Effects of interannual precipitation variability on aboveground net primary production in the Chihuahuan desert

Climate change will result not only in changes in the mean climate state but also in the variability of climate. The IPCC-A1B scenario, predicts a decrease in precipitation of 5-10% for the Southwestern US and an increase in interannual variability. Consequences of increasing precipitation variability for the functioning of ecosystems have received considerable less attention than assessments of the effects of changes in mean precipitation and temperature. We hypothesize that increased inter-annual precipitation variability on will increase aboveground net primary production (ANPP). The rationale of this hypothesis results from the existence of legacies in the response of ANPP to precipitation. Legacies are the difference between the expected ANPP based on current year precipitation and the observed ANPP. Legacies resulting from a dry previous year are expected to have a negative impact on ANPP and legacies resulting from a wet previous year are expected to have a positive effect on ANPP.  If legacies are symmetrical; and wet legacies have the same absolute value than dry legacies then variability would have no effect on productivity. In order to test this hypothesis we installed a well replicated experiment in the Jornada LTER where we manipulated interannual precipitation variability; increasing it by 50 and 80 % relative to controls. Increased variability treatments received a fraction of incoming precipitation in odd years and enhanced precipitation in even years.

Our results indicate that increased interannual precipitation variability decreases grass ANPP (F _{4, 48} P=0.02) but does not have an effect on shrub ANPP (F _{4, 48} P=0.44) resulting in a slight decrease in total ANPP (F _{4, 48} P=0.04). Dry legacies have a greater negative effect on grass ANPP than positive legacies. Dry years produce a drop in the grass meristem density that limits grass growth in subsequent years. Shrub ANPP limited response to precipitation interannual variability may be explained by less variable soil water content in deeper layers of the soil where shrub roots are more abundant or by a different growth strategy  that depends on dormant meristems that persist through dry years. Our results disagree with our hypotheses and showed that increased precipitation interannual variability has a negative impact on ecosystem functioning that is independent from annual amount of precipitation.