Cheatgrass (Bromus techtorum), leafy spurge (Euphorbia esula), spotted knapweed (Centaurea stoebe), and sulfur cinquefoil (Potentilla recta), have invaded millions of hectares of grasslands in the Western United States. While dramatic increases in productivity following invasion are well documented, less is known about potential underlying mechanisms, such as enemy release, greater invasive success in fertile areas, or a unique ability of invaders to increase soil fertility.
To assess if invaders can modify soil conditions and to see if these changes are invader-specific, we planted 3 week-old seedlings of all four invaders into established experimental plots that harbored monocultures of each invader or a mixed native community of grasses and forbs. Seedlings were kept within small rodent exclosures and watered as needed. Soil nitrogen availability and respiration were measured after two months, and we destructively harvested seedlings after three months and quantified shoot and root biomass.
Soil nitrogen availability and respiration differed among established communities. Specifically, monocultures of spotted knapweed and leafy spurge−but not cheatgrass and sulfur cinquefoil−increased nitrogen availability compared to native mixed communities. Leafy spurge also increased soil respiration relative to native communities and sulfur cinquefoil, and soil respiration and total seedling biomass were positively correlated (R=0.51; p<0.01). Total seedling biomass was highest in leafy spurge monocultures (p<0.001) and increased tenfold relative to native communities. However, while all other seedlings grew best in leafy spurge communities (p<0.05), biomass of leafy spurge seedlings did not differ among community types (p=0.66). Cheatgrass and sulfur cinquefoil seedlings were also larger in spotted knapweed soils relative to native communities.
These results clearly show that some invaders can modify soil conditions, and that effects are strongly invader-specific. While we observed no differences among cheatgrass, sulfur cinquefoil and native communities, spotted knapweed−and to an even greater extent−leafy spurge promoted soil nitrogen availability, respiration and seedling growth. However, leafy spurge seedlings did not grow well in their own soil, possibly due to build-up of species-specific pathogens. The relationship between soil respiration and seedling biomass suggest that differences in soil carbon input may influence site productivity and contribute to the success of some invaders.