Precipitation-NDVI relationships in eastern and southern African savannas vary along a precipitation gradient

Precipitation is a major determinant of vegetation production at the regional scale, especially in tropical areas. Recent works, however, have shown that this relationship is weak at the interannual temporal scale, particularly in mesic ecosystems, where limitations other than water constrain vegetation production. We investigated whether this holds true for eastern and southern African savannas, by studying the relationship between precipitation and the normalized difference vegetation index (NDVI) at the interannual time-scale in 33 protected areas. We also used extreme precipitation and NDVI events to reveal the overlooked influence of precipitation along the mean annual precipitation (MAP) gradient. Only the semi-arid ecosystems showed significant precipitation-NDVI relationships. We found that maximum NDVI was associated with opposite precipitation conditions along the MAP gradient; maximum NDVI was associated with some of the largest precipitation in semi-arid sites and with some of the lowest precipitation in mesic sites. Although untested, these results are consistent with the hypothesis of an interaction between water and nutrient limitations along the MAP gradient. Our results extend to African savannas the previous finding that ecosystem sensitivity to annual precipitation decreases with increasing MAP, and highlight that, even in mesic ecosystems, precipitation patterns condition the likelihood of reaching maximum NDVI.