BotanicalJournal of the Linnean Society (1996), 122: 9-34. With 15 figures
I s there a future for mahogany? Edited ly Andrew Madellan
@
Ecology and management of mahogany
(Swietenia rnacrophyZZa King) in the Chimanes
Forest, B e d , Bolivia
R.E. GULLISON, S.N. PANFIL, JJ. STROUSE AND S.P. HUBBELL
Department of Ecology and Evolutionary Biology, Princeton Univen$, Princeton
08544-1003, USA.
Mahogany (Sruiclenia marrophrlla King) regenerates in areas of erosion on high terraces and in forest killed
by flooding and deposition of alluvial sediments in the Chmanes Forest, Bolivia. These hydrological
disturbancesare patchy, and only one of five stands of mahogany that we inventoried was regenerating.
Mahogany survives these disturbances significantly better than the common tree species. The long time
between disturbances appears to favour late maturation. Mahogany trees allocate little photosynthates
to reproduction until they are very large emergents, at least 80 cm in diameter. The episodic nature of
the regeneration sites means that mahogany stands are composed of one or a few cohorts, which are
vulnerable to overharvesting, particularly with the current use of a minimum cutting diameter to
regulate harvest. The delayed onset of fecundity means that the small trees that escape harvest are not
very fecund, resulting in minimal seed input to logged forest. Only 7-9% of the gaps created by logging
contain natural regeneration after 20 + yr. A successful management plan for mahogany would entail a
monocyclic harvest, with a rotation age of 100 + years, the estimated time that it takes for trees to
achieve commercial size in natural forest. Since the number of seed trees that will be left is small, they
should be concentrated in sites that are likely to be conducive to natural regeneration, such as near rivers
and flood damaged forest. Seed production will be maximized for a given basal area (opportunity cost
to loggers)if trees c. 110cm dbh are selected as seed trees. The mahogany stocks in the Chimanes Forest
are nearly exhausted, but the fmdings of this study could be used to help rebuild the mahogany
populations, or to design management plans for the commercial species that have similar ecologies to
mahogany.
01996 The Linnean Society of London
ADDITIONAL KEY WORDS: -Amazon - conservation - demography - deposition - fecundity flood - growth - life history - sustainable - tropical.
CONTENTS
Introduction . . . . . .
Study area . . . . . . .
Forest and site description
Previouslanduse . . .
Present land use . . .
Description of mahogany . .
Morphology . . . .
Phenology . . . . .
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R.E. Gullison, current address: Renewable Resources Assessment Group, Centre for Environmental Technology,
Imperial College of Science, Technology & Medicine, 8 Princes Gardens, London SW7 1NA
S.N. Panfil,current address: Department of Botany, University of Georgia, Athens, GA 30602-7271, U.S.A.
J.J. Strouse, current address: 2018 Master Drive, Baltimore, MD 21209, USA.
00244074/96/090009+26 $18.00/0
9
01996 The Linnean Society of London
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R. E. GULLISON E T A .
Life history . . . . . . . . . . . . . . . . . . . .
Methods . . . . . . . . . . . . . . . . . . . . . .
Population structure . . . . . . . . . . . . . . , . .
Post-harvest stand structure . . . . . . . . . . . . . .
Spatial patterns and regeneration . . . . . . . . . . . . .
Natural regeneration of logged populations . . . . . . . . .
Fecundity . . . . . . . . . . . . . . . . . . . .
Seed dispersal . . . . . . . . . . . , . . . . . . .
Growth and mortality rates . . . . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . . . . .
Population structure . . . . . . . . . . . . . . . . .
Post-hawest stand structure
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Spatial patterns and regeneration . . . . . . . . . . . . .
Natural regeneration of logged populations . . . . . . . . .
Fecundity . . . . . . . . . . . . . . . . . . . .
Seed dispersal . . . . . . . . . . . . . . . . . . .
Seed mortality and germination . . . . . . . . . . . . .
Growth and mortality rates . . . . . . . . . . . . . .
Discussion . . . . . . . . . . . . . . . . . . . . . .
Hydrological disturbances and regeneration . . . . . . . . .
Population structure . . . . . . . . . . . . . . . . .
Life history and demography . . . . . . . . . . . . . .
Effects of current management and recommendations for improvement
Conclusions . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . .
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INTRODUCTION
High rates of deforestation in the Amazon and concern over the conservation
status of some commercial timber species have drawn attention to the current poor
understanding of the dynamics of Amazonian tree populations. The dynamics of
these populations will be determined by the interplay of their life histories with the
dynamics of the disturbance regimes that create regeneration sites (Clark, 1991a, b).
The knowledge of these processes is of critical applied importance because it will
form the basis of successful conservation and management plans.
The majority of research on neotropical forest disturbances has taken place in
Mexico, Central America and the Caribbean. Various studies have identified the role
in forest dynamics of treefall gaps (Lang & Knight, 1983; Hubbell & Foster, 1986;
Brokaw, 1987; De Steven, 1988; Martinez-Ramos, Sarukhan & Pmero, 1988;
Hartshorn, 1989), hurricanes (Crow, 1980; Frangi & Lugo, 1991) and fire (Lamb,
1966; Snook, 1993).
The agents of large scale forest disturbance in the Amazon differ from those in
Central America. Fires are uncommon in Amazonian forest, at least in recent history
(Uhl & Kauffman, 1990). Hurricanes are not present, but strong winds may cause
considerable damage (Nelson et al., 1994; E. Ortiz, pers. comm.). Recent studies
demonstrate the importance of river bend migration as a major disturbance in
Amazonian forests (Salo et al., 1986; Kalliola et al., 1991, 1992). Stream capture by
rivers can change flooding and drainage patterns and result in conversion of forest
to marsh along kilometre stretches 0. Terborgh, pers. comm.). There is also
palynological and sediment evidence for catastrophic flooding events caused by
periodic volcanic activity or changes in local or Andean precipitation regimes
(Colinvaux et al., 1985; Frost & Miller, 1987).
Studies that relate Amazonian forest dynamics to these disturbance regimes are
rare. The only example known to the authors is an excellent body of work on the life
MAHOGANY IN BOWIA
11
history characteristics of vegetation regenerating on primary successional habitats
caused by the lateral migration of rivers (Foster, Arce & Wachter, 1986; Kalliola,
Makinen & Salo, 1988; Kalliola et al., 1991; Salo & Kalliola, 1991).
Coupled with this lack of basic ecological knowledge is a noticeable absence of
forests managed for sustained yield in the Amazon (Poore et al., 1989).While in itself
not sufficient for good forest management, a sound ecological understanding of the
population dynamics of Amazonian tree species is certainly a first step, and poor
knowledge partially explains the current lack of successful forest management.
The current controversy over broad-leaved mahogany (Swietenia macrophylla King)
illustrates how little is known about Amazonian tree populations. S. macrophylla
(hereafter referred to as mahogany) occurs from Mexico to Bolivia (Pennington,
1981), but has been exhausted commercially in the northern parts of its range.
Mahogany was proposed for listing on Appendix Two of the Committee for
International Trade of Endangered Species in 1992, and again in 1994. Both
attempts were unsuccessful. A successful listing would require producer and
consumer countries to document all international trade in mahogany.
While the ecology of northern populations is well understood (Lamb, 1966; Snook,
1993),Virtually nothing is known about the ecology of Amazonian populations. Even
basic inventories are lacking for mahogany over most of its range. An informed
decision about the conservation status of mahogany depends heavily upon these
data.
Here we present the results of a four year study on the ecology and management
of mahogany in the Chimanes Forest, in the Bolivian Amazon. The goals of this
study were (1)to identlfy the disturbances that lead to mahogany regeneration in the
Chimanes Forest, (2) to obtain data on the basic demographic parameters and life
history of the Chimanes populations, (3) to document the effects of current harvesting
and management on mahogany, and (4) to use these data to suggest improvements
to management.
STUDY AREA
Forest and site description
The Chimanes Permanent Timber Production Forest (66" 00' to 67' 00' West and
14" 30' to 16" 00' South) is located in the Chimanes Region, in the state of Beni,
Bolivia (Burniske, 1994).In addition to the Chimanes Permanent Timber Production
Forest (hereafter referred to as the Chimanes Forest), the Chimanes Region contains
a protected area, the Beni Biological Reserve, and two Indigenous Territories. The
Chimanes Forest was officially established in 1986 with the granting of seven
concessions, although logging has taken place for at least the last 20yr in this
area.
The 449000ha Chimanes Forest contains small pockets of savanna, and is
bounded by savanna to the south and east. The forest can be divided into five
categories with aerial photography (Government of Bolivia, 1993). Non-flooded
alluvial plains forest (Bal) is situated on high terraces and areas of low hills, and has
a standing volume of 150-180m3 ha-' (all species, dbh > 40cm). Temporarily
flooded alluvial plains forest (Ba2) is situated on low terraces and may be flooded
during the rainy season. The standing volume is c. 100m3 ha-'. Transitional
R.E. GULLISON E7AL
12
TABLE
1. Description of the study sites where 100 ha plots were installed
to study the stand
structure of mahogany in the Chimanes Forest. Bal = non-flooded alluvial plains forest. Ba2 =
temporarily flooded alluvial plains forest. Tree species diversity is for all trees >10 cm dbh. (Source:
del A g d a , unpublished Bachelor's thesis)
~~
~
Fatima/
Monte Grande
Aguw Negras
Jaimanche
ForestType
Bal
Ba2
Altitude(m)
190
Ptot Size (ha)
1
No. of Species
Four Most
Abundant
Species
Cuberene
Ba2
Ba2
215
190
170
1
2.25
1
85
73
92
66
Iriaztca deltoids
(Palmae) n=9S
Poulsenia armuta
(Moraceae) w S 5
Pseudolmedia h i s
(Moraceae) -11
Unwnopsisjoribundu
(Annonaceae) -106
Ashocaryum sp.
I&?f.mdeltOidea
(Palmae) -56
Swatea emrhiza
(Palmae) -97
Ashocaryum sp.
(Palmae) -74
(Palmae) -76
Lununia pann$m
(Flacourtaceae) -52
oroboparvijbra
Guam mm@hyUa
(Myristicaceae)~ 5 1(Meliaceae) nn=76
Pseudolmedia laeub
(Moraceae) n=73
Sacruiea exmhka
UnOnl?tlSiSjlOribunda
Richoia sp.
(Annonaceae) 7 ~ 3 7 (Euphorbiaceae) -52
Rheedia sp.
(Palmae) -44
(Guttiferae) -32
temporarily flooded alluvial plains forest (Ba3) is similar to Ba2 but lower in height,
and with a lower standing volume of c. 73 m3 had. The other two types of forest, low
hill forest and mountain forest, are not of major importance in the area.
Botanical plots 1-2.25 ha in size have documented a tree diversity of 66-92 species
> lOcm dbh (Table 1). Soils are derived from Tertiary and Late Quaternary
Andean sediments, and are sandy/loam or clayish in nature (Government of Boiivia,
1993). The annual mean temperature is 26°C. Median precipitation at the two
nearest weather stations (San Ignacio de Moxos and San Borja) is 202 1 and 23 16 mm
yr-' for the period between 1980-1992 (Fig 1). July is the driest month of the dry
season (defined as the months with median precipitation < = 100mm), which lasts
from May to September. The wettest months are December through March.
Prevbus Land use
Previous agricultural activity may have been recent enough to have influenced
current forest structure and composition, and may be responsible for the relatively
low tree species diversity of the Chimanes Forest. Pre-historic agricultural fields
(camellones), typically 4.5 m wide and up to 200 m in length, are found on many of
the savannas and throughout much of the Chimanes Forest. The cultures that
created the camellones are poorly known, but are thought to date between 500-2300
B.P. (C. Erickson, pers. comm.). Camellones are present under mature forest in four
of our six study sites, implying that much of the current forest was once either
savanna or cleared for agriculture.
Present Land use
The seven timber concessions of the Chimanes Forest harvest mahogany almost
exclusively (Goitia, 1990; Burniske, 1994). All the timber companies follow a similar
MAHOGANY IN BOLIVIA
13
h
f
W
5
I
I
I
I
I
12Y-
T
l
I
I
I
I
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4
I
maximum
I
T
I
I
I
Figure 1. Precipitation between 1980 and 1992 in the Chimanes Forest. Also shown is the timing of
phenological events for mahogany (Precipitation data from the San Ignacio de Moxos weather
station).
extraction procedure. Throughout the year, areas with a high density of commercial
trees are located by plane or by employing indigenous people who know the areas
well. Groups of 15-20 people then search these areas on foot, cutting trails to the
trees as they find them. The trees are felled, and the trunks (up to the first branching
point) are cut into c. 5 m sections. In the dry season, the logs are dragged by wheeled
skidders to the main haul roads. From there, the logs are taken by truck to nearby
sawmills.
Extraction of mahogany results in relatively little structural damage to the forest.
A recent study estimated that in a 600 ha study area where 74 mahogany trees had
been extracted, only about 4.4% of the area of the forest had been damaged by road
building and tree felling (Gullison & Hardner, 1993).
The original management plan for the Chimanes Forest specified a 30yr cutting
cycle (Goitia, 1990). Harvest was to be regulated using a minimum cutting diameter
of 80cm dbh. The undersized trees, and 10% of commercial sized trees, were to
serve as seed trees and to provide stock for future harvest.
DESCRIPTION OF MAHOGANY
Morphology
Mahogany trees are emergents, attaining heights of up to 50 m and diameters up
to 2 m (Fig. 2). Mahogany trees emerge from the canopy when they are c. 22 m in
height and 30cm dbh.
The crowns of the largest trees are up to 20 m in radius, and are made up of a few
thick main branches. Buttress production begins at a size of 1&12 cm dbh, and their
height increases linearly with tree size above this point, reaching up to 3 m on the
largest trees. Snook (1993) attributed the low mortality of mahogany trees in
R. E. GULLISON ETAL.
14
hurricanes to their heavy buttresses which provide strong support, and reduced
branching and sparse foliage of the crowns that decreases wind resistance and
consequently helps mahogany survive hurricanes with less damage than other
species. Low mortality of adult trees is probably also due in part to a high allocation
of photosynthates to secondary compounds that prevent rot and hence increase
longevity (Lamb, 1966).
Mahogany trees are monoecious. The flowers are pollinated by bees and moths
(Styles & Khosla, 1976). In the Chimanes Forest, flower and leaf production occur
simultaneously in September at the onset of the rainy season (Fig. 1). The fmits are
woody capsules and take 10-1 1 months to mature. The species is deciduous, and
leaves f d at the onset of the dry season, leaving the mature capsules on the tree. As
the capsules dry, the valves drop off, leaving the seeds exposed but still attached to
the columella. They are released when a strong wind breaks the columella. Strong
wind may also knock mature capsules off the tree before they have shattered and
dispersed their seeds. In extreme cases, almost the entire crop of capsules can be
knocked off the tree before dispersing their seeds (Gullison, pers. obs.).
[us
1
10
5
0
BUTTRESS
AVERAGE
HEIGHT 32
(m)
~~,
1
0
na
0
25
50
75
100
125
150
175
200
DIAMETER (cm)
Figure 2. Relationship between tree diameter and other morphological measurements for mahogany in
the Chimanes Forest. A, height (y = (25.1)lodx)- 12.6, 2 = 0.88). B, crown radius (y = 0.139~2.82(104)x2, ? = 0.97). Some of the largest trees have broken crowns. C, buttress height (y = 0.017~0.0014, ? = 0.88).
MAHOGANY IN BOLIVIA
15
Laj2 histoy
Mahogany is considered a light demanding climax species. Mahogany seedlings
and saplings are shade intolerant, exhibiting their fastest growth with overhead light
and lateral shading (Lamb, 1966).They are intolerant of deep shade, but can survive
in a suppressed state for years in partial shade.
Mahogany is adapted to regenerate after fires and hurricanes in Central America
and Mexico (Lamb, 1966; Snook, 1993). Adult trees survive these ephemeral
disturbances better than most other tree species in these communities. As a result,
they can play a dominant role in post-disturbance regeneration.
A recent comparison among mahogany and two pioneer and one other climax
species showed that although photosynthetic rates did not differ much between
species, mahogany had slower height growth and higher rates of relative biomass
increase than the other three species (Ramos & Grace, 1990). The authors concluded
that differences in height growth were due to species differences in allocation. Given
its requirement for episodic disturbances to regenerate, it seems reasonable to assume
that mahogany allocates more of its photosynthates to structural support and to
secondary compounds that will increase longevity relative to other species, at the
expense of height growth. Negative correlations between growth rates and
investment into secondary compounds have been documented among temperate tree
species (Loehle, 1988).
METHODS
Population structure
Density and stand structure of mahogany were investigated in Bal and Ba2 forest
types, which are the most common in the Chimanes Forest. Five plots of 1 km by
1 km (100ha) were established (Table 1). The Bal plots (Fatima and Monte Grande)
were located on a terrace 3 4 m above the current river flood plain. The three
remaining sites were located in Ba2 forest. The Jaimanche plot was located
approximately 1 km away from the Jaimanche River, in the Fatima concession. The
Aguas Negras plot was placed so that it abutted a flooded savanna area. The
Cuberene site was located next to the Cuberene River.
Within each plot, a central 1000m transect was cut, and then 10 randomly located
500 m transects were cut perpendicular to this. The transects were searched for all
mahogany > 2.5 cm dbh within 10 m each side of the transect, yielding a total area
searched of 10 ha (10%)per plot. AU mahogany trees found within the transect were
tagged, mapped and measured (dbh, estimate of height to first branching, and total
height). In addition, one of the 10 transects was selected at random and searched for
all mahogany seedlings and saplings (individuals < 2.5 cm dbh). The locations of any
topographical features were noted in the Bal plots.
Post-hanlest stand structure
The timber companies logged three of the five plots (Fatima, Monte Grande, and
Jaimanche) during or shortly after installation. We noted the fates of all mahogany
16
R.E. GULLISON l 9 A L
trees located in the general area of the plots. Trees were classified as ‘seed trees’ if
they were marked to be retained for seed production. Trees were classified as
‘missed’ if they were of commercial size but had not been found by the workers that
searched for the trees. Trees were classified as ‘standing rotten’ if they had been left
standing after trial cuts with a chainsaw had revealed rot. Trees were classified as ‘to
be felled’ if they were marked for felling, but because of logistical difficulties
(primarily bad weather), had not been felled yet. ‘Felled’ trees were trees that had
already been felled by the company. Trees that had died naturally and that had been
removed by the companies were classified as ‘natural deaths’.
Spatial pattans and regeneration
To help identlfjr the disturbances that led to their recruitment, several mahogany
populations were mapped in relation to their local topography. A 15 ha plot (300 m
by 500m) was installed in the Bal forest (Chirizi) in the area of highest density of
commercial trees. All mahogany trees > 2.5 cm dbh were mapped and measured,
and the topographical features were noted.
Four plots were installed at the Cuberene site (Ba2 forest) because of the extensive
regeneration found there. Three plots (Bolson #1-3-4, Bolson #5, and El Pichi)
contained river bends, and the fourth plot (La Esperanza) was installed about 300 m
from the river. Plots were installed by cutting a baseline parallel to the river, and then
cutting perpendicular trails every 20m along the baseline to the river. AU
mahoganies > 2.5 cm dbh were mapped and measured.
Natural regeneration of logged populations
Gullison and Hubbell (1992) found that many mahogany seedlings survive the
felling and removal of parent trees. To determine whether these seedlings reach
commercial size in Ba2 forest, 28 plots were installed in the area of old gaps created
by mahogany removal 18-20 yr ago. The partially rotted stumps and branches were
easily discernible, and each plot was located so that it bounded the old gap by 10m
on all sides. Each plot was systematically searched for all mahogany individuals
> 2.5cm dbh.
The survey of the Chirizi plot described in the previous section provided an
opportunity to examine natural regeneration in Bal forest because it included 1 1 old
gaps formed by the felling of commercial mahogany trees 20-25yr ago.
Fecundig
The relationship between tree size and fecundity (the number of fruits or capsules
produced by the tree in one year) was determined by counting the capsules on trees
5-175 cm dbh. Trees drop their leaves before the capsules open and the seeds
disperse, so it was possible to get good counts from the ground with binoculars. Trees
with many lianas that impeded viewing were excluded. The ground below the
crowns was searched to detect any mature capsules that had fallen, and these were
added to the count. The capsules of 106 trees were counted in 1992. Counts were not
MAHOGANY IN BOLIVIA
17
possible in 1993 because the logging companies felled many of the trees, and access
was impossible to the remainder. In 1994, we counted the capsules in 89 trees. Only
trees in intact forest were included for the study. Trees by roads had unusually high
fecundities, perhaps due to increased lateral exposure to light. To convert the capsule
counts to total number of seeds produced, the seeds in 88 capsules from a dozen trees
were counted.
Seed dispersal
To measure seed dispersal, seeds were counted in 1 m by 80 m transects extending
in eight compass directions from the base of five trees (99, 122, 144, 145, and 160cm
dbh). The seeds were counted twice over a four week period. Seeds were marked
with a felt pen the first time to prevent double counting. The data are presented as
the average number of seeds m-* for 4 m intervals along the transects.
ofowth and mortalip rates
The growth and mortality of 117 mahogany trees 2.5-200 cm dbh were measured
each year between 1992 and 1994. The growth and mortality data are presented as
annual rates for doubling diameter categories. To determine the relative ability of
mahogany to survive flooding and deposition, the mortality rate of the mahogany in
B# 1-3-4 plot was compared to the mortality rates of the five most abundant species
in this plot( > 5 cm dbh). B#1-3-4 experienced the most flooding and deposition of
all the plots.
RESULTS
Population structure
The density of mahogany trees > 80 cm dbh (legal commercial size) was 0.1-0.2
trees had on the five 100 ha plots (Fip 3). The density of trees < 80 cm dbh was
similar among plots (0.1-0.2 trees ha- ), with the exception of the higher density of
small trees in the Cuberene plot. Erosion gullies formed an average of 10.9% and
18.3% of the area of the transects of the Monte Grande and Fatima sites. There were
no other distinct topographical features.
To determine whether a higher sampling intensity would have improved our
estimate of the mean density of mahogany, we repeatedly calculated the standard
error of the sample mean density as the data from an increasing number of transects
was included. The standard error levelled off at a sampling intensity of 6-7%,
demonstrating that our sampling intensity of 10% was more than adequate.
averaging 174 seedlings
Seedling densities were also similar between plots (Fig. 9,
had. The only exception was the Cuberene site, which despite the high density of
individuals > 2.5 cm dbh, had a low density of 17 seedlings had.
R. E. GULLISON E 7 A L .
18
lo 1
cutting
limit
Cuberene
0
#OFTREESIN
21
Aguas Negras
I
I
I
I!
I
10haSAMPLE 0
2
01
I
Jaimanche
I
=
I
Monte Grande
I
I
I
I
I
2
0
I
Fatima
I
.m. . . . . I . .=. .=. .
+ 9 d 9 ++d +\ff
I
.
$5
Q
\
.
+Q@@\bp,%Q&j@
bp
DIAMETER CLASS (cm)
Figure 3. Diameter histograms of the mahogany trees > 2.5 cm dbh inventoried in 10% samples of five
100 ha plots. Only the Cuberene plot, located adjacent to a river that is flooding and depositing alluvial
sediment. is regenerating.
Post-haruest stand structure
There were 75 alive or recently felled mahogany trees > 60 cm dbh in the three
harvested 100 ha plots (Fig. 5). Seven of the 1 1 trees < 80 cm dbh had been illegally
felled. Of the 65 trees > 80cm dbh, only one had been designated as a seed tree
200
-
AEWW
Nkras
180 -
Fatima&
Monte
Grande
Jaimanche
160 -
-
T4
140 120-
100-
5;
En
801
60:
40 20 -
I
I,
,
0- < m 20-,-40-
40 80
I
Ig-g-
b
Cuberene
.
<20
.
I
,
.
J
80+
SEEDLING HEIGHT CATEGORY (cm)
Figure 4. Height histograms of mahogany seedlings in four 1 ha plots. The Cuberene plot had the lowest
density of seedlings, even though it is the only site where the population is regenerating.
MAHOGANY IN BOLIVIA
121 Jaimanche
m-seedtree
19
--tobefell
2 12F Grande
Monte
DIAMETER CLASS (cm)
Figure 5 . The fates of mahogany trees in three of the five 100ha plots that were logged during the course
of the study. Only two of the 75 live trees were marked and designated as seed trees, and seven of the
11 trees 60-80 cm dbh were felled against regulations.
(1.5%). Five of the trees were left standing after trial chainsaw cuts in the trunk
revealed rot. The companies did not respect the minimum cutting diameter of 80 cm
dbh, nor the regulation requiring that 10% of commercial trees should be left as seed
trees.
Spatial patterns and regeneration
Chi&$
The mapping of the Chirizi plot revealed that topographical relief in the area was
caused by erosion (Fig. 6). Erosion gullies originated in the plot, joining to form
larger gullies that drained into the Chirizi River. The erosion gullies covered 28% of
the area of the plot. The presence of large trees on the banks of the gullies suggests
little current erosion. Stakes placed in the corners of five erosion gulhes showed that
the erosion gullies had not enlarged between 1992 and 1994. A prehistoric raised
land form approximately 2 m in width crossed the northwest corner of the grid (not
shown). The land form was bisected by an erosion gulley, indicating that at least
some of the erosion had occurred after abandonment of the area.
The 15ha plot contained 13 trees of commercial size, and three smaller trees. In
addition there were 11 mahogany stumps of commercial size judged to be 20-25 yr
old. The trees had been felled by axe, most likely by local boat builders. There was
no obvious gradient in the size of the individuals in relation to their position along
the erosion gullies.
Cuberene
The bed of the Cuberene River is 25-30 m wide. Xn the dry season, the river is as
little as 5-8 m wide. In the wet season the river floods its banks up to heights of c. 1 m,
R. E. GULLISON ETAL.
20
as evident by water marks on trees. The forest is heterogeneous, with no clear
successional gradients with increasing distance from the river. Small differences in
elevation determine whether a patch of forest is flooded or not.
Regeneration was present in three of the four plots in this area (Figs 7, 8). Only
the unflooded Pichi plot had no individuals in the smaller size classes. There appear
to be at least three different microhabitats where the smaller size classes occur. First,
a few seedlings were present in the primary successional habitat created by river
bend migration. Second, some of the seedlings were located along the banks of the
river bends where presumably the extra light permits survival and growth. Many of
these seedlings were of bad form and overgrown by vines. The third and most
important microhabitat where regeneration occurred was low in areas on the plots
that experienced annual flooding and deposition of alluvial sediment. Flooding and
deposition had opened up large areas of forest by killing all size classes of trees.
Flooding can reach areas quite distant from the river, as demonstrated in the
Esperanza plot that is 3 0 N 0 0 m distant.
4m
Terrace
Depth of Erosion Gullies
t
100 m
Diameter
~tegory
(cm)
0
0
*
50.t
Erosion
Gulley
0 -Stump
-
10-25
2.5 - 10
Figure 6. A map of the 15ha plot at Chirizi, located between the Fatima and Monte Grande 100ha plots.
A; the plot is located on a high terrace that is cut by erosion gulhes that network together. Where the
gullies leave the west side of the plot they have attained a depth of 4m. The erosion gullies cover 28%
of the area of the plot. B, the sizes and locations of live trees, and the locations of old stumps, axe shown
in relation to the erosion gullies. The high density of trees and stumps in this plot is associated with the
large area of past erosion.
MAHOGANY IN BOLIVIA
21
Natural regaeration of logged popuhtions
Natural regeneration was found in only three of the 28 plots established around
old tree fall gaps in Ba2 forest (Fig. 9). One of the trees was > 49 cm dbh. From the
growth data presented in Figure 1 1, a 20yr old tree can be expected to have a
diameter between 8.2 cm (median growth throughout Me) to 31.6 cm (maximum
growth throughout Me), and so this tree is too large to have regenerated after the
commercial tree f d . Excluding this tree, only two of the 28 old gaps in Ba2 forest
(7%) had natural regeneration.
In the 15 ha plot in Chirizi, there were only three small mahogany trees (dbh’s of
8, 8, and 22 cm) that could possibly have regenerated after the removal of the 11
A
N
BOLSON #1-3-4
Diameter
\
/
O
*
25-50
10-25
2.5- 10
Figure 7. The locations of mahogany trees are shown in relation to the Cuberene River in the Bolson
#1-3-4 plot (1 1.5 ha). The plot was installed on only one side of the river because the other side is an
indigenous territory with restricted access. This plot had the highest density of saplings inventoried in the
Chimanes Forest.
R. E. GULLISON ETAL.
22
trees from the plot 20-25yr ago. Only one of the s m d trees is close enough to a
stump (42 m) to potentially have regenerated in an old gap (Fig. 6), meaning that at
most 9.1 YO of gaps (1/ 1 1) had natural regeneration.
Fecund+
Trees start producing capsules at 30 cm dbh, when they begin to emerge from the
canopy (Fig. 10). Fecundity is relatively low for trees 30-80 cm dbh, with a maximum
N
f
c
Esperanzaa
9*
Olson #5
1oc
840
*8
Bolson # I 3 4
014
89 0
660
c
9**3
56.
$
J.jY
*6
Bolson #5
36.
013
*5
3*
-
Diameter
Category
100 m
(cm)
50-100
0
*
o
25-50
10-2.5
2.5- 10
Stump
Figure 8. Slap of the locations of the mahogany in El Pichi (2.7 ha), Esperanza (4.1 ha), and Bolson #5
l3.2 ha) plors along the Cuberene River. T h e Esperanza plot, located away from the river but in a low
area that was flooded during the wet season, had the most regeneration. The other two plots were located
adjacent to the river but were in high forest, and did not experience flooding in the wet season.
MAHOGANY IN BOLIVIA
23
regeneration in box)
450 500 550 600 650 700 750 800 850 900 950 lo00
GAP SIZE (mL)
Figure 9. Size histogram for 28 plots that were installed around 20 yr old mahogany gaps formed by the
felling and removal of adult trees. Three of the plots had a single sapling in them (dbh noted in box). The
remaining 25 plots had no natural regeneration.
production of 129 capsules. The fecundities of trees > 80 cm dbh are much higher,
with some trees producing as many as 600 capsules. Fecundity peaks at c. 130 cm
dbh. The capsules contained an average of 55.7 seeds (SD = 7.3, n = 88).
Therefore, the most fecund trees produce c. 33 000 seeds in a year.
Seed dispersal
Seed density is highest directly beneath the crown (Fig. 1 1) and drops rapidly with
distance from the tree. The median dispersal distance for the seeds was 32-36m.
Since seeds were occasionally found in the farthest quadrats, the maximum dispersal
distance is probably > 80m. More seeds fell toward the south and southeast.
s
DIAMETER (cm)
Figure 10. The relationshipbetween diameter and fecundity for mahogany trees in the Chimanes Forest
(y = 85.4-5.04~+ 0.085~'-0.00029x3, 12 = 0.45). (1992, n = 106; 1994, n = 89).
R. E. GULLISON ETAL.
24
2.0
T
N
1.5
@ w sw
%
SE
3
1.0
m
En
a
0.5
0.0
0
4
8 121620242832364044485256606468727680
DISTANCE FROM TREE (4 m intervals)
Figure 1 1. Relationship between the average density of dispersed mahogany seeds and the distance to the
adult tree (n = 5 trees) in 1992 (standard error shown as bars). The density of dispersed seeds drops very
quickly beyond the crown. The inset shows the average total number of the seeds dispersed in each of the
eight compass directions. More seeds are dispersed to the south and the southeast.
Seed mortulip and germination
Only 63% of the seeds in the 88 capsules counted were judged viable by
inspection. The majority of the unviable seeds were incompletelyformed. Of the 827
seeds found in the seed dispersal transects, 348 (42.1 %) were viable at the end of the
second census, with the majority of the unviable seeds having been attacked by
insects or fungus.
Germination rates of seeds are higher in nurseries than in forest. A local nursery
in Bolivia reported germination rates of 80-90% (Claudio Leaflo, pers. comm.).
Germination rates of seeds planted in greenhouses in Princeton ranged from 20 to
90%, the lower rates the result of excessively long storage and rough handling in the
field.
Seeds placed on the ground under the forest canopy take 2-16 weeks to germinate
(Strouse, unpublished data), depending on the amount of precipitation they
receive.
Growth and rnortalig rates
Growth rates were highly variable for all size classes (Fig. 12).Median growth rates
in the two smallest and two largest size categories were very low compared to the
maximum growth observed in those categories. Median growth rates are highest for
trees 2&80cm dbh.
Mortality rates observed in this study are low. Five of 153 trees died over the two
year period of 1992-1994 (1.6%yf'), but it would be misleading to extrapolate this
level of mortality to the whole population as four of the five deaths occurred in
MAHOGANY IN BOLIVIA
25
n
2.0
8
W
-9Operrmtile
1
-50 percentile
-10 percentile
--minimnm
DIAMETER CATEGORY (cm)
Figure 12. Annual growth rates for 117 mahogany trees in the Chimanes Forest from 1992-1994
(samples sizes: 2.5-10, n = 33; 10-20, n = 15; 2 M 0 , n = 18; 4 W 0 , n = 17; 80-160, n = 27; 160+,
n = 7).
flooded areas at Cuberene. Mortality rates for mahogany and for the five most
common species on B# 1-3-4, the plot that was experiencing the most severe flooding
and deposition, are shown in Table 2. Mortality rates are significantly different
between species (X2= 18.5, P < 0.001) and post hoc multiple comparisons among
proportions (Zar, 1984: 402) show that the mortality rate of mahogany is significantly
lower than for all five common species (P< 0.025).
DISCUSSION
Hydrological disturbances and regeneration
Mahogany regenerates after hydrological disturbances in the Chimanes Forest. In
the Chirizi site (Bal forest), mahogany is associated with areas of past erosion on high
terraces. The three plots installed in this area showed a trend between increasing
density of mahogany with increasing area of erosion on the plots. The density of
mahogany ranged from a low of 0.3 1 trees ha-' on Monte Grande plot (Fig. 5) that
had erosion gullies covering 1 1% of the area, to a high of 1.6 trees ha-' on the Chirizi
plot (Fig. 6), where erosion gullies covered 28% of the area. The Fatima plot was
second in mahogany density and in erosion. The mahogany trees are generally
located along the periphery of the erosion gullies (Fig. 6; Gullison, pers. obs.). Little
regeneration was found anywhere in these sites, and we found no areas that were
currently eroding. The lack of current erosion implies that precipitation and/or local
run-off was higher in the past when the large trees recruited.
In contrast to the erosion at the Chirizi site, flooding and deposition of sediment
by the river were responsible for the current regeneration of mahogany in the
Cuberene plots. Although the mortality of all species and size classes is high in these
26
R. E. GULLISON ETAL.
TABLE2. Mortality rates of mahogany and the five most abundant species (dbh 25cm) on the
flooded B#1-34 plot adjacent to the Cuberene River. Mortality rates are significantly different
between species (X2 = 18.5, P <0.001),and post hoc multiple comparisons among proportions
(Zar, 1984 402) show that the mortality rate of mahogany is significantly lower than for all five
common species ( P <0.025). Note that the sample size of mahogany represents the complete
population on the plot, while the sample sizes of the common species represent a 5-10% sample
of plot area
Species
Family
UnonopSisflorihundn
Protinurn sagotianurn
Mabe0 sp.
Inga sp.
Annonaceae
Burseraceae
Number
Mortality (%)
38.2
5.3
Pseudolmedia laeii,zs
Mimosaceae
Moraceae
34
30
31
33
35
Sriidenia onarrOphj.ll0
Meliaceae
19
Euphorbiaceae
80.0
35.5
24.2
62.9
areas (Gullison, unpublished data, and Table 2), the individuals that survive grow
well, presumably because of reduced root or shoot competition.
The flooding is caused by a logjam 6 km downstream that began forming in 1985.
The logjam is caused by the accumulation of organic debris that falls into the river
when river bends enlarge. In the wet season, the logjam causes flooding and slows the
flow of water. The reduction in velocity reduces the load that the river can carry
(Carling, 1992),resulting in the deposition of sediment on the forest floor. Most trees
in the area can survive flooding, but not heavy deposition of sediments that buries
their roots (Gullison, pers. obs.). On the plots in this study, the elevation of a site was
more important than distance from the river in determining how much alluvial
deposition occurred. High areas near the river, like the Pichi plot (Fig. 8) escape
deposition entirely, while low areas some distance from the river can experience high
levels of deposition, as in the Esperanza plot. Forest mortality caused by the
deposition of alluvial sediments preceding erosion by rivers has been observed in the
Peruvian Amazon 0. Terborgh, pers. comm.), but is caused by high sinuosity of the
river bend, and the disturbed areas are short-lived.
Large areas of forest can be killed by flooding and deposition. In the immediate
area of the logjam downstream, an area of 48000ha has changed from forest to
savanna in this short time period.
To the authors' knowledge, logjam-induced flooding and deposition have not been
previously reported as a disturbance agent in the Amazon. This type of disturbance
is larger and more ephemeral than the slow creation of primary succession sites
caused by river bend migration reported for the Peruvian Amazon (Salo et al., 1986;
Puhakka et al., 1992). Superficially, the hydrological disturbances responsible for
regeneration of mahogany in the Chimanes Forest differ from the disturbances (fire
and hurricanes) that lead to the regeneration of mahogany in Mexico and the
Yucatan (Lamb, 1966; Snook, 1993). The fact that mahogany is found regenerating
after these varied disturbances suggests either an underlying similarity in the
temporal and spatial patterns that allow mahogany to persist, and/or differences in
life histories between the mahogany populations.
Population structure
The ephemeral nature of these disturbances should cause peaks of recruitment
that will be reflected as peaks in the size distribution of the population. The position
MAHOGANY IN BOLIVIA
27
of the peak or peaks will depend on the time since they regenerated (Clark, 1991a).
Few of the plots have enough trees to demonstrate the size distributions well. Two
of the harvest plots do, Fatima and Monte Grande, and they have unimodal size
distributions with the modes considerably above the minimum cutting diameter (Fig.
5). The size distributions for trees to be harvested over larger areas also show
multiple peaks (Fig. 13). These distributions support the hypothesis that these stands
have undergone episodic regeneration.
The overall density of mahogany in the Chimanes Forest may be lower than the
density of 0.1-0.2 trees had that we found in our plots, as the plots were installed in
areas that were known to contain mahogany. The low density of mahogany in the
Chimanes Forest (Fig. 3) may be due to the age of the stands, or to low seed input
at the time of regeneration. It seems unlikely that density dependent processes are
regulating the population at such a low level, given that densities of up to 6-8 trees
ha-' > 80 cm dbh occasionally occur in the Chimanes Forest (Gullison,pers. obs.),
and that the density of mahogany is low compared to other regions. For example,
Verissimo et al. (1995), in the state of Para, Brasil, report densities of 1.O commercial
tree ha-', while Lamb (1966) gives densities of large mahogany emergents in the
Yucatan Peninsula of Mexico ranging 2.2-22 trees ha-'.
Lij%history and demograply
The temporal allocation of photosynthates between growth and reproduction (the
life history) will determine how a tree species responds demographically to the
regeneration sites created by the local disturbance regime (Stearns, 1976; Bell, 1980;
Clark, 1991b). Here we interpret the demographic parameters of mahogany
50 40 &
3
30-
5z
20 10n
I
DIAMETER CATEGORY (cm)
Figure 13. Diameter distribution of 245 mahogany trees inventoried prior to harvest by the Programa
Chimanes forestry staff. The census area was not recorded. Multiple peaks are present in the distribution,
suggesting several regeneration events. The current management plan is poorly followed, with the
forestry department's own data showing that only 3.7% of commercial trees were left as seed trees,
instead of the required 10%. These seed trees, and the 10 smaller trees, will supposedly provide the next
harvest. (Data provided by Programa Chimanes/Subprograma Forestal).
R.E. GULLISON ETAL.
28
observed in this study in light of the observation that mahogany regenerates after
large scale disturbances. Ideally, experimental manipulations of investment into
growth or reproduction are used to demonstrate tradeoffs, but this is diacult for long
lived organisms such as trees. Instead, we are limited to examining and interpreting
the variation present in these parameters in natural populations (for an example see
Eis, Garman & Ebel, 1965).
To examine how allocation to growth and reproduction changes with size for
mahogany, the median growth and fecundity values for increasing diameter
categories, divided by the maximum values attained in each category, are shown in
Figure 14. These two curves are simply a standardized way of presenting a subset of
the data shown in Figures 10 and 12. The individual fecundity counts shown in
Figure 10 are broken down into the same size categories used for the growth data in
Figure 12. Then, the median value in each size category is shown as a percent of the
maximum value (PMV) which is used here as an indication of the median allocation
to that function by individuals in the specified size class.
Ph4Y to growth is highest for the two smallest size classes (20-80cm dbh), rising
to almost 60% (Fig. 14). In contrast, most trees of this size produce no or few
capsules, even though they are physiologically capable of doing so (Fig. 10). For trees
> 80cm dbh, allocation of photosynthates switches from growth to reproduction.
This late onset of reproduction may be an adaptation to the long time between
disturbances (an example is provided by Clark, 1991b).
An additional component of life history theory not often considered is allocation
of photosynthates to structures and compounds that will increase longevity (Loehle,
1988), the period over which growth and reproduction can occur. There is some
evidence that mahogany invests less of its photosynthates in height growth than other
I
40
1
20
I
I
I
80160 +
80
160
DIAMETER CATEGORY (cm)
40-
Figure 14. Allocation to growth and fecundity by mahogany trees in the Chimanes Forest. The points are
(median/maximum)* 100% for growth and fecundity in each size category for those categories where
trees are capable of producing capsules. The majority of trees 2W30 em dbh are growing at > 50-60°/o
of the possible maximum, while allocating virtually nothing to reproduction. This pattern reverses for
trees > 80 cm dbh, where growth slows down, but reproduction increases drastically. (Data are from Figs
10, 12).
MAHOGANY IN BOLIVIA
29
early successional species, and may invest instead in secondary compounds to
decrease rot (Ramos & Grace, 1990). Snook (1993) suggests that mahogany invests
in buttresses to increase its ability to survive through hurricanes. Adaptation to
episodic disturbances would reward allocation of photosynthates to increasing
longevity for two reasons. First, the return time of the disturbances that permit
regeneration could be very long and unpredictable. Therefore, the longer a tree
survived, the higher the probability of it experiencing the disturbance necessary for
regeneration. Second, the episodic disturbances that create regeneration sites for
mahogany may destroy the seed and seedling banks (Snook, 1993; Gullison, pers.
obs.). Reproductive trees must survive through the disturbances to ensure that their
seeds and seedlings are present after the disturbance. Snook (1993) has shown that
mahogany trees survive fires and hurricanes better than other species. Similarly, we
have observed that mahogany trees survive flooding better than the common species
in the Cuberene area (Table 2).
In summary, mahogany is a long lived, late maturing species that regenerates after
large scale hydrological disturbances in the Chimanes Forest. This leads to the
development of stands composed of one or a few cohorts, very different from the
‘reverse-j’ shaped size distribution of a species that readily regenerates in tree fill
gaps or under closed canopy. The frequency and magnitude of past large scale
flooding and erosion events in the Chimanes Forest, and their role in influencing
forest dynamics in the Chimanes Forest, is still poorly understood.
Eflects of current management and recommendationsfor improvement
Haruest &dub
The time for mahogany trees to reach commercial size in natural forest can be
estimated from the growth data in Figure 12. If a tree grows at the maximum rate
throughout its life, it will take 52yr to reach commercial size, while growing at the
median growth rate it will take 148yr. The actual time to achieve commercial size
should fall within this range. Gullison and Hubbell (1992) estimated from growth
rings that the existing commercial trees took on average 105yr to reach the 80 cm
size limit. The strong seasonality in the Chimanes Forest, and some preliminary
evidence from scarring growth windows (cuts made to the cambium in consecutive
years which identlftr when growth rings are formed), suggest that these growth rings
are annual.
The growth rates of mahogany appear remarkably consistent across its range.
Lamb (1966) reports average growth rates of 3.6 to 9.1 mm yr-’, nearly identical to
the median growth rates found in this study of 2.6 to 9.Omm yr-’. Snook (1993)
found a similar growth rate of 2.0-10.9mm yr-’.
Unless silvicultural treatments are undertaken to increase growth, it should be
assumed that it will take somewhere between 105 and 148 years for mahogany trees
to reach commercial size in natural forest.
The slow growth rates and episodic nature of regeneration events make it clear
that the current management plan for mahogany of a 30yr polycyclic harvest is
inappropriate. Even if the companies obeyed the 80 cm minimum diameter and left
10% of trees > 80 cm dbh as required, harvest would often result in the removal of
entire stands, with no or too few survi\;ing trees to permit subsequent harvests, or to
provide postharvest seed input to the site.
30
R. E. G L U S O N ETAL.
The episodic nature and large scale of the disturbances that mahogany requires to
regenerate mean that a monocyclic harvest is more appropriate for this species. The
rotation age would be longer than the current 30yr cutting cycle, more likely to be
on the order of 100 + years. At least at current stand density, it is unlikely that the
reduced annual harvest volume that this would imply would be economically viable,
so the companies would need to augment their harvest with other timber species (R.
Rice, pers. comm.) or have much larger concessions.
Seed trees
Size. Under the current management plan, the population of seed trees is composed
of the 10% of trees > 80 cm dbh that is left by the companies, and all trees < 80 cm
dbh. This results in a large decrease in seed input to logged forest, as trees < 80 cm
produce relatively few seeds (Fig. 10) and the reduction of trees > 80 cm is so great.
The reduction in seed input will be even greater when the smaller trees are illegally
felled and < 10% of commercial trees are not left, as is happening in the Chimanes
Forest (Figs 5, 13).
The impact of harvest on seed input into the three harvest plots (Monte Grande,
Fatima and Jaimanche) can be estimated by multiplying the pre- and post harvest
diameter distributions by their estimated fecundity from the regression in Figure 10.
Combined pre-harvest fecundity (n = 75 trees) on the three sites was 13489 capsules.
After harvest, only six healthy trees (seed trees and trees missed by the company) and
five trees left standing because of heart rot remained. Assuming that the rotted trees
do not deviate from the fecundity regression, postharvest capsule production is 1492,
or 1 1.1YO of preharvest values. The proportion of total seeds produced by obviously
inferior (rotted) trees increases from 6.2% before harvest to 60% after harvest.
Postharvest fecundity can be increased by requiring companies to leave larger seed
trees. Requiring a percentage of total basal area of the commercial stand to be
preserved, rather than a percentage of the number of trees, will eliminate the
incentive to leave the smallest trees possible. The size of seed trees should be chosen
to maximize fecundity for a given amount of basal area (opportunity cost to loggers).
For mahogany in the Chimanes Forest, this occurs between 90 and 130 cm dbh (Fig.
15). Smaller trees in this range are preferable because of their higher reproductive
value.
Location. The existing management plan for seed trees does not account for the
localized nature of regeneration sites. Seed trees are left at a uniform low density
throughout the forest, rather than being concentrated in areas with the highest
potential for regeneration. If the number of seed trees is limited, there is little point
in leaving seed trees in logged forest where regcncration rarely occurs. Not only do
seedlings that survive harvest in general not survive until commercial size (Fig. 9), but
the post harvest density (0.036 trees ha-') results in a spacing of trees so much greater
than median dispersal distances (spacing of 527 m between seed trees vs median seed
dispersal distance of 36m) that it seems unlikely that they could colonize most
regeneration sites should they appear.
Seed trees should be concentrated in areas that are most suitable for regeneration.
Our results suggest that suitable sites are near rivers, particularly active rivers that are
flooding, or in zones where the forest has been recently killed by floods. These areas
can be identified easily with satellite photos. An additional benefit is that secondary
water dispersal of seeds should be greater for trees near rivers. Reducing the felling
MAHOGANY IN BOLIVIA
31
0.020l
o.ooo!,
I
I
I
,
I
I
,
I
I
,
,
I
,
I
9 9 4Q eo +,$$10@@.5P@Q\9+@@@$@
DIAMETER (cm)
Figure 15. The relationship between relative fecundity (No. of capsules divided by basal area) and
diameter for mahogany trees in the Chimanes Forest. This measure of fecundity allows a comparison of
the capsule production for a given amount of basal area for trees of different sizes. In this case, for a fixed
basal area, capsule production will be maximized by setting aside seed trees with diameters of c. 110 cm
(Fecundity is calculated from the regression equation presented in Fig. 10).
of mahogany trees near rivers by the timber companies would also preserve the trees
that are most useful to the local indigenous groups, who use them to make canoes,
but lack the means to haul logs or canoes any great distance on land. A disadvantage
is that these trees are the hardest to protect from illegal felling because access is
possible throughout the year.
Densip. The optimum density of seed trees may be even higher than the density of
mahogany in unlogged stands, given that the median dispersal distance of seeds
( C 36 m) is so low, unless secondary dispersal by water is very effective. Mahogany
can reach densities much higher than the average densities in the Chimanes Forest.
There are only a few examples of tropical tree species being regulated by densitydependent factors (Hubbell, Condit & Foster, 1990; Boot & Gullison, 1995), and
indeed recent theoretical work has shown that density-dependence is not required to
maintain tree species in species rich forests (Hurtt & Pacala, 1955).In the absence of
density-dependent regulation, future regeneration will be proportional to the density
of reproductive adults, and every tree that is harvested will reduce future
regeneration. The lower limit on the density of seed trees is not known either,
although it is quite possible that residual trees in logged forest that survive logging
will suffer reduced outcrossing rates with densities as low as one tree per 27ha
(Murawski, Gunatilleke & Bawa, 1994). The effect of mahogany density on
outcrossing rates in the Chimanes Forest is currently under investigation (M.
Loveless, pers. comm.).
CONCLUSIONS
Two previously undescribed types of large scale disturbances, deposition of alluvial
sediment caused by flooding, and past erosion on terraces, shape the stand structure
32
R. E. GULLISON ETAL.
and population dynamics of mahogany in the Chimanes Forest. Mahogany
regenerates after these episodic disturbances, and its stands are made up of one or a
few cohorts. The long time between disturbances favours late maturation. Mahogany
trees allocate their photosynthates to growth until they reach 80cm dbh, at which
point they increase allocation to reproduction. Unfortunately, this coincides with the
minimum cutting diameter, and the postharvest seed production is substantially
reduced. The polycyclic selective harvesting is misguided given the even-aged stands.
This study shows why management plans must be based on a sound understanding
of the ecology of the target species.
It is too late to manage for the sustained production of mahogany in the Chimanes
Forest. One company finished its supply of mahogany three years ago, and two
others admit to haLing only a one or two year supply left. AU three companies are
in the process of buying illegally felled mahogany trees from the Multiethnic
Indigenous Territory located in the middle of the Chimanes Forest. A management
plan that included a much lower cut of mahogany, the harvest of other commercial
species, and the concentration of seed trees in areas conducive to mahogany
regeneration would have been more appropriate. Luckily the density of mahogany is
low, and little structural damage has been done to the forest even though the harvest
levels are unsustainable at the species level (Gullison & Hardner, 1993).
The same larger scale hydrological disturbances that structure mahogany
populations may determine the population dynamics of many other tree species. If
this is so, the principles from this study can help design management plans for future
commercial species in the Chimanes Forest should there be political will to do so.
ACLWOWLEDGMENTS
Thanks to Jhonny Galloso, Valentin and Rudolf0 Garcia, Lucio and Joselo Isita,
Raul Gonzales, Claudio Leafio, Gregory Dicum, Jared Hardner, Corine Vriesendorp, and The Grand Chimanes Council for valuable help in the field. Dick Rice
and Hector Claure handled the logistics of the project. Henry Horn, Lyn Loveless,
Jake Overton, Laura Snook, Kristina Stinson, Corine Vriesendorp, John Terborgh
and two anonymous reviewers read and commented upon previous drafts of this
paper. Ruth Silva translated the paper into Spanish. Steve Pacala, Don Stratton,
Andy Dobson and Henry Horn have provided sound advice and supervision
throughout the project. The following institutions or companies have provided
financial assistance: National Science Foundation and the Office of Forestry,
Environment and Natural Resources, Bureau of Science and Technology, of the
U.S. Agency for International Development under NSF grant No. BSR-9100058;
BOLFOR Bolivia’s Sustainable Forest Management Project funded by USAID and
the Government of Bolivia through the Ministry of Sustainable Development and
the Environment; Thompson Mahogany Company; Industria Maderera ‘San
Francisco’ S.R.L.; Hermann Miller, Inc.; the U.S.D.A. Forest Service, US. Agency
for International Development/Bolivia; International Tropical Timber Organization PD 88/90; and the National Science and Engineering Research Council
(Canada).
MAHOGANY IN BOLIVIA
33
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