MAYA PROJECT RESEARCH
Description and Results
The following account of our research activities follows the
outline given. For each topic, we describe the associated
conservation challenges, give some background knowledge on the
topic, describe our research activities, and give a brief
synopsis of our results.
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Coarse-textured tall, upland forest on well-drained
sites gives way to a finer-textured forest of much
lower stature in
seasonally-flooded bajo areas.
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5. Forest Vegetation Ecology
Tikal's Forest Vegetation and Plant Ecology Studies
Vegetation of northern Petén's extensive forests had
received little detailed study since the 1930s (Lundell
1937).
With increasing conservation efforts being invested in this
area, there has been a need for a detailed description of the
forest vegetation. In addition, much remains to be learned of
the ecology of individual forest tree species, and of the
processes that govern tree community patterns. For these reasons
and because of our interest in vegetation ecology, we (mostly
Mark Schulze) studied in some detail the forest vegetation at
Tikal.
Although topographic diversity at Tikal is quite limited, we
found that the subtle gradient from hill tops to low-lying areas
produced dramatic differences in species composition and
structure of the forest vegetation, related in part to
differences in edaphic (i.e., soil) characteristics as well as
soil moisture and drainage characteristics along the topographic
gradient.
Against this subtle environmental gradient, we conducted a
classical "direct gradient analysis", to discover the
relationship between the abundance patterns of individual tree
species and various factors of soil and topography.
We found that low-lying areas with deep, clay-rich soil host
vegetation that is dramatically different from that occurring on
better-drained slopes just a stone's throw away. These low-lying
basins, known locally as bajos (literally "low
areas"), support a low forest of small, stunted trees with
quite open canopy but extremely dense understory of saplings and
vines. It is often impossible for a human to pass through such a
bajo or scrub swamp forest without liberal use of a
machete.
The tree species that reach their greatest abundance in these
scrub swamp forests are in large part a different set of tree
species than those that reached greatest abundance at other
portions of the predominant topographic gradient. Because of
their low-lying position and clay-rich soil, the bajos
are subjected to yearly extremes of flooding (often waist-deep
standing water in the rainy season) and dry-season
drought--during which water binds so fiercely to clay particles
that plants cannot access it, while the ground surface breaks
into deep fissures.
Hillside and hilltop sites had better soil drainage, more
organic matter, and less clay, compared to the bajos.
These better-drained sites experienced neither water-logging nor
severe drought, and a different set of tree species reached peak
abundance in these sites. Other tree species reached peak
abundance in intermediate conditions near the base of hills.
Many vegetation structural characteristics also varied along
this environmental gradient.
These results are described in detail in:
Mark D. Schulze and David F.Whitacre. 1999. A
classification and ordination of the tree community of Tikal
National Park, Petén, Guatemala. Bulletin of the Florida
Museum of Natural History, Vol 41, No. 3:169-297. University
of Florida, Gainesville.
A limited number of these bulletins are available for
free distribution by The Peregrine Fund.
Advanced Vegetation Ecology at Tikal
One question of perennial interest to tropical ecologists has
been this: how can so many tree species coexist in tropical
forests? Traditional ecological dogma invokes the
"competitive exclusion rule", which states that no two
species can rely on precisely the same resources, otherwise one
will drive the other to extinction via competition. This school
of thought holds that coexisting species must coexist largely
through differences in species attributes. Another school of
thought holds that the environment is too changeable for this to
occur--the competitive apple cart gets upset by other factors
before such competitive exclusion processes can lead to
exclusion of one or more species.
Tropical forest ecologists have thus asked--are all these
tree species able to coexist because they are really that
different? Or is the composition of the tree community a result
of more random processes?
For trees in a tropical forest, there are only so many
"niche axes" along which they may partition resources
in order to minimize competition. For seedlings in the deep
shade of the tropical forest understory, a major problem is
getting access to light. Hence many plant ecologists have
emphasized the role of "tree-fall gap dynamics" in
permitting seedling establishment, and/or the subsequent growth
of saplings and young trees. While some trees can germinate in
the shade, many species need the bright light provided by a
tree-fall gap in order for seedlings to survive and race upward
toward the light.
Some tropical ecologists in the past have recognized only two
or three strategies of trees with respect to their regeneration
ecology, especially as regards their reliance on tree-fall gaps.
One novel aspect of our vegetation studies was that Mark
Schulze described many different variations on the theme of how
trees use or do not use gaps to regenerate.
Moreover, each tree species at Tikal appears to be most
successful along a certain portion of the gradient from
well-drained slopes to seasonally-flooded and drought-stressed bajos.
When Mark took into account tree species' differing responses to
topographic position in combination with their different
regeneration strategies with respect to tree-fall gaps and light
levels, he mapped out an amazing variety of overall ecological
strategies.
 Click here for
Figure 38 from Vegetation Monograph
(Figure 38 copyright Florida Museum of Natural History--not to
be used for commercial purposes.)
Literature Cited, Forest Vegetation Ecology
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