Ecosystem and Restoration Consequences of Invasive Woody Species Removal in Hawaiian Lowland Wet Forest

A removal experiment was used to examine the restoration potential of a lowland wet forest in Hawaii, a remnant forest type that has been heavily invaded by non-native species and in which there is very little native species regeneration. All non-native woody and herbaceous biomass (approximately 45% of basal area) was removed in four 100-m² removal plots; plots were followed for a three-year period. Removal plots had a lower leaf area index, higher air temperatures, higher afternoon soil temperatures, and lower relative humidity than control plots. Removal plots had 40% less litterfall mass and similarly reduced nutrient inputs. Leaf litter decomposition rates were much slower in the removal plots, due more to site quality than litter quality. However, soil N and P were not different between treatments. Native species had a distinct suite of leaf traits (greater integrated water use efficiency, lower mass-based leaf nutrient concentrations, and lower specific leaf area). Despite major environmental changes in the removal plots, native species' diameter growth and litterfall productivity were not significantly greater after removal, testifying to the slow response capabilities of native Hawaiian trees. Our results are consistent with the expectation that native species are conservative in regards to resource use and may not strongly respond to canopy removal, at least at the adult stage. Management strategies will have to incorporate the slow growth rate of Hawaiian species and the fact that weeding may be required to suppress expansion and nutrient inputs of introduced species.