Controles ambientais e bióticos da dinâmica de plântulas em uma floresta de terra-firme da Amazônia central

Abstract

The transition of seedlings to advanced development stages is probably the most important bottleneck defining plant distribution in the tropics. Thus there is a need to better understand the mechanisms acting at this life stage. As with adult distribution, edaphic features greatly influence seedling establishment, but with different strength. This study comprises three different approaches on seedling establishment along a clay soil gradient. (1) In order to evaluate how seasonality interacts with soil texture in influencing seedling establishment, seedling mortality and recruitment were monitored at 30 plots along the environmental gradient, during two rainy seasons and three dry seasons. The time needed for rainfall decrease to affect seedling mortality was assessed by comparing the time series of rainfall with the time series of seedling mortality and recruitment with cross-correlation analysis and multiple lagged-regressions. We found that the effects of the extreme dry season on seedling mortality continued into the rainy season. Drought effects had a delay of approximately 1 month in seedlings of clay soils and an immediate effect on seedlings in sandy soil, indicating that this environment is more vulnerable to drought stress. That may be one of the reasons the plant composition in the valleys (always with sandy soils at Ducke) has a higher turnover than on the plateaus and slope. (2) Observing adult trees and seedlings distribution across a topo-edaphic gradients it was possible to describe patterns of restriction of species to different portions of the environmental gradient. Most of the species were restricted at the adult stage, suggesting a contraction of the suitable habitat as ontogenetic development proceeds. Two species were restricted only during seedling stage, which might indicate temporal and/or spatially fruiting asynchrony across the soil gradient. (3) Searching for evidence of density-dependent effects on seedling establishment, the genetic structure of a tree species population was estimated, and the changes of the structure across different stages of development was assessed. Seedlings spatially close to each other tend to be more genetically similar, but the similarity decreased as distance was augmented and as the plant grew older, but juveniles and adults did not follow that trend, as relatedness values were higher among neighboring individuals in this size class. That suggests a trade-off between the negative effects of being near closely-related adults (e.g. Herbivore and pathogen attack) and the advantage of being in a site favorable to establishment. We also found young seedlings in clay soils (slope and plateau) were more related to each other than were seedlings in sandy soils (valley), suggesting that the outcomes of conspecific negative density-dependent effects that maintain tree community diversity may interact with environmental features.