Competition between exotic grasses and native forbs: the role of soil resources in mediating restoration success in an upland prairie in Oregon’s Willamette Valley
In prairies, higher soil nutrient levels are thought to allow dominant species, “demons”, to competitively exclude weaker competitors, leading to low plant diversity. If the mixed exotic-native communities now typical of most Western prairies behave like natural grassland systems, dominant exotic species should competitively exclude native ones, and that this elimination of diversity should be most intense and rapid on high resource sites. We tested this hypothesis in an upland prairie restoration site in Western Oregon. Prior to restoration, we installed 100 1m x 1m permanent monitoring plots spaced 15m apart along evenly-spaced transects. Percent cover of all vascular plant species was recorded in May or June for three years following restoration. Soil depth was measured for each plot, TDR estimates of soil moisture were obtained across the growing season, and nitrate levels were estimated using Hach cadmium reduction test packs. Single transplants of five native perennial forb species were transplanted 1m below the permanent sampling points in March of 2012, 2013, and 2014, into subplots with existing vegetation and subplots from which competition had been removed using Glyphosate. Transplant survival, size, and reproductive effort were measured in June of each year.
The community of exotic annuals that dominated the site in the year following restoration quickly shifted to one dominated by exotic perennials, with some presence of restored native grasses. Ordination of third year post-restoration data revealed that the strongest gradient in composition was correlated with soil depth and nitrate. Species richness was highest at the midpoint of this gradient, with low species diversity both in plots with extremely shallow soils, where abiotic stress was high, and in plots with deep soils where competition was more intense. Multiple linear regression of size of transplanted forbs against soil variables revealed significant linear relationships for some species. Size of Achillea millefolium transplants, for example, was larger as soil moisture increased and larger without competition, but there was no evidence for increased competition intensity at high resource levels. Fragaria vesca, in contrast, did show intensified effects of competition at high resource levels, with plant size decreasing as nitrate levels increased, but only when exotic competitors were present. Our results so far suggest that plot-level variation in abiotic stress and in resource-mediated competition intensity both play a role in determining prairie diversity.