Water use of native and invasive trees in a lowland tropical rainforest in Hawaii

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 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 for eleven months in 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 decreased steeply with sapwood depth for all tree species, and these trends were 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, native M. polymorpha had the lowest sap flow rates per unit sapwood, but the highest sap flow rates per whole tree, likely because the native trees tended to be larger in diameter than the invasive trees. Stand-level water use declined by half after invasive species were removed, and the removal of invasives caused a slight but significant increase in water use by the remaining natives in the removal plots. While M. polymorpha species may be too slow growing and conservative in its overall resource use to respond in a measurable way to increases in water availability, the species nevertheless took up more water when more was available.