Elizabeth Hammond Pyle

CARBON BALANCE AND VEGETATION DYNAMICS IN AN OLD-GROWTH AMAZONIAN FOREST

Amazon forests could be globally significant sinks or sources for atmospheric carbon dioxide, but carbon balance of these forests remains poorly quantified. We surveyed 19.75 ha along four 1-km transects of well-drained old-growth upland forest in the Tapajos National Forest near Santarem, Para ´, Brazil (2 8519 S, 548589 W) in order to assess carbon pool sizes, fluxes, and climatic controls on carbon balance. In 1999 there were, on average, 470 live trees per hectare with diameter at breast height (dbh)

Dynamics of carbon, biomass, and structure in two Amazonian forests

10 cm. The mean (and 95% CI) aboveground live biomass was 143.7 6 5.4 Mg C/ha, with an additional 48.0 6 5.2 Mg C/ha of coarse woody debris (CWD). The increase of live wood biomass after two years was 1.40 6 0.62 Mg C·ha 21 ·yr 21 , the net result of growth (3.18 6 0.20 Mg C·ha 21 ·yr 21 from mean bole increment of 0.36 cm/yr), recruitment of new trees (0.63 6 0.09 Mg C·ha 21 ·yr 21 , reflecting a notably high stem recruitment rate of 4.8 6 0.9%), and mortality (22.41 6 0.53 Mg C·ha 21 ·yr 21 from stem death of 1.7% yr 21 ). The gain in live wood biomass was exceeded by respiration losses from CWD, resulting in an overall estimated net loss from total aboveground biomass of 1.9 6 1.0 Mg C·ha 21 ·yr 21 . The presence of large CWD pools, high recruitment rate, and net accumulation of small-tree biomass, suggest that a period of high mortality preceded the initiation of this study, possibly triggered by the strong El Nino Southern Oscillation events of the 1990s. Transfer of carbon between live and dead biomass pools appears to have led to substantial increases in the pool of CWD, causing the observed net carbon release. The data show that biometric studies of tropical forests neglecting CWD are unlikely to accurately determine carbon balance. Fur- thermore, the hypothesized sequestration flux from CO 2 fertilization (,0.5 Mg C·ha 21 ·yr 21 ) would be comparatively small and masked for considerable periods by climate-driven shifts in forest structure and associated carbon balance in tropical forests.

Carbon in Amazon Forests: Unexpected Seasonal Fluxes and Disturbance-Induced Losses

(1) Amazon forests are potentially globally significant sources or sinks for atmospheric carbon dioxide. In this study, we characterize the spatial trends in carbon storage and fluxes in both live and dead biomass (necromass) in two Amazonian forests, the Biological Dynamic of Forest Fragments Project (BDFFP), near Manaus, Amazonas, and the Tapajos National Forest (TNF) near Santarem, Para´. We assessed coarse woody debris (CWD) stocks, tree growth, mortality, and recruitment in ground-based plots distributed across the terra firme forest at both sites. Carbon dynamics were similar within each site, but differed significantly between the sites. The BDFFP and the TNF held comparable live biomass (167 ± 7.6 MgCha � 1 versus 149 ± 6.0 MgCha � 1 , respectively), but stocks of CWD were 2.5 times larger at TNF (16.2 ± 1.5 MgCha � 1 at BDFFP, versus 40.1 ± 3.9 MgCha � 1 at TNF). A model of current forest dynamics suggests that the BDFFP was close to carbon balance, and its size class structure approximated a steady state. The TNF, by contrast, showed rapid carbon accrual to live biomass (3.24 ± 0.22 MgCha � 1 � a � 1 in TNF, 2.59 ± 0.16 MgCha � 1 � a � 1 in BDFFP), which was more than offset by losses from large stocks of CWD, as well as ongoing shifts of biomass among size classes. This pattern in the TNF suggests recovery from a significant disturbance. The net loss of carbon from the TNF will likely last 10-15 years after the initial disturbance (controlled by the rate of decay of coarse woody debris), followed by uptake of carbon as the forest size class structure and composition continue to shift. The frequency and longevity of forests showing such disequilibruim dynamics within the larger matrix of the Amazon remains an essential question to understanding Amazonian carbon balance.