Abstract
Tropical rainforests store enormous amounts of carbon, the protection of which represents a vital component of efforts to mitigate global climate change. Currently, tropical forest conservation, science, policies, and climate mitigation actions focus predominantly on reducing carbon emissions from deforestation alone. However, every year vast areas of the humid tropics are disturbed by selective logging, understory fires, and habitat fragmentation. There is an urgent need to understand the effect of such disturbances on carbon stocks, and how stocks in disturbed forests compare to those found in undisturbed primary forests as well as in regenerating secondary forests. Here, we present the results of the largest field study to date on the impacts of human disturbances on above and belowground carbon stocks in tropical forests. Live vegetation, the largest carbon pool, was extremely sensitive to disturbance: forests that experienced both selective logging and understory fires stored, on average, 40% less aboveground carbon than undisturbed forests and were structurally similar to secondary forests. Edge effects also played an important role in explaining variability in aboveground carbon stocks of disturbed forests. Results indicate a potential rapid recovery of the dead wood and litter carbon pools, while soil stocks (0-30cm) appeared to be resistant to the effects of logging and fire. Carbon loss and subsequent emissions due to human disturbances remain largely unaccounted for in greenhouse gas inventories, but by comparing our estimates of depleted carbon stocks in disturbed forests with Brazilian government assessments of the total forest area annually disturbed in the Amazon, we show that these emissions could represent up to 40% of the carbon loss from deforestation in the region. We conclude that conservation programs aiming to ensure the long-term permanence of forest carbon stocks, such as REDD+, will remain limited in their success unless they effectively avoid degradation as well as deforestation.
Highlights
Anthropogenic forest degradation is the reduction in the overall capacity of a forest to supply goods and services including carbon storage, climate regulation, and biodiversity conservation
In tropical forests, the degree of degradation of carbon stocks depends on the type of disturbance, the intensity and frequency of disturbance events, and the time elapsed since their occurrence (Laurance et al, 2006; Barlow et al, 2012; Arag~ao et al, 2014)
We show that forest disturbance, when resulting from a combination of both fire and logging, can severely alter forest structure (Fig. 3; Fig. 4), resulting in acute degradation of carbon stocks, with losses being more pronounced in live aboveground carbon than in dead organic matter or soil (Fig. 2)
Summary
Anthropogenic forest degradation is the reduction in the overall capacity of a forest to supply goods and services including carbon storage, climate regulation, and biodiversity conservation. A lack of field data from disturbed forests means that our knowledge of the relative importance of these different factors in explaining changes in overall carbon stocks is very poor (Aguiar et al, 2012). We still have a limited understanding of the combined effects of multiple forms of disturbance on different carbon pools, which constrains our ability to identify management priorities for avoiding further losses and restoring already degraded forests
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