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Carbon Cycle & Ecosystems

There is concern that in a warming climate the ensuing moisture stress could result in Amazonian rainforests being replaced by savannas, in which case the large reserves of carbon stored in these forests, about 100 billion tons, could be released to the atmosphere, which in turn would accelerate global warming significantly. Hence, the drought sensitivity of these forests is a subject of intense study – recent articles on the response and vulnerability of these forests to droughts illustrate the various complexities. Severe droughts such as those associated with the El Niño Southern Oscillation (ENSO), when the plant‐available soil moisture stays below a critical threshold level for a prolonged period, are known to result in higher rates of tree mortality and increased forest flammability.

During this decade, the Amazon region has suffered two severe droughts in the short span of five years – 2005 and 2010. Studies on the 2005 drought present a complex, and sometimes contradictory, picture of how these forests have responded to the drought. Now, on the heels of the 2005 drought, comes an even stronger drought in 2010, as indicated by record low river levels in the 109 years of bookkeeping. How has the vegetation in this region responded to this record-breaking drought? Here we report widespread, severe and persistent declines in vegetation greenness, a proxy for photosynthetic carbon fixation, in the Amazon region during the 2010 drought based on analysis of satellite measurements. The 2010 drought, as measured by rainfall deficit, affected an area 1.65 times larger than the 2005 drought – nearly 5 million km2 of vegetated area in Amazonia. The decline in greenness during the 2010 drought spanned an area that was four times greater (2.4 million km2 and more severe than in 2005. Notably, 51% of all drought-stricken forests showed greenness declines in 2010 (1.68 million km2 compared to only 14% in 2005 (0.32 million km2). These declines in 2010 persisted following the end of the dry season drought and return of rainfall to normal levels, unlike in 2005. Overall, the widespread loss of photosynthetic capacity of Amazonian vegetation due to the 2010 drought may represent a significant perturbation to the global carbon cycle. Figure 1 shows the spatial patterns of July to September (JAS) 2010 standardized anomalies of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) in vegetated areas of drought. The drought affected region, as depicted by the spatial patterns of JAS 2010 standardized anomalies of precipitation, is shown in Figure 2.

VI_amazon Figure 1: Spatial patterns of July to September (JAS) 2010 standardized anomalies of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) in vegetated areas of drought (precipitation anomalies less than -1 standard deviation)

precip_2010

Figure 2: Spatial patterns of July to September (JAS) 2010 standardized anomalies of precipitation. Monthly precipitation rate (mm/hr) at a spatial resolution of 0.25degx0.25deg from The Tropical Rainfall Measuring Mission (TRMM) sensor is utilized for this study.

This project creates a platform to investigate the sensitivity of these Amazonian rain forests to extreme climatic anomalies (e.g. drought of 2005 and 2010), utilizing satellite-derived greenness data sets from MODIS on board the TERRA and AQUA sensor. Other ancillary data sets like precipitation and radiation are obtained from the TRMM and CERES sensors.

Collaborative research groups:
1. The Climate and Vegetation Research Group at Boston University, Boston, MA, USA.
2. Ecological Forecasting Lab at NASA Ames Research Center, Moffett Field, CA, USA.
3. Research Group on Biosphere-Atmosphere Interaction at the Federal University of Viçosa, Brazil.

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Started: Mar 10, 2011

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