Seasonal and interannual variability of climate and vegetation indices across the Amazon

Abstract

Drought exerts a strong influence on tropical forest metabolism, carbon stocks, and ultimately the flux of carbon to the atmosphere. Satellite-based studies have suggested that Amazon forests green up during droughts because of increased sunlight, whereas field studies have reported increased tree mortality during severe droughts. In an effort to reconcile these apparently conflicting findings, we conducted an analysis of climate data, field measurements, and improved satellite-based measures of forest photosynthetic activity. Wet-season precipitation and plant-available water (PAW) decreased over the Amazon Basin from 1996-2005, and photosynthetically active radiation (PAR) and air dryness (expressed as vapor pressure deficit, VPD) increased from 2002-2005. Using improved enhanced vegetation index (EVI) measurements (2000-2008), we show that gross primary productivity (expressed as EVI) declined with VPD and PAW in regions of sparse canopy cover across a wide range of environments for each year of the study. In densely forested areas, no climatic variable adequately explained the Basin-wide interannual variability of EVI. Based on a site-specific study, we show that monthly EVI was relatively insensitive to leaf area index (LAI) but correlated positively with leaf flushing and PAR measured in the field. These findings suggest that production of new leaves, even when unaccompanied by associated changes in LAI, could play an important role in Basin-wide interannual EVI variability. Because 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.

Fig. 1.

(Top) Average dry-season EVI across central South America for the period 2000–2008. Overlaid circles represent average VPD measured at 280 meteorological stations across the region for the period 1996–2005. (Middle) Coefficient of variation in annual EVI for the period 2000–2008. (Bottom) Average annual PAW at 10-m depth, expressed as a percentages of the maximum, for the period 1996–2005. Note that the scale here is amplified relative to Fig. 2.

Fig. 2.

Spatial–temporal patterns of dry-season EVI across the Amazon for areas with high percentage of canopy cover (EVI >0.4 and MODIS canopy cover product >70%). The panel at the top left shows average EVI from 2000–2008 (referred in the figure as '00-'08). Other panels show the EVI anomaly, calculated as EVI_i − EVI_mean, for each year, as indicated.

Fig. 3.

Monthly climate patterns over the Amazon region based on data from 280 meteorological stations distributed across the basin. (A) Monthly averages of precipitation. (B) Monthly modeled PAR. (C) Modeled monthly PAW at 10-m depth. (D) Monthly VPD. Blue lines represent a smooth curve based on a loess method, and red lines represent a local regression model (spline).

Fig. 4.

Temporal patterns of environmental and biophysical correlates of EVI. (Top) EVI derived from MODIS NBAR. EVI derived from the collection 5 MODIS product screened to include only good- or best-quality control flags and LAI (Methods). (Middle) Monthly PAR and bimonthly litterfall and new leaf production (Methods). (Bottom) PAW (% of maximum) at two depths (0–2 m and 2–11 m) and daily precipitation (in mm).