Hurricanes, fire, and coastal pine forests in a rising sea: The Florida Keys

Slash pine (Pinus elliottii var densa) forests in the Florida Keys are restricted to relatively well-drained sites on several lower Keys islands, where the presence of fresh ground water enables an open pine rockland assemblage to thrive close to the coast.  Fire also plays a critical role in the maintenance of the pine rockland community, by controlling the expansion of shrubs, thereby maintaining the thin soils and open light conditions that favor pine regeneration and the persistence of a rich herb flora.  In recent years, sea level rise has emerged as a threat to the continued existence of Florida Keys pine forests, by elevating both ground water level --- thereby reducing fire frequency and intensity --- and ground water salinity --- thereby favoring halophytes over fresh water species. Over the last century, rising seas have been implicated in the retreat of pine forest toward the center of Sugarloaf Key, where 2/3 or more of the original area in pine forest is now dominated by either hardwood forests or halophytic communities. Based on recent observations, we suspect that this transformation in community type may be event-driven, in association with periodic hurricanes that flood the islands with salt water that drains or dissipates slowly from low-lying areas.  Such an event occurred in November 2005, when Hurricane Wilma (November 2005) flooded the extensive pine forests on Big Pine Key. Mortality of pine trees varied from more than 90% to less than 20%, and was concentrated in stands of slightly lower elevation, where storm surge waters from adjacent Florida Bay were retained longest.  In the near term, reestablishment of pines in these dieoff areas may depend on carefully planned prescribed fire with the intent of preparing a mineral seedbed.  In the longer term, the prospects for lowlying pine forests on oceanic islands are not rosy.