Sensitivity of South American subtropical forests to precipitation variability at multiple time scales

Background/Question/Methods: Seasonal and interannual variability in climate is a major driver of temporal patterns in primary productivity.  Although forest sensitivity to mean annual rainfall in humid climates is minimal, precipitation variability can have major impacts on primary productivity of humid forests.  We address 1) How does the productivity of subtropical humid forests respond to precipitation variability at multiple time scales? and 2) how do upland and riparian forest and savanna differ in sensitivity to precipitation anomalies?  We used the normalized difference vegetation index (NDVI) from SPOT imagery in 1998-2012 to analyze how temporal variation in precipitation and consecutive dry months affect productivity of subtropical forests and wooded savannas of Uruguay in southeastern South America over increasing time scales from 1-24 months. 

Results/Conclusions: Long-term precipitation trends played an important role for the productivity of humid subtropical forests in Uruguay, despite a lack of correlation between mean annual rainfall and net annual primary productivity.  The sensitivity of subtropical forest NDVI to precipitation anomalies enhanced with increasing time scales, such that both riparian and upland forests were equally susceptible to long periods of water deficit and excess despite a relatively humid climate.  Meanwhile, wooded savanna was most sensitive to medium time scale (3-9 months) precipitation variability and showed more rapid declines in productivity than forests over increasing number of dry months.  The linear increase in greenness with increasing rainfall and declining drought risk found here may a potential precursor for the expansion of forests in the region, as predicted by future climate change models.  While increasing forest productivity with increasing precipitation poses potential benefits for carbon sequestration and wildlife conservation in the South American Pampean region of the La Plata River Basin, this study emphasizes that climate variability is a key driver of ecosystem services provided by riparian and upland forests in a landscape undergoing rapid land-use change.