Hawaiian ecohydrology: Water use of native and invasive trees in a lowland tropical rainforest

Background/Question/Methods While the supply of clean water in Hawaii is expected to decline in the coming decades, invasive plant species are greatly extending their ranges across the islands. To quantify the ecohydrological impacts of invasive species, we compared the water use of co-occurring native and invasive tree species in a heavily invaded forest on the Big Island of Hawaii. We also compared native tree water use in the presence and absence of invasive species. This study was based within the framework of an ongoing project investigating the impacts of invasive species on a lowland tropical rainforest ecosystem with respect to water, light, nutrients, and regeneration, where all invasive species were removed from four 10 x 10m plots, and four additional 10 x 10m plots were established as controls. We measured soil moisture and tree water use in two of the established removal plots and two of the control plots. Twenty native Metrosideros polymorpha and ten trees each of three invasive species: Macaranga mappa, Melastoma septemnervium, and Cecropia obtusifolia were instrumented with heat dissipation sap flow probes from February to December 2008. Twelve of the fifty trees were instrumented with additional probes to investigate the variation in sap flow with sapwood depth. 

Results/Conclusions Sap flow varied with sapwood depth for all tree species, and this information was subsequently used to model sap flow rates for the rest of the trees which had probes installed at a single depth. In the control plots, where both natives and invasives were present, preliminary data show that native M. polymorpha had the lowest sap flow rates per unit sapwood overall. Conversely, M. polymorpha had the highest sap flow rates per tree, likely because the native trees tended to be larger in diameter than the invasive trees. Stand level water use declined after invasive species were removed, which was confirmed by a substantial increase in soil moisture in the removal plots. There was, however, no discernible difference in native M. polymorpha sap flow in the presence vs. absence of invasive species, possibly because water is not a limiting resource for this native species, even during periods of drought. Alternatively, M. polymorpha species may be too slow growing and conservative in its resource use to respond in a measurable way to increases in water availability.