Native seed mix functional composition determines native plant recruitment and competition against invasive annual grass Bromus tectorum
Amy S. Gill, Mary Van Dyke, Emma Enebo, Hunter Geist‐Sanchez, Troy Ocheltree, Caroline A. HavrillaAbstract
Introduction
The invasion of the winter annual grass Cheatgrass ( Bromus tectorum L.) reduces native plant biodiversity and increases wildfire risk in western North America. Seed‐based restoration often fails to increase native plant recruitment in degraded, Cheatgrass‐invaded areas. Co‐seeding native grasses and forbs enhances niche complementarity and biotic resistance to invasion and may improve restoration outcomes.
Objective
In a greenhouse experiment, we tested whether native seed mixes differing in functional composition enhance native recruitment and suppress Cheatgrass and identified functional traits and diversity metrics associated with biotic resistance.
Methods
In mesocosms, we tested three perennial native seed mixes: grasses (G), forbs (F), and grasses and forbs (G + F) with (+E) and without Cheatgrass. We measured native and Cheatgrass recruitment and growth, and quantified functional traits (specific leaf area, leaf dry matter content, root:shoot ratio, and specific root length) to assess how native community‐weighted trait means (CWMs) and functional diversity indices related to competitive outcomes.
Results
The grass + forb (G + F) mix produced the highest native density and biomass with and without Cheatgrass competition. However, the lowest Cheatgrass density and biomass were associated with the grass “only” treatment, as compared to forb “only” or grass plus forb treatments. CWM traits were not significantly linked to community outcomes, but functional evenness showed a parabolic relationship with Cheatgrass relative biomass.
Conclusion
Co‐seeding grasses and forbs may improve restoration outcomes, but the strength of biotic resistance may depend on seed mix composition and functional trait diversity, and the environmental context in space and time.