The composition of plant communities depends on interactions between propagule availability (seed limitation) and propagule establishment (microsite limitation), both of which can be mediated by biotic and abiotic conditions (nutrient availability, climate, competition from resident species). Successful restoration of native plant communities degraded by a long history of non-native invasions as well as altered land use and disturbance patterns require understanding the interplay between all these factors. However, most ecological restoration studies are limited in duration and spatial extent, reducing the inferences that can be drawn and their applicability to different locations. Prairies in the Pacific Northwest of North America provide an opportunity to test these questions with the hope of improving restoration strategies for a highly endangered ecosystem. We tested the importance of removal of exotic dominants, microsite and seed limitation, and disturbance in determining community composition, with the aim of improving restoration success. To examine how biotic and abiotic conditions can alter these relationships, we replicated experiments at 10 sites along a 500km latitudinal gradient, spanning a wide range of environmental conditions and degree of invasion.
Results/Conclusions
We found propagule availability was the key factor limiting diversity of native plant communities; only seed addition increased native diversity. Addition of 7 commonly occurring native species increased diversity by an average of 3.4 species/m2, regardless of disturbance or invasive plant control. However, after 3 yr the abundance of seeded native species was higher with disturbance (fire), but controlling dominant invasive grasses (herbicide) in the absence of disturbance had no effect. The response of seeded species was strongest at high productivity sites (higher soil nitrogen, NPP, and soil moisture). Removal of non-native dominants through fire and herbicide did not lead to the expected increase in native species at most sites, although sites with higher initial quality (greater native diversity and relative abundance) exhibited the strongest treatment effects. Overall we found restoration of native plant communities to be constrained primarily by propagule limitation, as well as the effects of both biotic and abiotic constraints on propagule establishment.