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ESD Tackles Tropical Forest C Balance

SourceAlan DiVittorio, and Dan Hawkes

AmazonforestThe world's forests have been identified as the primary terrestrial carbon sink, with tropical forests assimilating about half of the total forest carbon uptake. However, these and similar estimates of forest carbon balance have high levels of uncertainty, partially due to undercharacterized mortality regimes. Scientific debate continues over the adequacy of existing field plots to sufficiently capture Amazon forest dynamics in efforts to estimate regional forest carbon balance. While advances in remote sensing have increased scientific understanding of mortality regimes, questions remain regarding statistical distributions of disturbance size and their effects on spatial patterns of forest dynamics. Answering such questions is critical for understanding how the relationship between annual mortality and disturbance size distribution influences estimates of forest carbon balance. 

Tree mortality dynamics are particularly uncertain because of the difficulty in observing large, infrequent disturbances. Following up on a recent paper reporting that Central Amazon plots missed 9–17% of tree mortality; ESD climate scientists Alan DiVittorio, Robinson Negrón-Juárez, and Jeff Chambers (along with Niro Higuchi of Brazil’s National Institute for Amazonian Research) recently addressed why this should be the case, by elucidating two distinct mortality components: (1) annual average mortality and (2) the spatial clustering of annual average mortality. 

Using a stochastic-empirical tree growth model, the team showed that a power law distribution of event size (based on merged plot and satellite data) is required to generate spatial clustering of mortality that is consistent with forest gap observations. They argue that the spatial clustering associated with the additional mortality due to large events further explains why existing plots do not adequately capture disturbance losses, leading to potential overestimates of Central Amazon forest carbon uptake. They conclude that existing plots do not sufficiently capture losses because their placement, size, and longevity assume spatially random mortality, while mortality is actually distributed among differently sized events (clusters of dead trees) that determine the spatial structure of forest canopies. 

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Di Vittorio, A.V., R.I. Negrón-Juárez, N. Higuchi, and J.Q. Chambers (2014), Tropical forest carbon balance: Effects of field- and satellite-based mortality regimes on the dynamics and spatial structure of Central Amazon forest biomass. Environmental Research Letters, 9 (3) 034010; DOI: 10.1088/1748-9326/9/3/034010.

Funding: BER