Grazing and solar irradiance drive patterns of change in two grassland plant communities in southern British Columbia

Abstract

We compared the recovery of two plant communities over 14 years following a switch from spring to fall grazing (G), and in ungrazed exclosures (UG), at six sites in Lac du Bois Grasslands Provincial Park, British Columbia. Both communities showed similar large increases in cover of their respective dominant grasses – rough fescue (Festuca campestris), and bluebunch wheatgrass (Pseudoroegneria spicata) – in both G and UG treatments. As expected, the more productive rough fescue community had higher amounts of soil polysaccharides, and total soil C and N. Repeated measures ANOVA indicated significant declines in species richness and Shannon diversity over time, especially in the ungrazed rough fescue community. Non-metric multidimensional scaling of species cover data showed continued divergence between communities, largely due to an increase in abundance of the two dominant grasses. Community differences were mainly determined by aspect and its impact on soil temperature and water content. We concluded that, from a management perspective, switching to light fall grazing was as effective as removing grazing to assist in recovery from previous overgrazing, and that the relationship of grazing combined with the fine-scale pattern of variable solar irradiance was responsible for the closely interspersed mosaic of the two grassland communities.