Decadal scale vegetation response of metrosideros polymorpha-dominated communities to wildfire on Hawai'i Island

Within the last 50 years wildfire has become an increasingly destructive and anthropogenic driven disturbance in Hawai`i, and is a serious threat to remaining native vegetation communities. To examine vegetation response to wildfire, I surveyed 14 sites on Hawai`i Island that experienced fire between ten and 45 years ago. These sites span a diverse environmental gradient and encompass different vegetation communities, but are all dominated by the native tree Metrosideros polymorpha (`ohi`a). Vegetation at each burned site and a corresponding unburned control site was measured to analyze vegetation response following wildfire. Native vegetation levels were suppressed across all sites regardless of time since fire, environmental conditions, the density of alien species, or vegetation community type. Measures of alien vegetation for cover, species richness, shrub density, tree density and basal area in burned plots were 135-165% of unburned controls, whereas native measures in burned plots were 7-109% of unburned control values, with native basal area experiencing the sharpest reduction. Recoveries were not consistent between study sites, and I postulate that the post-fire trajectories of these sites encompass a gradient between sites that will complete a slow recovery to pre-fire native vegetation composition and structure to sites that have entered an alternate stable state and will never return to their pre-fire vegetation conditions. Site rainfall and the density of alien species explained a small amount of the variation in recoveries between sites, with wetter and less-invaded sites generally recovering better. The best case scenario for these vegetation communities following fire appears to be a slow recovery that may take centuries, while the worst (and more common) outcome is a permanent replacement of the native vegetation community with one dominated by alien species.