Changes in Forest Structure and Biomass over Ten Years in a Lowland Amazonian Forest

The topographic effects on biomass and its increment were evaluated for a terra-firme forest near Manaus, Brazil. We established three plots (60 × 60 m) on a plateau and at the bottom of a valley (namely baixio), and measured stem diameter at breast height D (> 10 cm) for tree and palm species in 2002 and 2012. Small trees (10 cm > D > 5 cm) were measured in subplots (20 × 60 m) within each plot. We also estimated tree height with the D – tree height H relationship model based on a hundred samples of the tree species in each topography. The aboveground biomass AGB was estimated as 315.4 ± 33.2 (average ± SE) Mg ha-1 on the plateau, which was higher than 224.2 ± 20.3 Mg ha-1 in baixio though the difference was statistically marginal (p = 0.09). The difference in AGB was partly ascribed to the decrease of H for large trees and a lack of large trees whose D > 80 cm at the baixio site. The AGB of palm species in baixio was 2.7 ± 0.87 Mg ha-1, which was higher than 0.24 ± 0.08 Mg ha-1 on the plateau though the difference was statistically insignificant (p = 0.11). On the other hand, the increment of AGB did not differ significantly between the two sites (p > 0.05), and the variation of AGB increment among the plots was mainly ascribed to the variation of biomass loss caused by mortality.