The determinants and significance of stomatal responses to vapor pressure deficit in native Hawaiian wet and dry forest species

Background/Question/Methods Vapor pressure deficit (VPD) responses may be a fundamental determinant of species distribution and tolerance to drought. By hypothesis, stomatal responses of more drought tolerant species should have more responsive stomata to dry air. Using a gas exchange system we measured stomatal conductance at two different VPDs for 21 native Hawaiian wet and dry forest species grown in a common garden, and calculated indices of VPD response. We tested whether species with high maximum rates of gas exchange showed stronger VPD responses. We also aimed to test whether species with higher stomatal density and/or with smaller stomata showed stronger responses to vapor pressure deficit.

Results/Conclusions We found a strong variation of VPD responses across species from wet and dry forests, and that dry forest species were significantly more responsive to VPD than wet forest species. We explore hypotheses for the determinants of stomatal vapor pressure deficit responses, their relationship to leaf anatomy, and their role in integrated leaf function and ecology. These data may also be useful for conservation and restoration projects given the need for ecophysiological information for plants of threatened dry and wet forests in Hawaii.