The hydroclimate--vegetation relationship in the southwestern Amazon
during the last 20 years
Abstract
The relationship between multiple hydroclimatic variables and vegetation
conditions in the upper Madeira Basin (southwestern Amazon) has been
analyzed. Vegetative dynamics are characterized using NDVI dataset as an
indicator of the photosynthetic capacities of vegetation. Hydroclimatic
variability is analyzed using satellite-based precipitation datasets,
observed river discharge and satellite measurements of terrestrial water
storage (TWS). Our results show that the vegetation in the Basin varies
from energy- to water-limited. During the peak of the wet season
(January-February), rainfall, discharge and TWS are negatively
correlated with NDVI (r=-0.48 to -0.65), suggesting that during this
period the vegetation is mainly energy-dependent. Outside this period,
these correlations are positive (r=0.55 to 0.9), suggesting that
vegetation depends mainly on water availability. This higher water
dependence is more noticeable during the vegetation dry season (VDS;
June-October). Considering the predominant land cover types, differences
in the hydroclimate-NDVI relationship are observed. Evergreen forests
remain energy-limited during the beginning of the VDS, but they become
water-dependent almost at the end. Savannas show a different behaviour,
where water dependence occurs months before the onset of the VDS. On the
other hand, unlike the other variables, the TWS better explains the NDVI
in evergreen forests during the VDS (r=0.7 to 0.85). This is probably
because evergreen forests are more dependent on deep soil water. A
spatial analysis between hydroclimatic variables and the NDVI shows the
predominance of positive correlations in most of the basin. However,
specific areas do not show significant correlations. The weak
relationship in these areas is explained by two factors i) very wet
conditions during most of the year in the “rainfall hotspot” regions,
where the vegetation is not water-limited, and ii) recent land-use
changes (deforestation) that break the natural response in the
hydroclimate-vegetation system. These findings provide new evidence on
the impacts of the land cover changes on the natural relationship
between vegetation and hydroclimatic variability, which is particularly
relevant given the increasing rates of deforestation in this region
during the recent years.