The Effect of Disturbance and Freshwater Availability on Lower Florida Keys’ Coastal Forest Dynamics

Abstract

th century has been attributed to a combination of sea level rise and hurricane storm surge impacts, but the interactions between these two disturbances leading to forest decline are not well understood. The goal of my research was to assess their effects over a period spanning more than two decades, and to examine the relationships between these press and pulse disturbances and freshwater availability in pine rockland, hardwood hammock, and supratidal scrub communities. Impacts and recovery from two storm surges, Hurricanes Georges (1998) and Wilma (2005), were assessed with satellite-derived vegetation indices and multiple change detection techniques. Impacts were greater at lower elevations, and in hardwood hammock, spectral signatures indicative of plant stress and productivity returned to predisturbance levels within a few years. In pine rockland, impacts were predominately related to Hurricane Wilma, however, a similar return to pre-disturbance conditions was absent, suggesting that trajectories of disturbance recovery differed between the two communities. Long-term monitoring of forest composition, structure, and groundwater salinity showed that compositional shifts in the low shrub stratum were associated with ix salinization of the freshwater resource attributable to sea level rise. Throughout the course of twelve months of climate and groundwater monitoring (2011)(2012), groundwater salinity generally decreased in response to large precipitation events.Modeling of geophysical data indicated that groundwater salinity was an important predictor of community type. Isotopic analysis of δ 18 O in plant stem water and foliar δ 13 C was used to determine temporal and spatial patterns in water use and plant stress in two community dominants, slash pine, Pinus elliottii var. densa, and buttonwood, Conocarpus erectus. Both species relied heavily on groundwater, and plant stress was related to increasing groundwater salinity. The results of this work suggest that the interaction of press and pulse disturbances drive changes in community composition by causing mortality of salt-sensitive species and altering the freshwater resource.x forests that require freshwater inhabit a unique position on low-elevation limestone islands as glycophyte-dominated forests located in close proximity to saline water.Coastal forests in the greater Caribbean basin are also subject to periodic disturbance from hurricanes and tropical storms, but which typically are not stand-replacing events (Brokaw and Walker, 1991;Yih et al., 1991;Bellingham et al., 1992). Collectively, these forests are high in species diversity , 2006). These conditions would favor a combination of disturbance, 2 drought, and salinity tolerant species, as groundwater salinity increases (Guha and Panday, 2012) and coastal forests are exposed more frequently to major storms, with shrinking recovery periods between such events (Lugo, 2000). Additionally, the interaction of press and pulse events will increase the likelihood of surface inundation by salt w...

Evidence for trait‐based community assembly patterns in hardwood hammock forests

“…Trees are more susceptible to occasional salt water brought by storm, tide, and wave activities in coastal environments, and salt deposition in soil may occur in hurricane-prone areas such as the Florida Keys. The effect of salt stress is even more pronounced where water availability is limited such as in the Florida Keys forests (Ogurcak 2016, Subedi et al 2018. Therefore, the ability of trees to survive under salinity exposure, drought, and hurricanes clearly contributes to environmental filtering in the Florida Keys.…”

“…We acquired ground elevation, soil organic matter, soil nitrogen, soil phosphorus, soil pH, and groundwater salinity data from our earlier studies (Ross et al. , Ross and Sah , Ogurcak ), as our permanent plots were established adjacent to transects for earlier studies. Unfortunately, for a few sites (three in the Florida Keys and two in Long Pine Key), similar environmental data were not available.…”

“…Among these, soil depth was measured at 20 equidistant locations along a 20-m transect laid out through the middle of the permanent plot by probing to bedrock with a metal rod for all the plots. We acquired ground elevation, soil organic matter, soil nitrogen, soil phosphorus, soil pH, and groundwater salinity data from our earlier studies (Ross et al 2003, Ross and Sah 2011, Ogurcak 2016, as our permanent plots were established adjacent to transects for earlier studies. Unfortunately, for a few sites (three in the Florida Keys and two in Long Pine Key), similar environmental data were not available.…”

Variation in Stomatal Characteristics of <i>Bursera simaruba</i> (L.) Sarg., a Dominant Tree Species of Tropical Hardwood Hammock Forest across a Habitat Gradient in the Florida Keys

“…Moreover, stomatal traits coupled with other physiological stresses will affect the discrimination against 13 C uptake during photosynthesis. The carbon fixed in the leaves of physiologically stressed plants including those suffering from salinity stress have been shown to be enriched in the heavier isotope 13 C compared with the lighter isotope 12 C [10]. When plants are stressed, the rate of photosynthetic CO 2 assimilation decreases due to reduced stomatal conductance and consequent restriction in the availability of CO 2 for carboxylation.…”

“…When plants are stressed, the rate of photosynthetic CO 2 assimilation decreases due to reduced stomatal conductance and consequent restriction in the availability of CO 2 for carboxylation. This will cause a lower discrimination against 13 C during the biochemical fixation of CO 2 in the Calvin cycle [11]. Therefore, 13 C/ 12 C ratios (expressed as δ 13 C, after standardization) may track the trajectories of stomatal density and size as well as other factors in response to hydrologic conditions.…”