Leaf wax isotopic response to seasonal changes in climate in the Sonoran Desert

The North American Monsoon (NAM) supplies 50-80% of total annual rainfall in the Sonoran Desert and is of critical importance to the health and survival of native plant species. The precipitation delivered by late summer NAM convective storms is necessary for growth and reproduction. However, little is known about the species-specific responses to monsoon storms, or to inter-annual variability in precipitation. Given that the Sonoran desert is predicted to face severe water limitation as a result of anthropogenic climate change, we investigate plant physiological responses to changes in the seasonal timing and amount of precipitation, focusing on the onset of the monsoon. We use isotopic tracers recorded in leaf-wax biomarker molecules (n-alkanes and alkanoic acids): d²H signals in plant tissue track the isotopic value of precipitation; and d¹³C represents a plant's water use efficiency. We sampled species of three diverse plant functional types across the Sonoran Desert landscape. New growth was sampled on a bi-weekly basis from native plants between May, 2016 and June, 2017. We extracted leaf-wax biomarkers from each plant sample's total lipid extract, and measured d²H and d¹³C using a GC-IRMS system, correlating these results with changes in local weather conditions. Results indicate differences in water use efficiency between plant functional types that is related to individual storm frequency, which has implications for adaptation of different species to increased drought stress in the future.