Tree seedling recruitment in Amazon secondary forest: Importance of topography and gap micro-site conditions

Abstract

Deforested lands in Amazonia are usually converted into pastures and maintained through annual prescribed burning which depletes the soil seed bank. Here, we assess the effect of topography and micro-site conditions on the seed bank and recruitment success of tree species in 20-year old secondary forests developing on abandoned pastures in Central Amazonia, Brazil. Seedling emergence, mortality, and growth were monitored in four 1×1-m sub-plots located systematically in the center of 21 10×10-m artificial canopy gaps, seven each on three different topographic positions – plateau, slope, and bottomland. The 84 seedling sub-plots were assigned to four different treatments generated by the combination of two litter treatments, litter intact and litter removed, and two soil treatments, soil turned and soil unturned. Sixteen soil samples were collected from the four corners of each sub-plot for analysis of the seed bank. There was no significant effect of topography on the number of seeds, although on the average, densities on the plateaus and the bottomlands were more than double that on the slopes. Seedling emergence increased 200% with litter removal and 50% with soil turning relative to respective controls. Seedling emergence was significantly higher in bottomlands than in slopes, and seedling growth was significantly higher in bottomlands and slopes than in plateaus, indicating that water availability may be the limiting factor for the recruitment success on the higher parts of relief. There were no effects of topography and litter removal on seedling mortality. Management tools that can accelerate succession on intensively used land offer options for fostering reforestation. Based on this study, manipulating litter and soil micro-environment provide viable methodological tools.