Wimpy leaves and sturdy stems: Decoupling of drought tolerance at the stem and leaf levels in island chaparral
We do not yet have good models for predicting the vulnerability of island plants to climate change. One reason for this is that we do not understand how the physiological tolerances of island plants differ from mainland relatives. Plants exposed to highly uniform climates (i.e. tropical and/or maritime environments) may evolve narrow physiological stress tolerance compared to organisms exposed to climates that are more variable and extreme. This hypothesis is generally supported by comparisons of organisms in tropical vs. temperate and low vs. high altitude environments. Island vs. mainland systems offer an additional, understudied system to test this hypothesis. Recent work examining the water relations of shrubs in chaparral communities on Santa Catalina Island and the adjacent southern California mainland suggests that island plants are buffered from the extremities of the seasonal drought experienced by mainland relatives. Has this exposure to a less stressful environment selected for reduced tolerance of drought and high temperatures in island plants? To address this question we measured a suite of drought-related functional traits including: minimum seasonal water potential (Ymin), specific leaf area (SLA), leaf size, turgor loss point (TLP), wood density, stem-specific hydraulic conductivity (Ks), and cavitation resistance (P50) on 10 congeneric species pairs from Santa Catalina Island and the adjacent southern California mainland.
Results suggest that island plants exhibit a more mesomorphic and less drought tolerant strategy than mainland relatives at the leaf level - i.e. island leaves have higher SLA (P < 0.05), larger size (P < 0.05), and higher TLP (P < 0.05). However, Ks and P50 are not different between mainland and island plants. These findings suggest that while the leaves of island plants exhibit higher mesomorphy and lower drought tolerance, their stems are as tolerant of drought as mainland relatives exposed to more arid conditions. These results also suggest that the stems of island plants are 'overbuilt' for their current environment and may reflect phylogenetic conservatism in lineages that have recently diverged from mainland ancestors. Further work is needed to understand how this decoupling of drought tolerance at the stem and leaf levels is likely to affect the sensitivity of island plants to climate change.