Recent studies have highlighted the diversity of functional strategies by which plants in seasonally-dry habitats cope with drought. Phylogenetic analyses have shown that while resistance to drought-induced xylem cavitation seems to be correlated with low annual rainfall, patterns of correlated evolution with other plant hydraulic traits may be more complex. To determine the degree to which resistance to xylem cavitation correlates with the individual drought conditions experienced by a plant, a survey of the literature was conducted to find studies in which woody plants from Mediterranean climates around the globe had been measured for pre-dawn water potential (Ψpd) during the dry season concurrently with the water potential at which 50% of xylem hydraulic conductivity was lost (Ψ50).
Results/Conclusions
Decreasing Ψpd was found to be significantly related to decreasing Ψ50 across all species. Two phylogenetically independent contrasts of the data yielded different results. The first, using recent molecular phylogenies of all species but assuming equal divergence times on all branches, showed a much less significant relationship between the two traits than when ignoring the effects of phylogenetic relatedness. The second, using a slightly less well resolved tree but including estimates for divergence times failed to show a statistically significant relationship between Ψpd and Ψ50. These results offer weaker evidence for convergent evolution between experienced drought and Ψ50 than intuition may suggest. It is likely that many independently evolving traits such as rooting depth, leaf-area sapwood-area ratios, deciduousness, and even gas exchange patterns coordinate a species' fitness in drought-prone habitats.
