How grazing and soil quality affect native and exotic plant diversity in Rocky Mountain grasslands

Abstract

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7–60 yr old (31.2 ± 2.5 yr, mean ± 1 se ). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m 2 plots and 780 1-m 2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m 2 subplots was significantly higher ( P < 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m 2 plots in exclosures (31.5 ± 2.5 native and 3.1 ± 0.5 exotic species), adjacent grazed plots (32.6 ± 2.8 native and 3.2 ± 0.6 exotic species), and randomly selected grazed plots (31.6 ± 2.9 native and 3.2 ± 0.6 exotic species). We found no significant differences in species diversity (Hill’s diversity indices, N 1 and N 2), evenness (Hill’s ratio of evenness, E 5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m 2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 ± 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m 2 plots) in the same vegetation type overlapped just 48.6 ± 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 ± 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m 2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient = 0.647, t = 5.107, P < 0.001), elevation (coefficient = 0.012, t = 5.084, P < 0.001), and total foliar cover (coefficient = 0.110, t = 2.104, P < 0.039). Only 12.8% of the variance in exotic species cover (log 10 cover) was explained by percentage of clay (coefficient = −0.011, t = −2.878, P < 0.005), native species richness (coefficient = −0.011, t = −2.156, P < 0.034), and log 10 N (coefficient = 2.827, t = 1.860, P < 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soil N at the 1000-m 2 plot scale. Our research led to five broad generalizations about current levels of grazing in these Rocky Mountain grasslands: (1) grazing probably has little effect on native species richness at landscape scales; (2) grazing probably has little effect on the accelerated spread of most exotic plant species at landscape scales; (3) grazing affects local plant species and life-form composition and cover, but spatial variation is considerable; (4) soil characteristics, climate, anddisturbances may have a greater effect on plant species diversity than do current levels of grazing; and (5) few plant species show consistent, directional responses to grazing or cessation of grazing.