Professor University of Oregon Eugene, Oregon, United States
Abstract: California annual grasslands are floristically diverse hotspots that provide many ecosystem services including forage production, soil erosion control, habitat provision, and cultural value. Variable precipitation and more frequent deluge and drought events may restructure grassland community composition. Compost amendments are a proposed adaptive management strategy to sequester carbon, retain soil moisture, and buffer grassland productivity during drought. A concern, however, is that increased soil nitrogen (N) from organic compost amendments and wetter soil (via rainfall or retained soil moisture) may promote resource-acquisitive non-natives and further reduce native flora, as has been shown with inorganic N.
To test whether and how compost amendments affect native diversity we analyzed the abundance response of four native species (two grasses, two forbs) seeded in a randomized-block field experiment that manipulated non-native background neighbors (reduced via herbicide, control), annual precipitation (drought, irrigated, control), and soil nutrients (compost, fertilizer, control). We used multivariate analyses to assess species- and community-level patterns by treatment, and Bayesian generalized joint-attribute modeling to quantify direct and indirect abiotic effects and competitor effects on native species.
Collective results revealed a hierarchy of dominance by non-native grasses, followed by non-native forbs and native grasses that competed with one another when released from non-native grass competition (via herbicide or drought). Native forbs co-occurred in modest amounts with native and non-native grass species–perhaps exploiting niches available in the absence of exotic forbs. Nutrient amendment and annual precipitation jointly affected subdominant competitive outcomes and community composition via differential effects on native species and background competitor functional groups. Both native grasses and one native forb preferred drought or relatively dry ambient conditions, while one native forb and dominant non-native grasses preferred irrigation. We did not find strong evidence of compost favoring non-native flora over natives, particularly in the scenario of a wet year. Compost in fact advantaged a seeded native grass under drought conditions. In contrast, fertilizer treatment favored exotic forbs under drought, suggesting that the effect of organic versus inorganic N inputs on composition are fundamentally different. Two other natives seeded were unaffected by soil nutrient treatment and were more sensitive to neighbor identity.
Our findings support compost as an adaptive management tool for mitigating drought effects on grassland services while promoting native plants on the landscape. Working with variable climate, drier years that inhibit dominant grass growth may be windows to strategically restore native flora by reestablishing native annual grasses then seeding native forbs.