Seasonal and inter-annual variability of climate and vegetation indices across the Amazon

Background/Question/Methods

The responses of tropical forest metabolism and carbon stocks to drought influences the flow of carbon to the atmosphere. Satellite-based studies suggest that Amazon forest productivity increases during droughts because of higher light availability, whereas field studies show that forest productivity is inhibited by drought, with tree mortality increasing when droughts are severe. We conducted an analysis of climate data, field measurements, and improved satellite-based measures of forest photosynthetic activity to better understand these apparently conflicting findings. .

Results/Conclusions

While dry-season precipitation (PPT) and plant available water (PAW) decreased over the Amazon during1995-2005, photosynthetically active radiation (PAR) and air dryness (expressed as vapor pressure deficit, or VPD) increased during 2002-2005. Using an improved MODIS enhanced vegetation index (EVI) measurements (2000-2008), we show that gross primary productivity (expressed as EVI) declined with VPD and PAW in regions of lower canopy cover across a wide range of environments for each year of the study. In densely forested areas, no climatic variable significantly explained the Basin-wide, inter-annual variability of EVI (2000 to 2005). Based on a site-specific study, we show that monthly EVI was relatively insensitive to leaf area index (LAI) but positively correlated with PAR and leaf flushing measured in the field. These findings suggests that production of new leaves, even when unaccompanied by associated changes in LAI, could play an important role in Basin-wide, inter-annual EVI variability. Because inter-annual EVI variability was greatest in regions of lower PAW, we hypothesize that drought could increase EVI by synchronizing leaf flushing via its effects on leaf bud development.