area, site-specific variation in one parameter can be masked by an offsetting variation in the other parameter. Thus, the
same LAI could result from either a small number of large leaves or a large
number of small leaves.
In mangrove forests, reported LAI values range between 0.2 and 5.1 with lowest values in dwarf forests and highest values in riverine forests (Cintr6n and
Schaeffer-Novelli, 1983). In a sample of eight mangrove forests, Cintr6n et al.
(1980) reported a mean of 3.8 :!: 0.6 m2 m-2• These relatively low values for
undisturbed mangrove forests, compared to other tropical forest types with LAI
values sometimes above 10 (Cintr6n and Schaeffer-Novelli, 1983), have been
attributed to the intrinsic shade intolerance of mangroves, the dense aggregation
of foliage in the upper part of the canopy, and the typical absence of an understory
(Cintr6n and Schaeffer-Novelli, 1985; Snedaker and Lahmann, 1988).
The purpose of this study was to compare LAI, canopy light transmission, and
643
644
BULLETIN OF MARINE SCIENCE, VOL. 60, NO.3, 1997
Isoo
{)
~ Matheson Hammock
Chapman Field
Biscayne Bay
25°30'
Dade County
'l!J
J
Atlantic
Ocean
Figure I, Location of the dwarf mangrove study site (Matheson Hammock) and the basin mangrove
study site (Chapman Field) in southeast Florida, USA.
leaf sizes in the canopy foliage of two distinct mangrove forest types, dwarf and
basin (Lugo and Snedaker, 1974), dominated by R, mangle.
METHODS
Dwarf forest measurements were made in a 1.5 ha study-site located in southeast Dade County at
Matheson Hammock (25040' N, 80° 16' W) (Fig. I). The dwarf mangroves had a mean height of l.l
± 0,32 m and the study area was surrounded by tall R. mangle and Avice/mia germinans L. (black
mangrove), Australian pine (Casuarina equisetifolia L.), and Brazilian pepper (Schinus terebinthifolius
Raddi). The small trees in the dwarf forest were confirmed to be dwarfed, and not simply growth
supressed, since all branches were orthotropic with a spiral phyllotaxis and the emergent branches
exhibited no evidence of a hypopodium. The basin forest measurements were made in a study site of
similar size at Chapman Field Park (25 37' N, 80° 17' W). This basin forest had a mean height of
2.9 ± 0.62 m and was surrounded by a similar mixture of native and exotic tree species as recorded
for the dwarf site.
LAI, canopy height, leaf size and light transmission through the canopy were measured at 25
randomly-determined points in each study site. LAI was measured using the plumb line method
described by Pool (1973). Light transmission measurements were made in the open and beneath the
canopies with a LI-COR® LI-190SA quantum sensor and a LI-IQOO datalogger . Subcanopy light
levels, expressed as a percent of ambient light in the open, were calculated for each point as an
estimate of light transmission through the canopy following the field technique described by Snedaker
et al. (1992). Approximately 30 mature leaves (i.e., fully expanded, non-senescent) were collected at
each point from the distal ends of branches exposed to full sunlight. A total of 749 leaves was collected
at the dwarf site and 840 leaves at the basin site. Length and width was measured for each leaf from
the two sites and the area of a subsample (n = 50) of randomly-selected leaves from both sites was
determined using planimetric techniques (leaf tracings on millimeter grid paper), Since the leaf shapes
were similar based on length:width ratios (dwarf leaves 2,382 and basin leaves 2.385), the leaf area
data were pooled. The equality of variances assumption was verified and a one-way ANOYA was
then used to evaluate differences in the variables leaf length, width, and area. An unpaired t test was
0
645
ARAOJO ET AL: RED MANGROVE FOREST TYPES
Table I. Summary of subcanopy light transmission and leaf area index (LAI) in Rhizophora mangle
dwarf and basin forest types. Light transmission is expressed as the percent of ambient light in the
open.
Dwarf
Light
forest
transmission (%)
25
17.2 :t 5.9
35.5
7.4
0.343
N=
Mean (:tSD)
Max.
Min.
Coef. Var.
Basin
forest
LA!
Light transmission (%)
LA!
25
3.0 :t 1.8
9.0
0.0
0.593
25
7.2 :t 1.5
20.6
1.6
0.765
25
5.7 :t 1.8
10.0
3.0
0.316
used to evaluate differences in LAI and light transmission means between the two sites. In addition,
a regression analysis provided an equation that was used to estimate the total leaf area of the sample
populations from both forest types based on leaf length and width.
RESULTS
AND DISCUSSION
The LAI and subcanopy light transmission values for the dwarf and the basin
forests are shown in Table 1. The percent light transmission was significantly
higher (p < 0.05) in the dwarf forest (17.2 ± 5.9) relative to the basin forest (7.2
± 1.5), and expectedly, LAI values showed a significant (p < 0.05) opposite
trend. The LAI was higher in the basin forest (5.7 ± 1.8) than in the dwarf forest
(3.0 ± 1.8). To determine whether the differences might be influenced more by
leaf number than by leaf size, a regression analysis was performed to generate an
algorithm for calculating the area of all samples leaves (n = 1589). Leaf area was
significantly correlated (r2 = 0.97) with the length and width (Equation 1); the
resulting values for the two forest types are reported in Table 2.
y
=
-18.261
+ 1.911(length, em) + 6.250 (width, em)
Eq. 1
The number of leaves per one LAI (i.e., 1 m ) was then calculated for each
forest type. The basin forest had a lower number (400) of leaf area units per m2
relative to the dwarf forest (427) indicating that a slightly smaller number of
larger leaves per unit LAI contributed to the difference in canopy light transmission in the basin forest. The leaf size difference (i.e., dwarf forest leaves < basin
forest leaves) was confirmed by ANOYA (length: Fl. 1587 = 6.78, P = 0.093; width:
Fl. 1587 = 42.67, P = 0.0001; area: Fl. 1587 = 28.488, P = 0.0001) and found to be
significant (Table 2). In this regard, Russell et al. (1989) reported that the number
of leaves in a canopy is the structural characteristic that has the largest effect on
the interception of radiation and therefore its penetration through the canopy.
The variability found in the LAI of mangrove ecosystems has been recognized
2
Table 2. Summary of length, width, and area measurements and descriptive statistics for the leaf
sample in the dwarf and basin forests. Area estimates obtained from the equation y = -18.261 +
1.911 cm (length) + 6.250 cm (width).
Basin forest
Dwarf forest
Length
N=
Mean (:tSD)
Ma'(.
Min.
Coef. Var.
(em)
749
9.4 :t 1.2
13.1
5.3
0.13
Width
(em)
749
3.8 :t 0.6
5.5
1.8
0.16
Area
(cm2)
749
23.4 :t 5.6
38.7
3.1
0.24
Length
(em)
840
9.6 :t 1.3
13.6
5.5
0.14
Width
(em)
840
3.9 :!: 0.7
5.7
2.0
0.17
Area (cmZ)
840
25.0:!: 6.5
43.4
5.4
0.26
646
BULLETIN OF MARINE SCIENCE, VOL. 60, NO.3,
1997
in the literature (Pool, 1973; Cintr6n and Schaeffer-Novelli, 1983), and Pool attributed the LAI variability to the age of the stands and to distrubances such as
hurricanes and tree falls in the Caribbean region. Lugo and Snedaker (1974) and
Lugo et al. (1981) have shown a reduction in leaf area associated with different
type of stressors including high salinity, latitudinal location, and exposure to pollutants. If we assume that, in general, dwarf forests are under environmental stress,
such as limiting nutrient resources (Lugo and Snedaker, 1974) it is not surprising
to find both lower LAI values and smaller leaf sizes in the dwarf vegetation
relative to the basin forest. When the total leaf biomass per unit land area is a
uniform over a broad range of environmental conditions (Ovington, 1962; Rodin
and Basilevich, 1967; Odum and Pigeon, 1970), i.e" LAI tends to be a relatively
constant and predictable value for specific forest types, such as temperate oak
(LAI 3.1; Mj1jller,1945) and Florida hardwood (LAI 6.2; Lugo, unpubl. ms. in
Pool 1973). The variation in LAI in mangrove ecosystems reflects the different
environmental forces acting upon each location and reinforces the practical ecological value of the mangrove forest-type classification (Lugo and Snedaker 1974,
Twilley et al. 1986).
ACKNOWLEDGMENTS
Sincere appreciation is expressed to M. Medina and J.P. Lagoueyte who volunteered to assist with
the field work. A portion of this work was supported by the U.S. Environmental Protection Agency
(CR 820667).
LITERATURE
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647
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DATEACCEPTED: December 4, 1995.
ADDRESS: (R.J.A., S.C.S.) Division of Marine Biology and Fisheries, Rosenstiel School of Marine
and Atmospheric Science, University of Miami. 4600 Rickenbacker Causeway, Miami, FL 33149-1098.
(J.e.J.) Harbor Branch Oceanographic institution, 5600 U.S. 1 North, Fort Pierce, FL 34946.