Rainfall is a limiting factor for above-ground net primary production (ANNP) for almost all terrestrial ecosystems. Average rain-use efficiency is defined as the ANNP per unit precipitation. Huxman et al. (2004) report that in dry years, rain use efficiency is similar in deserts, forests and grasslands. These results suggest that regions with low precipitation resulting from reduced mean annual precipitation or more frequent droughts due to increased precipitation variability under global climate change might experience decreases in above-ground net primary production in a future climate.
Reference
Huxman, T. E., and Co-Authors, 2004: Convergence across biomes o a common rain-use efficiency. Nature, 429, 651-654.