Citation:

Magee, T., K. Blocksom, A. Herlihy, AND A. Nahlik. Characterizing nonnative plants in wetlands across the conterminous United States. ENVIRONMENTAL MONITORING AND ASSESSMENT. Springer, New York, NY, 191:344, (2019). https://doi.org/10.1007/s10661-019-7317-3

Impact/Purpose:

A critical need for continental-scale wetland assessment is an effective biological indicator of ecological stress, comprised of easy-to-measure metrics, that is relevant across large scales and wetland types. To address this need, we developed the Nonnative Plant Indicator (NNPI), a categorical indicator of potential stress (low, moderate, high, or very high) from the set of nonnative plants occurring at a given location. NNPI potential stress categories are based on a decision matrix using exceedance values for three metrics: richness, relative frequency, and relative cover of nonnative plants. We applied the NNPI to wetlands across the conterminous United States using data from the USEPA 2011 National Wetland Condition Assessment (NWCA). NNPI status from 967 NWCA probability sites was used to estimate wetland area within the four NNPI categories at national, ecoregional, and wetland type scales. Estimates of wetland area with low, moderate, high, and very high NNPI will be useful to policy makers and resource managers in identifying where potential stress from nonnative plants may be most extensive, prioritizing management actions, and informing future research. To help understand NNPI extent patterns, we conducted and reported on exploratory analyses characterizing: 1) the complement of individual nonnative species observed at sampled sites, 2) ecoregional and wetland type patterns for NNPI metrics and nonnative growth habit groups, 3) ecoregional and wetland type characteristics (native vegetation, abiotic properties, and human-mediated disturbance) and 4) potential ecological predictors of NNPI status. This research also contributes to work under SSWR Task 3.01A, subtask 1.1.

Description:

Nonnative plants are recognized as stressors to wetlands and other ecosystems, competing with native plant species or altering ecosystem processes and may affect ecological condition, posing challenges to resource managers. We devised the Nonnative Plant Indicator (NNPI) as a categorical indicator of potential stress to ecosystems from the collective set of nonnative plant taxa occurring at a particular location. The NNPI indicates categories of potential stress from nonnative plants (low, moderate, high, or very high), based on a decision matrix using exceedance values for three metrics: richness, relative frequency, and relative cover of nonnative plants. We applied the NNPI to wetlands across the conterminous United States (US) using plant data collected during the US Environmental Protection Agency’s 2011 National Wetland Condition Assessment (NWCA). Based on the NWCA probability design, wetland area of the sampled population occurring in each NNPI category was estimated at the national-scale, and within five large ecoregions and four broad wetland types. Estimates for wetland area in different NNPI categories are expected to be useful to policy makers or resource managers in identifying situations where potential stress from nonnative plants may be most extensive, which may help inform and prioritize management actions and future research. At the national-scale, potential stress from nonnative plants as indicated by NNPI category was low for approximately 61% (~15.3 million ha), moderate for about 20% (~5.2 million ha), high for about 10% (~2.48 million ha), and very high for about 9% (~2.2 million ha) of the sampled population wetland area. The percent of wetland area in the sampled population with high and very high NNPI was greater within interior and western ecoregions (~29 to 87%) than within ecoregions in the eastern half of the nation (~11%). Among wetland types, greater percent area with high and very high NNPI was also observed for herbaceous vs. woody wetland types, and for inland vs. estuarine wetland types. To aid in understanding the patterns described by the NNPI extent results, we also conducted, and report on results for, a series of exploratory analyses to: 1) characterize the complement of individual nonnative species (n = 443) observed across 1138 sampled sites, 2) examine ecoregional and wetland type patterns for the three NNPI metrics and for growth habit groups of nonnative plants, 3) describe ecoregional and wetland type characteristics based on (native vegetation attributes, abiotic properties, and human-mediated disturbance) to broadly characterized the areas into which nonnative plants are invading, and 4) identify potential predictors of NNPI status.

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