Fenceline contrasts: grazing increases wetland surface roughness

Abstract

A warming earth has lost substantial mountain-stored frozen fresh water, thus generating a pressing need for greater liquid–water storage within upper-elevation riparian systems. Liquid–water storage can be enhanced by avoiding microtopographic channels that facilitate land drainage and rapid runoff. A number of authors have attributed certain forms of wetland hummocks and inter-hummock channels to grazing livestock but there is little evidence in the scientific literature for a cause and effect mechanism. We used comparisons at six fencelines on four meadow and wetland complexes to test the null hypothesis that grazing management makes no difference in hummocks and inter-hummock channels measured as surface roughness. Surface roughness was measured both photogrammetrically (photo) and with an erosion bridge (EB), and the measurements expressed as surface roughness indices (SRIs). Wetland surface roughness inside fenced areas was 44 (EB) and 41 (photo). Wetland surface roughness outside fenced areas was more than 50 % higher (p < 0.0001), measuring 76 (EB, n = 6) and 62 (photo, n = 4). The site with the longest period of conservation management (50+ years) had the lowest inside EB SRI at 27. The two independent measurement methods, EB and photo, yielded similar, correlated results (R = 0.71, n = 8). Historical aerial photography provides supporting evidence for our findings. We reject the null hypothesis and while we suspect macrotopography, hydrology, soil type, and climate are factors in hummock formation, our evidence supports the thesis that hummocks formed surface-down by inter-hummock channels result primarily from grazing by domestic livestock.