Dissecting biomass dynamics in a large Amazonian forest plot

Above-ground biomass (AGB) is increasing inmost of the Amazon forests. One hypothesis is that forests are responding to widespread and intense human intervention prior to the European conquest (> 500 y ago). In this study we confront this hypothesis with changes in AGB over 6.3 y in a large western Amazonian forest plot (> 150 000 shrubs and trees and 1100 species with dbh >= 10 mm in 2 5 ha). We examined AGB flux in different habitats and across diameter classes. The forest lost small stems (4.6%), gained large trees (2.61%), and gained biomass (0.7%). The change in AGB stock was due entirely to this upward shift in size leading to more canopy trees and fewer saplings after just 6 y. Across habitats, the biggest increment in biomass was in the secondary-forest patch (3.4% y(-1)) which we know was cleared about 27 y ago, whereas mature forest on ridges and valleys had small increases (0.10%, and 0.09% y(-1), respectively). In both censuses. AGB stocks were > 50% higher on the ridge than in the valley while relative growth and mortality were higher in the valley. Mean wood specific gravity (WSG) decreased with increasing diameter class: WSG did not change much between censuses in mature forests and did not contribute to the change in AGB stocks. Our forest increased its standing biomass, but far less than the average reported for other Amazonian forests (i.e. 0.30 vs. 0.98 Mg ha(-1) y(-1)). We find no evidence to support the notion that this forest is recovering from long-past human intervention. Instead of a long-term recovery. we believe the forest changed in response to natural fluctuations of the environment (e.g. changes in precipitation, higher CO2). windstorms or other more recent events. The significant differences in AGB stocks between valley and ridge suggest that the terra firme forests are a mosaic of natural habitats, and that this mosaic is in part responsible for the variation in biomass stocks detected in Amazonian terra firme forests.