Seasonal timing of rainfall influences hydraulic and leaf trait evolution in evergreen shrubs

The consequence of annual soil water deficit has been studied extensively in chaparral vegetation in mediterranean-type climates, characterized by hot, dry summers and cool, wet winters.  Comparisons with communities in semi-arid but non-mediterranean climates provide an opportunity to examine how timing of rainfall may influence plant water use and consequently carbon gain. In central Mexico, evergreen shrub communities are morphologically and taxonomically similar to California chaparral, yet climatically distinct: a winter drought is followed by summer monsoonal rains.  Sites in Santa Barbara, California, and Tehuacan, Mexico, were matched for mean annual temperature, precipitation, and duration of drought.  Six congeneric pairs, representing dominant members of the community in each site, were chosen for study.  Two suites of traits were measured, one related to hydraulic function (e.g. conductivity, vulnerability to embolism, wood anatomy), and the other to the leaf economic spectrum (photosynthetic rate, nitrogen concentration, specific leaf area, and carbon isotope composition). We tested for repeated directional divergences in response to the contrasting precipitation patterns as well as for evolutionary correlations within the suites of leaf and hydraulic traits.  Leaf specific conductivity was generally higher in Santa Barbara, consistent with an adaptive response of higher leaf water supply in response to increased evaporative demand during the period of soil water deficit.  In contrast, divergences in leaf traits revealed genera had less consistent evolutionary trends across the climatic contrast.  Evolutionary divergences in minimum seasonal water potential and vulnerability to embolism were correlated, suggesting the trait association arises from adaptive trade-offs.  The varied evolutionary trajectories suggest rainfall seasonality may have distinct selective effects on hydraulic versus leaf traits.