| The influence of drought on water relations has been studied extensively in chaparral vegetation of mediterranean-type climates, characterized by hot, dry summers and cool, wet winters. Comparisons with communities in non-mediterranean climates provide an opportunity to examine the consequences of the timing of soil versus atmospheric drought on plant water relations. In central Mexico evergreen shrub communities closely related to California chaparral experience a winter drought followed by a period of warmer temperatures and higher insolation along with summer monsoonal rains. High evaporative demand and soil drought thus coincide in California, and are offset in Mexico. I compared water relations of 13 evergreen shrubs in two sites, Santa Barbara, California, and Tehuacan, Mexico, which were matched for mean annual temperature, precipitation and duration of drought. Within both sites, species that experienced greater soil drought, as determined by minimum water potentials, had lower leaf-area-based whole plant hydraulic conductance (KWP). However, species in California, where atmospheric drought was higher, had significantly higher rates of K WP relative to species in Mexico. Wet season photosynthesis, specific leaf area, and leaf nitrogen did not differ between sites, and pairwise trait relationships were consistent with those found globally. KWP and maximum assimilation were poorly correlated, suggesting photosynthetic and hydraulic strategies maybe independent in semi-arid communities. The shift observed in whole plant hydraulic conductance demonstrates how soil and atmospheric drought may have opposing influences on hydraulic architecture.; Six congeneric pairs, which represent dominant members of the community in both sites, were chosen as the foci for a subsequent study. Using these taxa, I tested for correlated evolutionary divergences in water relations traits across these climate types. I examined detailed aspects of hydraulic architecture including sapwood and leaf specific conductivity and xylem vulnerability to embolism. 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. Evolutionary divergences in minimum seasonal water potential and vulnerability to embolism were correlated, with no consistent site association. This study demonstrates that seasonality of precipitation can have important effects on the evolution of hydraulic architecture. |
