Riparian Wetland Response to Livestock Exclusion In the Lower Columbia River Basin

The purpose of this study was to examine the characteristics of riparian plant communities along a succession gradient of livestock exclusion in the Lower Columbia River Basin (LCRB). Livestock exclusion is an example of a passive restoration practice throughout the region. However, few studies have focused on the effects of livestock or livestock exclusion on riparian wetland ecosystems in this area. Two passive restoration sites, 3 and 13 years since livestock exclusion, and a control site with a continued livestock grazing presence were examined. It was hypothesized that native plant species richness would be lower in the excluded wetlands than in the grazed wetland due to the competitive exclusion from an increase in non-native plant dominance in the absence of grazing. Data were collected along six (45-60m) randomly distributed transects which were aligned perpendicular to the wetland shoreline of each site, providing a total of 18 transects with an accumulative length of approximately 990 meters. Vegetation cover data were collected for 10 cm intervals along these transects using the line intercept method during low water periods in August and September of 2009. The Kruskal-Wallis one-way nonparametric analysis of variance by ranks and the Mann-Whitney U test were used to detect significant (p <0.05, p<0.0167 after Bonferroni adjustment) differences in native and non-native plant species richness, diversity indices and relative cover among sites.;A total of 58 plant species were identified among all three study sites: 27 native, 27 non-native and 4 species of unknown origin. The grazed wetland had significantly (p<0.0167) greater average total species richness (23.3), native (10.2) and non-native (12) species richness than both the excluded wetlands. Average species richness did not differ significantly between the 3 year and 13 year excluded wetlands for both native (6.6, 2.8, p=0.088) and non-native (5, 2.7, p=0.064) species or total species richness (12, 5.5, p=0.063). However, native species abundance was significantly (p<0.0167) lower on the 13 year excluded wetland (4.2%) than both the 3 year excluded (51.5%) and grazing (23.2%) wetlands. The invasive grass Phalaris arundinacea L., commonly known as reed canarygrass, was found to be the dominant vegetation cover in all three wetlands with average relative cover ranging from 95.2% at the 13 year exclusion site to 52.8% at the grazing site and 43.0% cover at the 3 year exclusion site. These results suggest that livestock exclusion alone may be an ineffective strategy for restoring riparian plant communities in the LCRB where invasive species like Phalaris arundinacea L. are abundant. Other more practical management strategies could include short-term livestock exclusion and re-introduced targeted grazing to reduce livestock impacts and control Phalaris arundinacea L. dominance.