Coastal forests in the lower Florida Keys inhabit a unique position on low-elevation limestone islands as glycophyte-dominated assemblages located within close proximity to saline water. Over the past sixty years, increases in groundwater salinity and subsequent shifts in species composition have been attributed to the 23 cm/century of sea level rise (SLR) recorded at Key West . Yet these forests are also periodically inundated with saline water from hurricanes, e.g., Hurricanes Georges (1998) and Wilma (2005), leading to extensive forest damage and mortality. How the press disturbance of SLR interacts with the pulse disturbance of hurricane storm surge to effect changes in community composition is not clear. Here, we attempt to identify a model of press-pulse disturbance to explain the decline of pine rockland forests in the Florida Keys. We addressed this question by using a combination of remotely-sensed data, groundwater monitoring, and measurement of plant physiological response. Vegetation indices were calculated from the Landsat TM time series (1986-2011). Stable isotopic analysis of plant water source using d18O, and plant stress, indicated by foliar d13C, were conducted for south Florida slash pine, Pinus elliottii var. densa, and buttonwood, Conocarpus erectus,located at sites across a salinity gradient.
Results/Conclusions
While vegetation indices decreased after Hurricane Wilma at all locations, decreases in vegetation index values prior to this hurricane were observed only at low elevations having groundwater salinities greater than 3 ppt. Additionally, while high elevation pine rockland locations returned to pre-pulse disturbance index values within several years of the hurricane, indices at low elevations did not recover to pre-disturbance levels. Both tree species accessed a high proportion of groundwater compared to water available in the unsaturated vadose zone during the dry season sampling events. In both species, plant stress, as indicated by d13C, increased throughout the extended drought that occurred over the study period. For buttonwood, foliar d13C was highly correlated with increasing groundwater salinity, while in slash pine foliar d13C enrichment was greater at sites having maximum annual groundwater salinities greater than 3 ppt. The reliance on groundwater as the primary source of freshwater has implications for the mechanism of community change. The effects from hurricane storm surge had more severe consequences for trees in suboptimal habitat as defined by higher groundwater salinities, leading to increased mortality among already stressed individuals while also altering the groundwater resource on which the species rely.