| Wood is a conspicuous feature of all forest ecosystems. A large portion of carbon fixed via photosynthesis is stored in woody tissues, and this carbon is returned to the atmosphere from the death and decay of twigs, branches and the trunks of trees. This study investigated the role of large wood (>10 cm diameter) in the carbon cycle of central Amazon rain forest. Decomposition (respiration, fragmentation, and leaching) rates for large dead wood varied by over 1.5 orders of magnitude (0.015--0.67 yr-1). Wood density and bole diameter accounted for a large part of this variability. A multiple regression model was developed that provides unbiased estimates of decomposition rates as a function of wood density and diameter. Comparing globally distributed wood decomposition studies, mean annual temperature was a strong control over rates (Q10 = 2.4). A study of dead wood respiration showed that CO2 production rates were highly correlated with dead wood moisture content, and this control was apparent in both forests and pastures. Respiration was estimated to account for about 70% of total decomposition carbon losses. An empirical-statistical model was developed that allows spatial and temporal extrapolations of the carbon cycling dynamics of total large wood (live and dead). The model required species-specific information on maximum trunk diameter, and growth suppression of small trees, to accurately simulate forests size structure. The model predicts that large wood acts as a carbon source during high mortality years, and a carbon sink during low mortality years. The model predicts that standing-stocks of dead wood are about 31 Mg ha-1, with a turnover rate of 0.14 yr-1, or a mean residence time of 7.4 years. The modeled frequency of 1000 year old trees was 0.05 ha-1 (1 per 20 ha). Results from the model suggest that to quantify regional carbon budgets, areas larger than 10 ha are required to representatively sample annual variability in carbon exchanges with the atmosphere. |
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