Impact of fire and invasive species on aboveground carbon pools along a precipitation gradient in Hawaiian tropical forests

Little is known about the evolutionary history of fire in shaping the structure and function of tropical rainforests. In addition, many tropical forests are now heavily impacted by nonnative species which can alter successional trajectories and disturbance regimes, and disrupt ecosystem services and processes. We examined the synergistic impacts of lava-ignited wildfire and nonnative species invasion on aboveground carbon pools in vegetation and detritus along a precipitation gradient (1630-2380 mm) in Hawaii Volcanoes National Park. Aboveground carbon pools were quantified in replicated, paired burned and unburned sites (n=5) in each of five communities. The two lowest precipitation sites were dominated by a native shrub (Dodonaea viscosa) with nonnative grass or nonnative fern understories. The remaining sites had intact native forest canopies (Metrosideros polymorpha), with understories dominated by a nonnative fern, a native fern, or a native tree fern. Live and detrital biomass were quantified with clip plots, dasometric measurements, allometric equations, and fuels transects. Across all communities, total aboveground biomass ranged from 931 to 38,203 g m^-2. Fire reduced total aboveground carbon storage by an average of 35%. In unburned sites, carbon storage was equally partitioned between live biomass and detritus. In burned sites, detritus accounted for >80% of total aboveground carbon pools. The lowest precipitation areas were the most severely impacted by fire and nonnatives, and these communities appear to have entered a nonnative species-fire cycle that increases fuel loads and fire frequency, precluding establishment of all but the most disturbance-adapted natives. Loss of forest cover has reduced carbon sequestration in these areas by >90%. Changes in carbon sequestration as a result of wildfire and nonnative species interactions are particularly important in light of the ubiquitous presence of invasive species and the need for better understanding of the role they will play in disturbance regimes and global C cycling.