Effects of grazing on plant communities and successional processes vary along an aridity gradient at a northern temperate grassland

Abstract

The intermountain grasslands of North America reach their most northern geographic extent in interior British Columbia’s Cariboo-Chilcotin region. Here, this study examined the long-term effects of livestock grazing exclusion and reductions in grazing severity on plant community characteristics including plant and litter cover, species richness and abundance of leading species of 33 grassland sites across a broad aridity and soil property gradient. Across the aridity gradient, grazing reduced species richness, plant cover, and litter cover. However, the effects of grazing on dominant species varied across the gradient. In more arid grasslands, historical grazing substantially reduced cover of late-seral native bunchgrass Psuedoroegnaria spicata, and repeated measurements indicate that very long time periods are necessary for successional processes associated with recovery of native bunchgrasses. At the cool-wet end of the aridity gradient, successional processes are more rapid but dominated by exotic species Poa pratensis and Tragopogon pratensis. Recent (past 20 years) light grazing and rest-rotation have favored Poa pratensis at the expense of native needlegrasses (Achnatherum spp. and Hesperostipa spp.). We suggest that absence of a dominant large-stature native bunchgrass for mesic grasslands was a key factor in the invasion and dominance of Poa pratensis.

Appendix

See Tables 7, 8 and 9; Figs. 5 and 6

Table 7 Results of t-tests for abundance of plant species and ecosystem characteristics (in percent cover) for the Interior Douglas-Fir (IDF) and Bunchgrass (BG) biogeoclimatic zones

Table 8 Results for linear regression between plant community characteristics (cover value) and climate variables

Table 9 Results for linear regression between soil properties and climate variables. AHM1 represents the relationship between log-transformed variables (y axis) and with the climate variable (x axis) and AHM 2 represents the non-transformed variable (y axis) with the climate variable (x axis)

Fig. 5

Percent cover values for the first time period (left side) and second time period (right side) for non-grazed and grazed treatments at 15 reference areas across an annual heat:moisture (AHM) gradient for A Litter, B Poa pratensis, C Koeleria macrantha, D Pseudoroegnaria spicata, and E Hesperostipa comata, and F Achnatherum richardsonii

Fig. 6

Relationships of soil characteristics: A organic carbon content, B total nitrogen, and C carbon to nitrogen ratio to annual heat:moisture index (AHM) across twelve non-grazed study sites. Relationships of soil characteristics to AHM include organic carbon content (Y = − 0.062x + 3.61; R² = 0.88; p < 0.001), total nitrogen content (Y = − 0.055x + 0.90; R² = 0.87; p < 0.001) and carbon to nitrogen ratio (Y = 0.007x + 2.71; R² = 0.82; p < 0.001)