Examining the current status of isolated wetlands in southwestern Georgia and the implications of 60 years of land use change

Background/Question/Methods

Remotely-sensed land use data are often effective predictive tools for gauging wetland quality. In this study, we describe the current status of isolated wetlands in southwestern Georgia through an examination of current and historic land use patterns surrounding each wetland. Specifically, we identify land use types dominating a 100 m buffer around each wetland through time, and assess the current condition of the wetlands. We focus on the Dougherty Plain, a 6,690 km² physiographic province containing over 11,600 isolated wetlands. In a subset of this physiographic region, we analyzed patterns of land use change associated with each wetland over six preceding decades. The 59 year span (1948-2007) includes a period of marked intensification of agriculture and silviculture, two important economic drivers in the region.  Land use data from 1998 were most suitable for coverage and analysis at the regional scale. Historical land use data layers were hand-digitized using geo-rectified aerial photography. We analyzed 999 isolated wetlands in a 10 percent sample of the region to identify trends in land use change. To measure relative wetland condition, we calculate the Landscape Development Intensity (LDI) index which evaluates land use based on the amount of external energy required to maintain it.

Results/Conclusions

Analysis of the spatial extent of existing isolated wetlands within the Dougherty Plain revealed the mean area to be 2.04 ha, with nearly half (45%) of all wetlands less than 1 ha in size. We estimated a density of 1.70 wetlands per km². We identified five major land use classes within the 100 m buffers surrounding wetlands in the region: natural forest, herbaceous fields, irrigated agriculture, dryland agriculture, and planted pine. Natural forested lands represent the largest proportion of land in all years.  However, we found a marked increase in the proportion of pine plantations and a simultaneous decrease in natural forest over the past 60 years, though the total area of forested land remained relatively stable. We found a similar pattern of intensification in row crop agriculture; the proportion of dryland agriculture declined precipitously as irrigated agricultural lands became more prevalent, while the total proportion of agricultural lands remained stable. Our assessment of wetland condition suggests that nearly 90% of wetlands are impacted by low to mid levels of development, a useful but conservative estimate. The lower categorical resolution of the LDI index does not differentiate between levels of silvicultural or agricultural intensity, as does our high-resolution historical analysis.