Introduction to the Maya Project Introduction to The Selva Maya Principal Agents of Tropical Deforestation The Maya Project Components - Activities and Results


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.

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.

5. Forest Vegetation Ecology

Tikal's Forest Vegetation and Plant Ecology Studies

Vegetation of northern Pet�n'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, Pet�n, 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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Last Revised:  12/10/08