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Neotropical lowland forests along environmental gradients

Neotropical lowland forests along environmental gradients
Marisol Toledo


C.T. de Wit Graduate School for Production Ecology and Resource Conservation, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.


Understanding spatial patterns of tropical forests and the environmental factors determining these patterns is important for forest management and for predicting responses of forests to climate change. The main objective of this dissertation was to evaluate how environmental factors shape tropical lowland forests in Bolivia. Specifically it assessed how climatic and edaphic factors affect 1) forest structure, 2) floristic composition, 3) tree growth rates, and 4) species distribution. It also assessed how disturbance factors affect tree growth rates.

For this research, I used a network of 220 1-ha permanent sample plots distributed along environmental gradients. For each plot, all stems ≥10 cm diameter were identified, evaluated and monitored; climatic data were interpolated from weather stations and soil samples were collected. In lowland Bolivia, rainfall seasonality increased from north to south; although drier forests had more fertile soils than moister forests, some plots in moister forests were also fertile. Environmental variables were summarized, using Principal Components Analysis, into four (composite) environmental factors reflecting rainfall, temperature, soil fertility and soil texture, respectively. A stepwise selection procedure identified how these environmental factors affect tropical forest.

It is shown that rainfall and soil texture positively affected most of the 15 forest structural variables. Forest height, palm density and total basal area increased with rainfall while liana infestation decreased. While forest height and liana infestation were more affected by soil texture, palm density was negatively affected by soil fertility. Surprisingly, tree basal area was not affected by the environmental factors. For documenting the variation in floristic composition I selected a set of 100 plant species. Floristic variation was more strongly shaped by the climatic gradient than by the edaphic gradient. Detrended Correspondence Analysis ordination, based on species abundance, divided lowland Bolivia into two major groups (Southern Chiquitano and Amazonia), and a Multiple Response Permutation Procedure distinguished five floristic regions. Additionally, I described diameter growth rates at tree and stand level and evaluated the effects of environmental factors and logging on growth rates. Growth rates at both tree and stand level increased with rainfall and temperature but no clear effects of soil fertility were found. Growth rates increased in logged plots, especially those which had a high logging impact. Finally, I analyzed the species distribution and environmental response curves for each of those 100 selected species. I found a positive trend between species abundance and occurrence. While 91% of the species were affected by climate, 47% of the species were affected by soil factors.

In summary, climate was the most important factor shaping lowland forests in Bolivia. The large variation among forests and the ecological differences along the main environmental gradients have to be taken into account when developing forest-specific management plans. The results of the gradient approach suggest that with future decreases in rainfall and increases in temperature, due to climate change, drastic shifts can be expected in forest structure, composition and dynamics in these tropical lowland forests.