Are island plants whimpy? Evidence for low stress tolerance in chaparral shrubs on Santa Catalina Island, California

Background/Question/Methods

It has been argued that exposure to high climatic uniformity (i.e. low seasonal variation in temperature) results in organisms with narrow physiological stress tolerance compared to organisms from habitats with more variable and extreme climates.  This hypothesis is generally supported by comparisons of organisms in tropical vs. temperate and low vs. high altitude environments.  Furthermore, organisms exhibiting low stress tolerance may have increased sensitivity to climate change.  Island vs. mainland systems offer an additional, understudied system to test this hypothesis primarily for two reasons: (1) the uniform climates of islands are contrasted by the more variable and extreme climates of adjacent mainland areas, and (2) islands are often inhabited by close relatives of mainland taxa that have colonized the island in the recent past.  We hypothesize that if island plants are undergoing strong selection due to local conditions, they will exhibit low stress tolerance and increased sensitivity to climate change compared to mainland relatives.  To test this hypothesis, we measured drought-related ecophysiological traits in 10 congeneric pairs of chaparral shrubs from matched sites on Santa Catalina Island, California and the adjacent southern California mainland.  This approach has allowed us to address two key questions related to our hypothesis: (1) are island environments less stressful as a consequence of low seasonal variation in temperature, and (2) do island plants exhibit low stress tolerances compared to mainland relatives?    

Results/Conclusions

Our results show that (1) island plants experience less severe seasonal drought compared to mainland relatives, and (2) island plants possess traits that are indicative of low drought tolerance.  Measurements of pre-dawn water potential (Y_pd) and leaf-to-air vapor pressure deficit (VPD_L) are significantly different between island and mainland sites when averaged across all species (P < 0.001).  In addition, island plants exhibit high specific leaf area (SLA), less negative minimum water potential (Y_min), larger leaf size, less negative turgor loss points (TLP), and high bulk tissue elastic modulus near saturation (ε_max) compared to mainland congeners.  These findings support the hypothesis that low seasonal variation in temperature is associated with low physiological stress tolerance.  Future plans involve quantifying freezing tolerance within the same system, and using common garden experiments to test the genetic component of the observed variation in stress related traits.