Impacts of the invasion of alien strawberry guava trees on the transpiration characteristics of a montane Hawaiian forest

Background/Question/Methods Many areas of Hawaiian montane forests are invaded by non-native trees. Due to the differences in leaf-level ecophysiological traits, which are responsible for transpiration rates and responses to the environmental variables, invasion is thought to alter hydrological processes. Among the invasive species, strawberry guava (Psidium cattleianum) exhibits extensive invasion into the forests composed of the endemic tree species, ohia (Metrosideros polymorpha). To understand the impacts of the invasion on the transpiration of the montane ecosystem, effects of leaf wetness on transpiration should be taken into consideration because leaves are often wetted by rain and fog. Further, previous studies showed the response of stomatal conductance (gsw) to leaf wetness is variable among species. In this study we investigated the leaf-level ecophysiological parameters related to gsw and measured gsw under wet surface conditions to examine whether strawberry guava had higher gsw and transpiration rates than ohia in a forest invaded by strawberry guava and a non-invaded forest where both evapotranspiration (ET) and carbon sequestration are monitored using eddy covariance methods. We calculated gsw of wet leaves using gross photosynthetic rates and electron transport rates, which were independently measured using chlorophyll fluorescence.

Results/Conclusions Despite their different ecological behaviors, ohia had photosynthetic traits similar to those of strawberry guava (photosynthetic capacity and stomatal sensitivity to the environmental variables). Measured gsw was reduced to lower levels under wet surface than under dry conditions both in ohia and guava.

Results suggested that strawberry guava has leaf-level ecophysiological traits identical to ohia, and that transpiration rates of the forests dominated by each species have transpiration rates and responses in transpiration to the environmental variables similar to the other. This suggestion is, however, different from the observed results by eddy covariance methods, which showed similar ET between sites despite the lower available solar energy in the invaded forest. The reason for the discrepancy between the leaf-level findings and the results from eddy covariance will be discussed in the presentation.