Effects of interannual precipitation variability on plant-species diversity in the Chihuahuan Desert

Background/Question/Methods

Climate change will result not only in changes in mean precipitation but also in its variability. For example, the IPCC-A1B scenario predicts decreased precipitation by 5-10% and increased interannual variability for the Southwestern US. 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. Arid and semiarid ecosystems are made up of a combination of drought tolerant and drought avoidant species. The former group usually has slow relative growth rate and shows small interannual variability in their abundance. In contrast, drought avoidant species are usually fast growing species and show high interannual variability in abundance. We hypothesize that increased inter-annual precipitation variability increases the evenness component of plant diversity as a result of the increasing importance of drought avoidant species. 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. 

Results/Conclusions

Our results indicated that increased interannual precipitation variability increased plant diversity calculated as the Simpson’s index (1/D) (F _{4, 47} = 14.76, P = 0.001). The two components of plant diversity, evenness (1/1/D) and richness (S), showed different responses; evenness increased with precipitation variability (F _{4, 47} = 14.95, P < 0.001) while species richness showed no response (F _{4, 47} = 0.88, P = 0.64). Observed responses in our experiment are associated with changes in abundance of drought-avoidant species. Although high precipitation variability increased evenness, we observed a decrease in aboveground net primary production in response to increased interannual precipitation variability. This result may be explained by the fact that drought avoidant species account for a small fraction of primary production. Drought avoidance traits are often associated to a reduction in biomass that may prevent an increase in productivity.