Rainfall partitioning and cloud water interception in native forest and invaded forest in Hawai'i Volcanoes National Park

In many Hawaiian forests, including cloud forests, native plant communities are being displaced by invasive tree species, possibly affecting rainfall partitioning and direct harvesting of cloud droplets by vegetation. In this study, the hydrological impacts of invasive species are examined, using measurements of rainfall (RF), throughfall (TF) and stemflow (SF), and estimation of wet-canopy evaporation and cloud water interception (CWI) by the canopy water balance approach in both native Metrosideros polymorpha-dominated and invaded, Psidium cattleianum-dominated forests within Hawai'i Volcanoes National Park (HAVO). Canopy water storage capacity was found to be more than twice as great at the native site (1.86 mm) compared to the invaded site (0.85 mm). Annual RF, CWI, TF and SF were 3233, 1188, 3700 and 261 mm, respectively, for the native site; and 3735, 734, 3033 and 1091 mm, respectively, for the invaded site. Net RF (TF + SF) was 123 and 110% of RF, respectively, at the two sites. Annual evaporation of water from the wet canopy was also greater at the native site (464 mm) than at the invaded site (347 mm). Low canopy water storage capacity and the exceptionally high SF total at the invaded site are related to morphological characteristics and high stem density of the invasive P. cattleianum tree, which favour efficient transport of intercepted water to the ground via the stems. Despite its more peripheral location near the edge of the orographic cloud, CWI at the native site was higher. The characteristics of the native M. polymorpha tree may facilitate more effective harvesting of cloud water droplets, enhancing CWI at the site. Species invasion results in a lower proportion of RF reaching the forest floor (110 vs 123%) and becoming available for groundwater recharge, suggesting that invasion by P. cattleianum may have significant negative effects on Hawai'i's aquatic ecosystems and water resources. Copyright (C) 2010 John Wiley & Sons, Ltd.