Occurrence, function, and conservation of playa wetlands: The key to biodiversity of the southern great plains
Johnson, Lacrecia A.
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Playas form the primary wetland system in the High Plains portion of the Southern Great Plains (SGP) and provide valuable ecosystem services and functions including being key sites for biodiversity. Current estimates of the number of playas within the SGP (Texas, New Mexico, Oklahoma, southwestern Kansas, southeastern Colorado) from historical soil surveys (pre-1970s), topographic maps, and field checks exceed 25,000. This number often gives the potentially mistaken impression that there are numerous, adequately functioning playas in the region that continue to meet ecological and societal needs. In addition, these historical estimates are used to generate samples of playas for a variety of natural resource survey and research efforts, which depend on the occurrence of functional playas to generate sound inferential results. In January 2001, the U.S. Supreme Court ruled on Solid Waste Agency of Northern Cook County (SWANCC) v. United States Army of Corps of Engineers, which eliminated the provisions of the Clean Water Act available for the protection of playas and other isolated wetlands. The 1985 Food Security Act enacted the Federal “Swampbuster” provision for conservation of wetlands, which has been maintained in subsequent Farm Bills and is currently the only remaining potential incentive for the conservation playas throughout the SGP. For a playa to be determined as a wetland under Swampbuster, there must be a prevalence of hydric soil. Hydric soils were used not only to characterize and define playa locations, but also as a necessary criterion for determination of wetland status. However, since 1994, soils of the SHP associated with depressions, including playas, have been subject to reclassification and subsequent remapping by the United States Department of Agriculture (USDA). Anthropogenic factors and lack of regulatory protection have negatively impacted soil reclassifications, accelerated wetland loss, and impaired delivery of ecosystem services in many playas. Revised estimates of playa number, location, level of function, landscape connectivity, and capability for restoration are needed to effectively implement conservation of playas and species that rely on them. Playas occur in perhaps the most agriculturally impacted landscape in North America, with numerous prominent landscape alterations resulting from changing cultivation and grazing practices since the late 1800s. As a consequence, sediment accumulation and other contaminants are negatively impacting playas and leading to both physical and functional loss. One method to reduce the impacts of altered watershed condition upon adjacent playas is to provide a surrounding buffer of native vegetation. Wetland buffers reduce the adverse impacts of adjacent land uses by preventing or reducing sediments, contaminants, and excess nutrients (e.g., fertilizer or manure) from reaching the wetland. Conservation agencies and organizations have installed a very limited number of playa buffers during the past 15 years, and Farm Bill Programs have served to incidentally (e.g., Conservation Reserve Program; CRP) and intentionally (e.g., CP23a) buffer playas. However, a field-based study is needed to quantitatively evaluate the effectiveness of this technique. Collectively, playa wetlands are the primary sites for biodiversity and the primary wetland habitat for numerous wetland-dependent species that breed, migrate through, and winter in the High Plains. The loss of biodiversity in the Great Plains and elsewhere is driven by habitat loss, degradation, and increasing fragmentation, with future biodiversity also subject to changes as a result of climate change. Combined, these ecological stressors will have a strong influence on habitat and ecosystem connectivity and serious implications for the conservation of future biodiversity. Habitat network analysis is a promising tool for conservation planning and management in fragmented landscapes facing climate change impacts. Using this approach it is possible to model the impacts of landscape change on connectivity and processes supported by connectivity. To estimate playa loss, determine scale of function reduction, and evaluate conservation strategies for playa wetlands, my study objectives were to (1) estimate physical loss and modifications of playas as a function of anthropogenic impacts, (2) utilize these data to estimate a rate of playa loss since 1970 and the number of playas remaining on the landscape in the Southern Great Plains, (3) develop the framework of a functional assessment for Great Plains playa wetlands, (4) present the data collected to characterize the chosen functions and apply the resulting Function Assessment to playa wetlands of Texas, Oklahoma, and New Mexico to assess the level of function for playas within the SGP, (5) evaluate impacts of the potential change in area and number of hydric locations (i.e., historical playas) as a result of the USDA soil reclassification and subsequent remapping of upland and depressional soils in the SHP of Texas, (6) evaluate implications of remapping for natural resource managers and other scientists involved in conservation of playas, (7) address the effectiveness and impact of vegetative buffers of different widths and vegetation structure around playa wetlands on concentrations of metals, nutrients, and sediment in precipitation runoff and total volume of water entering playas, and (8) develop a framework for network analysis of playa systems. To assess physical loss and modifications, a combination of GIS and field data were utilized. A Geographic Information Systems (GIS) representation of playas historically and currently present on the SGP landscape was developed by combining the Playa Lakes Digital Database for the Texas Portion of the Playa Lakes Joint Venture Region (PLDD) and National Wetland Inventory (NWI) spatial data. Two percent of the wetland polygons were randomly selected for assessment of water feature type, anthropogenic impacts, and physical presence. USDA National Agricultural Imagery Program (NAIP) and Google Earth imagery were used to categorize the current state of each polygon as a playa, other waterbody, or no visible water feature. This categorization of the water features within the combined layer allowed for an estimate of the number of historical and current playas represented in the combined layer. When a polygon was categorized as playa, anthropogenic modifications and impacts were evaluated. Anthropogenic modifications and landcover was described within a 500-m diameter surrounding the playa. Historical playa locations with no depression visible in recent imagery and/or with loss of ≥ 100% of the original hydric-soil defined volume due to sediment accumulation were defined as physically lost. Sediment loads were determined through field visits to randomly selected dry playas. Using sediment depth, percent volume loss due to sediment accumulation was calculated. The percent of playas with ≥ 100% loss of the original hydric soil defined volume was determined. The combined dataset contained 34,512 polygons representing potential water features in the SGP across 51 counties in three states. The status of 740 polygons (2% per county) was assessed and categorized. Six hundred and two were categorized as historical playas, 125 as other waterbodies, and 13 as no visible water feature. Only 1 playa (0.2%) had no playa or watershed modification. Twenty-eight playas (4.7%) had no playa modification. Three playas (0.5%) had no watershed modification. Across all landcover types, 51.5% of playas were tilled at the time the imagery was taken or showed signs of being tilled in the past. Mean volume loss within native grassland and CRP were <100% of the original hydric volume in Texas, New Mexico, and Oklahoma. A playa confirmation rate of 81.4% resulted in an estimated 28,092 depicted historical playa locations in Texas, Oklahoma, and New Mexico. During the time period of 1970-2008 an estimated 17% of playas have been physically lost from the SGP landscape. Including the impacts of sediment accumulation, playa wetlands were reduced by an estimated 60% (1.6% per year). These estimates are conservative in that cultivation of the playa floor is not included as a loss. However, during cultivation of the playa, the characteristic hydric soil layer is altered. The inclusion of past and present tilling as a physical loss increases this estimate to 85.7% and a rate of 2.3% per year, during the 38 years under study. The cumulative influence of these losses is unknown until the spatial distribution of lost playas is evaluated in relation to existing playas. The framework for a functional assessment was created through the use of field data, literature searches, and best professional judgment. It consists of 15 key elements or predictors for playa wetlands (i.e., model variables) and their associated measurement or condition important in evaluating the level of an individual playa’s function based on the three ecological functions or services of interest: hydrology/recharge, water quality, and biodiversity. Based on the outcome from this matrix, a continuous function scale was populated, producing a distribution of sampled playas. Cut-off points were established within this continuous function scale to place playas into one of five categories (i.e., fully functional, partially functional and restorable, partially functional and non-restorable due to cost, partially functional and non-restorable, and non-functional). In 2008-2009, playas were stratified by playa density/county and randomly selected. These playas were used to create the function matrix, adjust values within the function matrix, and to assess function delivered by the playas remaining on the landscape. For the function assessment, empirical data were combined with GIS-derived information, forming 15 model variables. Nine model variables were directly evaluated via either field sampling or GIS-based techniques: (1) Physical Modifications, (2) Reduction in Original Volume, (3) Sediment Depth, (4) Agricultural Impacts, (5) Ratio of Native to Non-Native Plant Species, (6) Buffer Zone, (7) Density of Wetlands, (8) Landcover in Watershed, and (9) Upland Land Use. The remaining 6 variables were assessed using a subset of playas, due to both time and financial constraints, through the use of field data, basic statistics and modeling: (1) Average Duration of Surface Water, (2) Measured Aluminum, (3) Measured Chromium, (4) Measured Iron, (5) Measured Total Suspended Solids, and (6) Biomass. Through the application of the function matrix, none of the sampled playas were estimated to function at full functional capacity in the SGP. Seventy-three (47%) of playas were estimated to be partially functional and restorable. Partially functional and non-restorable due to cost playas were estimated at 12.9% or 20 playas, and 61 (39.4%) playas were partially functional and non-restorable because effective restoration techniques do not exist. Lastly, 1 (0.01%) of the extant surveyed playas was estimated to be non-functional. For eight counties in Texas, I compared the occurrence of playas, as indicated by soils designated as hydric in original soil surveys, to designations in USDA-remapped soil surveys of upland and depressional soils. I estimate a 65% decrease in playa numbers and 50% decrease in playa area as defined by the presence of a hydric soil following soil remapping. An estimated 80% of small playas (<5 ha in area) will potentially lose protection due to soil remapping. Playas embedded in both grassland and cropland watershed cover types were remapped; however, the mean (±SE) size of playas where soil type changed was 7±0.40 ha and 3±0.06 ha, respectively, for each cover type. Excessive sediment accumulation and other anthropogenic factors, resulting in an altered hydrology and masking of hydric soil characters, are proposed as primary factors responsible for differences in playa numbers and area following soil remapping. Other factors potentially impacting the remapping results include current USDA methodology and correction of historical survey errors. Therefore, any playa in the SHP being considered for inclusion under USDA conservation programs must be individually and independently assessed on-site for wetland criteria, rather than relying on information provided by revised USDA-NRCS Soil Survey maps. Further, during on-site evaluations, effects of anthropogenic alterations on the ability of the playa soil to develop and maintain hydric characteristics must be considered. Finally, until the remapping effort is complete, the online USDA-NRCS Soil Survey maps will be comprised of a mixture of historical soil surveys and revised classification of historical surveys, which will cause confusion during interpretation by those unfamiliar with the status of soil survey reports for the Texas SHP. In 2008 and 2009, the effect of buffers surrounding playa wetlands on water quality was evaluated as functions of buffer width and vegetation cover. Precipitation runoff was collected from 36 playa buffers (Conservation Reserve Program=7, Fallow Cover=18, Native Grassland=11) using I-CHEM storm water samplers (n=238) placed at the outer edge of the buffer adjacent to cultivated watersheds and each 10 m to the playa floor. Samples were analyzed for total dissolved solids (TDS), total suspended solids (TSS), specific conductance (SC), pH, nitrate, phosphorus, and metals (i.e., Al, As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Sr, V, Na, Zn). Also evaluated was the influence of buffers on hydrology, using the NRCS-USDA Curve Number Method. TDS and TSS reached a combined maximum removal at 50 m, 49% and 72% respectively. Nitrate and phosphorus reached a combined maximum removal at a distance of 20 m, 49% and 33% respectively. Maximum removal of metals occurred at 40 m. Estimated percent reduction in runoff reaching the playas due to the presence of a buffer was greatest for the native CRP cover type (-5.8%). A minimum buffer width of 40-50 m is necessary to maximize contaminant removal from runoff entering playa wetlands. However, use of buffers does not completely negate the impacts of watershed cultivation on playa wetlands. Therefore, additional conservation practices are necessary in adjacent cultivated watershed (e.g., contour plowing, no-till agricultural) to minimize movement of contaminants from watersheds into playas. The proposed framework for an initial network analysis of the Great Plains playa system includes use of metrics and algorithms outside of currently available software and incorporates physical and functional loss data. For playa locations, I recommend the combined spatial data developed for use in assessing physical and functional loss of playas; for surrounding land use I recommend the National Landcover Database (http://www.mrlc.gov/). Indices for playa quality are probability of inundation and level of function. The utilization of this framework will address how physical and functional loss of playas, in concert with climate change, is currently influencing or going to influence connectivity and biodiversity.