Citation:

Herlihy, A., M. Kentula, T. Magee, G. Lomnicky, A. Nahlik, AND G. Serenbetz. Striving for consistency in the National Wetland Condition Assessment: developing a reference condition approach for assessing wetlands at a continental scale. ENVIRONMENTAL MONITORING AND ASSESSMENT. Springer, New York, NY, 191:327, (2019). https://doi.org/10.1007/s10661-019-7325-3

Impact/Purpose:

One of the biggest challenges when conducting a continental-scale assessment of wetlands is setting expectations of natural background reference condition for the assessed sites. This is usually done by the selection of regionally representative reference sites. This is very challenging because there is both tremendous natural environmental variability and variability in available reference site quality across the U.S. In this paper, we described the process we developed to select 277 reference sites for the US Environmental Protection Agency’s National Wetland Condition Assessment (NWCA). These reference sites were later used to develop biological condition indices and define good and poor condition thresholds for the NWCA. As part of the process of selecting reference sites, we also developed a wetland classification scheme for national reporting, assessment, and reference site development by classifying sites into 10 reporting groups based on vegetation type and ecoregion location. These reporting groups were used as a basis for setting criteria for defining least disturbed reference condition and also identifying the most disturbed sites. They were also used as the template for reporting on condition in the NWCA report. The NWCA provided a unique opportunity to improve our conceptual and technical understanding of how to best apply a reference-condition approach to assessing wetlands across the U.S. These results will enhance the technical quality of future national wetland assessments and can be adopted for use by states as part of wetland monitoring and assessment to support wetland protection and management. It also contributes to work being done under SSWR Task 3.01A, subtask 1.1.

Description:

One of the biggest challenges when conducting a continental-scale assessment of wetlands is setting appropriate expectations for the assessed sites. The challenge occurs for two reasons: 1) tremendous natural environmental heterogeneity exists within a continental landscape and 2) reference sites vary in quality both across and within major regions of the continent. We describe the process used to set reference expectations and define a disturbance gradient for the US Environmental Protection Agency’s National Wetland Condition Assessment (NWCA). The NWCA employed a probability design and sampled 1138 wetland sites across the conterminous United States to make an unbiased assessment of wetland condition. NWCA vegetation data were used to define 10 reporting groups based on ecoregion and wetland type that minimized the naturally occurring variation in wetland vegetation associated with continental-wide differences in biogeography. These reporting groups were used as a basis for defining quantitative criteria for least disturbed and most disturbed condition, and developing indices and thresholds for categories of ecological condition and disturbance. The NWCA vegetation assessment was based on a reference-site approach, in which the least-disturbed reference sites were used to establish benchmarks for assessing the condition of vegetation at other sites. Reference sites for each reporting group were identified by filtering NWCA sample data for disturbance using a series of abiotic variables. Ultimately, 277 least disturbed sites were used to set reference expectations for the NWCA. The NWCA provided a unique opportunity to improve our conceptual and technical understanding of how to best apply a reference-condition approach to assessing wetlands across the US. These results will enhance the technical quality of future national assessments.

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