Livestock exclusion alters plant species composition in fen meadows

Abstract

AbstractQuestionsOur study evaluated how species composition and plant traits that indicate functioning condition in fens responded to grazing cessation over time in an arid ecosystem of the western US. The specific questions addressed were: (i) how does livestock exclusion influence species composition in fens; (ii) is grazing cessation associated with shifts in species functional traits that indicate fen condition; and (iii) what is the pattern of response to livestock exclusion over time?LocationPlumas National Forest, California, US.MethodsWe studied paired fenced and unfenced study sites in two fens to examine the effects of livestock exclusion. Parallel transects were established at each site, and plant species and ground cover were repeatedly surveyed, once prior to and multiple times following treatment, using 0.01 m2 frequency frames. We used NMDS to analyse species composition, RLQ and fourth‐corner analysis to evaluate species functional traits and environmental variables, and linear mixed effects models to examine differences in responses between fenced and unfenced study sites over time.ResultsAfter fencing, we observed unexpected shifts in species composition and plant functional traits. Grazed sites were associated with peat‐forming obligate wetland, moss and sedge species, while fenced sites were characterized by non‐peat‐forming facultative upland, and upland forb, grass and early seral species. Species composition also varied between sites and sample years.ConclusionsWe found that livestock exclusion strongly affects plant species composition in fens, including promoting species with functional traits that indicate a loss of functioning condition, such as ruderal and upland species. Possible explanations for these observed shifts include: (1) biomass accumulation in the absence of herbivory, (2) competitive exclusion in fenced sites, (3) succession, (4) the abiotic conditions of our study sites, particularly hydrology and nutrient status, and (5) interactions among these factors. We conclude that degradation of fen wetlands caused by livestock grazing in the arid western US may not be reversed by excluding livestock alone.