Will Climate Change Alter Wildfire Behavior and Effects in Seasonally-Dry Wetlands?

The effects of edge on ecosystems is well documented on animal and plant species, as well as a number of ecosystem attributes. A substantial determinant of ecological edge effects is the effect of edge on microclimate parameters such as temperature and humidity. These effects have been described in detail in upland communities, but not in wetland forests. Of particular interest is whether hydrologic influence trumps edge effect; in other words, does the influence of inundation create a hydrologic “switch” that overwhelms edge effect? In a landscape with numerous wetland forest patches subject to seasonal hydrologic fluctuations, I measured microclimate in relation to edge distance during drought and inundation to determine the relative influence of edge effect on microclimate under contrasting hydrologic conditions. I also tested whether the occurrence of wildfire in the study area two years prior had lasting influence on edge-related microclimate. Predictions of edge effect on microclimate were observed in both wet and dry conditions, and in patches that were burned as well as unburned patches. While evidence did not indicate the presence of a hydrologic “switch” controlling the influence of edge, a stronger edge effect occurred during dry conditions than during inundation. Also, while sites experienced relatively little difference in edge influence on microclimate from fire after two years of recovery, the influence of edge effect under dry conditions compared to inundation was greatly magnified in burned patches compared to unburned patches. These findings suggest that under scenarios of warmer, drier conditions, the edge effects seen in this study could be increasingly important in providing refugia for humidity-sensitive plant species. Additionally, the gradient of sizes in which these measurements were taken may allow variably-sized cypress domes to be used as proxies for varying scenarios of future altered climate in seasonallyinundated wetlands. Background and purpose Edge effects in ecology are well accepted, and have been demonstrated across a variety of ecological systems and attributes (e.g., Harris 1988, Saunders et al. 1991). Among the numerous examples from forests ranging from the boreal (Schmiegelow and Mönkkönen 2002, Hamberg et al. 2009) to the neotropics (Laurance 1991, Laurance et al. 2000), the changing conditions from the edge of a patch toward its interior have been shown to affect animal populations (Paton 1994, Fletcher 2005) as well as plant populations (Gehrig-Downie et al. 2011, Obregon et al. 2011). While these relationships are explained by a number of factors that vary with the distance from the edge of a community or ecosystem edge (e.g., physical structure, and area of available habitat for animals; Margules and Pressey 2000), many components of edge effects are explained by changing microclimatic conditions. Microclimate, or atmospheric conditions at a particular point on the landscape near the ground (Geiger 1965), influence a range of ecological processes (Meentemeyer 1978, Waring and Running 1998, McDonald and Urban 2005, Turner and Chapin 2005). Often, microclimate differences are studied between areas of contrasting vegetation communities, such as between forested patches and areas that have been cleared by logging or for road building (Asbjornsen et al. 2004). In the case of the former, clear changes have been observed in microclimate with increasing distance away from the edge of the interface of opening and forest, proceeding perpendicular away from the open area (Chen et al. 1993, Chen et al. 1995). In general, at increased distances from an opening toward the interior of a forest patch, values and variances of temperature, humidity, and wind speed are attenuated (Vanwalleghem and Meentemeyer 2009). The reasons for these changes include decreased wind penetrance into remnant forest, increased shading of soil and low vegetation strata, and decreased lateral diffusion of moisture to the atmosphere exterior to the forest patch. In the same way that fragmentation results in changes to forest structure that affect microclimate attributes of resulting patches, other disturbance events that alter vegetation structure within a forest patch may exert control over edge effects on microclimate. Fire consumes standing live and dead biomass; and, by removing elements of forest structure and reducing forest canopy area (e.g., Cochrane et al. 1999), can increase insolation and air movement. Villegas et al. (2010) have shown seasonal effects on microclimate as conditions of canopy cover and evapotranspiration change substantially depending on leaf development and senescence. In addition to directly influencing microclimate, fire occurring in a forest patch may exert similar control over the influence of edge effect by reducing or eliminating those structural elements which attenuate microclimate with increasing distance from a patch edge. Despite progress in understanding effects of edge distance (Chen et al. 1999, Vanwalleghem and Meentemeyer 2009) on microclimate, studies heretofore have been limited to terrestrial forests. Few studies have examined microclimate in wetland forests, and none have been published concerning edge effect and microclimate in these communities. Thermal mass of the water inundating a wetland or saturating its soil would influence temperature (and certainly humidity), perhaps more so than the effects of canopy shading or attenuation of air mixing. Under these circumstances, hydrologic effects on microclimate could overwhelm the edge effects on microclimate that otherwise would be predicted under non-hydric conditions: in other words, edge effects on microclimate could be governed by a hydrologic “switch. ” In wetlands subject to strong seasonal variations in hydrologic regimes, is it possible that the occurrence of fires during droughts exerts control over edge effects on microclimate? Might seasonal inundation likewise create hydrologic control over the occurrence of edge effects on microclimate? I sought to investigate whether either wildfire or inundation govern the occurrence of edge effects on microclimate in seasonally-inundated forest patches. Hypothesized hydrologic control on edge-mediated microclimate variation leads to the following predictions: 1. Under non-inundated (e.g., drought) conditions, microclimate variation in wetland forest patches adjacent to nonforest communities would exhibit similar trends in response to patch and edge effect as those observed in terrestrial forests; i.e., attenuated temperatures and higher relative humidity with increasing distance from edges toward centers of patches. 2. Inundation degrades these relationships, resulting in a partial or complete loss of relationship between distance from edge (DFE) and microclimate. 3. Wildfire sufficient to cause substantial changes to vegetation structure results in losses of edge influence, resulting in higher temperatures and lower humidity among domes experiencing fire compared to unburned domes. 1 For purposes of this discussion, “patch” should not be assumed to indicate a small fragment of forest or forest remnant, but rather any spatially-discrete unit of continuous forest regardless of size. Hydrologic government of edge effect, if extant, could improve our understanding of the implications of severe droughts or future altered climate on wetland forest processes, particularly where issues of fragmentation or small patch size are concerned. Knowledge of fire impacts to microclimate and edge influence, meanwhile, may allow us to better forecast potential effects of future fires following wildfire events. Study Description and Location Big Cypress National Preserve in southwestern Florida USA (26° N, 81° W, Figure 1a) covers 300,000 ha of low-relief landscape. There, upland forests of south Florida slash pine (Pinus elliottii var. densa [Little and Dorman]) and prairies primarily composed of muhlygrass (Muhlenbergia capillaris [Lam.]) occur adjacent to swamps dominated by pondcypress (Taxodium distichum var. imbricarium [Nutt.]), a deciduous conifer characteristic of longhydroperiod wetland forests across large portions of the southeastern United States. Small swamp patches, called domes because of their structure, occur frequently across large areas of the preserve (Figure 1b), providing the opportunity to investigate wetland forest processes in a landscape with generous replication of patches at a range of sizes. Mean annual precipitation of 1360 mm y occurs primarily in the wet-season months of June – September (Duever et al. 1986). While the entire landscape experiences periodic inundation during periods of intense precipitation, standing water is generally confined during and shortly after the wet season to cypress swamps where soil elevations in are lower than those in the surrounding landscape by 1 m or more. Water infiltrates into the region’s karst limestone readily after rains cease, and the shallow depressions in which cypress domes occur may become dry for 3 – 6 months during the late dry season (Ewel 1995). As the first thunderstorms of the rainy season begin (usually in May), frequent lightning strikes occur on a landscape with plentiful available fuel. Naturally-ignited wildfires occur frequently during this seasonal transition, with the pyrogenic vegetation of flatwoods and prairie communities readily carrying fire to the edges of swamps and domes. Dry conditions can permit sufficient fuel continuity to allow fire to enter these wetland forests, where understory vegetation such as sawgrass (Cladium jamaicense [Crantz]) may burn readily. During drought conditions, large ground fuels, accumulated duff, or even the peat soils themselves may ignite, resulting in severe fires. In May 2009, during a severe regional drought, the Deep Fire was ignited by lighting and burned 12,000 ha in the northwestern corner of the Preserve. The fire burned through all vegetatio