Invasive non-native trees in Hawaiian forests could increase negative impacts of climate change on water resources

Abstract:

Invasion by non-native tree species affects forests globally, threatening native biodiversity and altering biogeochemical processes. In Hawai‘i, where non-native trees have invaded widely, loss of biodiversity and hydrological impacts are major concerns. Improving understanding of the effects of non-native tree invasion on the functioning of Hawaiian forests is important for resource managers, especially under a changing climate. But, the effects of species invasion on water cycling are not well known. We present the results for evapotranspiration (ET) based on a decade of eddy covariance measurements at a native forest site and a forest site invaded by Psidium cattleianum, the most prevalent invasive tree species in Hawai‘i, While mean ET was 4% higher at the native site, when normalized by net radiation, the native site was 7% lower than the invaded site. Sensitivity to available energy was greater for the invaded site and, hence, monthly ET of the invaded site exceeds that of the native site when net radiation is greater than about 120 W m^-2. This threshold is slightly lower than the mean net radiation at the invaded site and much lower than mean net radiation in drier areas. Recent historical climate trends and projections of future change suggest drier, sunnier conditions in some areas of Hawai‘i. Significant areas of forest are likely to experience greater reductions in streamflow generation and groundwater recharge due to the effects of invasive trees under a future sunnier climate.