A classification and ordination of the tree community of Tikal National Park, Petén, Guatemala

Abstract

We studied tropical forest composition and structure in relation to topographic and edaphic variation. with the goal of understanding the factors that determine species distributions and the degree to which species composition can be predicted by local environmental conditions. We sampled the tree community of Tikal National Park in Petén, Guatemala using 294 sample plots of 0.041 ha each, totaling 12.1 ha, placed systematically along topographic gradients. In addition to recording tree and sapling occurrences, we took data on environmental factors, vegetation structure, and disturbance history. Indirect and direct gradient analyses were performed (using DCA and CCA, respectively) to investigate variation along a predominant topographic/edaphic gradient and along gradients of disturbance and light availability. We used TWINSPAN analysis and ranked sorting of samples by environmental variables to produce a forest type classification practical for field use. Individual species distributions were examined with respect to edaphic conditions, understory light availability, and natural forest disturbance. Though topographic relief is moderate in the area, several edaphic factors varied strongly and predictably, though not monotonically, from hillcrests to low-lying depressions, and exerted a predominant influence on tree community composition. Most tree species distributions were strongly correlated with topography and associated edaphic conditions, and distribution patterns were consistent among widely separated topographic gradients, suggesting that environmental conditions rather than historical events, were largely responsible for these patterns. Many of the less shade-tolerant species also showed positive relationships with natural disturbance history and degree of canopy opening. Treefall gaps have different functional significance for regeneration at different points along the topographic/forest type continuum, due to correlated differences in canopy height canopy evenness, and light penetration. Hence, many species that appear to depend on treefall gaps for colonization and recruitment in upland portions of the continuum, are less confined in their exploitation of lower regions of the gradient, where canopy discontinuities may provide sufficient light for seedling persistence and recruitment. Thus many highly light-demanding species were not associated with treefall gaps, and appeared capable of colonizing relatively open lowland forests. Distribution patterns and disturbance/light responses are discussed for individual species, which are provisionally placed into ecological guilds based on apparent tolerances for light levels and edaphic and other environmental factors. In Tikal, much of the natural variation in forest composition and structure can be related to topography and edaphic conditions.