Physiological Ecology of Native and Alien Dry Forest Shrubs in Hawaii

Hawaii's dry forests are among the most endangered of all ecosystems in the archipelago. Invasion of alien plant species into these ecosystems is one of the most significant threats to on-going efforts to preserve and restore Hawaii's remaining dry forests. Comparing the physiological performance of alien and native species can offer causal explanations behind the relative success of alien plant invasions within Hawaiian dry forests and elsewhere. We compared maximum rates of net CO₂ assimilation, water-use efficiency (WUE), daily carbon gain, and leaf morphology for three native and two alien shrubby species growing within 1-m² plots under two natural light (sub-canopy shade relative to open full sunlight) treatments. Maximum rates of net CO₂ assimilation were similar between alien and native species (8.15 vs. 7.12molm^–2s^–1, respectively), however, native plants exhibited lower stomatal conductance and higher instantaneous WUE than alien plants in all treatments (0.13molm^–2s^–1 and 72.36mol CO₂mol H₂O^–1 against 0.23 and 58.78, respectively). Alien plants had approximately 65% more aboveground biomass than native plants. This result may reflect differential seed production and seed bank viability between native and alien species. We found an overall strong, positive correlation between species-specific physiological traits and final species aboveground biomass. Based on this type of information we can predict species-specific boundaries across light gradients, and focus restoration efforts accordingly.