Coordination between leaf and xylem function can define the hydrologic niches that plants occupy and influence ecosystem function. Variation in hydraulic traits is often assumed to be adaptive, but much of the evidence for this assumption has been derived from correlations between physiological variation and environmental resource gradients. Though valuable, this approach does not directly identify the causes or targets of natural selection.
Results/Conclusions

Using comparative phylogenetic methods, I examine whether plant hydraulic traits evolved in response to variation in moisture availability. My results indicate that increased hydraulic capacity evolved in combination with leaf shedding as a mechanism to escape aridity. Using phenotypic selection analysis, I examine whether increased leaf hydraulic conductance and photosynthesis are targets of natural selection in a drought escaping Mediterranean annual. I found that there is strong selection for increased photosynthesis at the time of flowering, but no direct selection on leaf hydraulic conductivity. Instead, the evolution of increased leaf hydraulic conductivity occurred because of genetic covariance between photosynthesis and leaf hydraulics. These results suggest that the inter-specific correlation between hydraulic function and moisture availability could have arisen via natural selection on a correlated leaf trait, photosynthesis, rather than through direct selection on hydraulic traits.