Interactions between plant community trait composition and environmental stress influence community resistance to a recent annual grass invader, Ventenata dubia

Claire Tortorelli and Meg A. Krawchuk, Forest Ecosystems and Society, Oregon State University, Corvallis, OR, Becky K. Kerns, Pacific Northwest Research Station, USFS, Corvallis, OR

Background/Question/Methods
Ecological theory posits that plant communities will be more resistance to invasion if they are highly productive, harbor species with similar functional traits to invaders, or support species with superior traits. However, the strength of competition may decrease with increasing abiotic stress if species more heavily invest in traits that confer stress tolerance over competitive ability, potentially influencing community trait-resistance relationships. Recent research examining how community traits influence invasion resistance has been predominantly focused on single vegetation types, and results between studies are often conflicting. Few studies have evaluated the extent to which abiotic factors and community traits interact to influence invasion along an environmental stress gradient. To address this knowledge gap, we used a two-year manipulative in-situ field experiment to examine how community traits and site condition interact to influence community resistance to invasion by an exotic annual grass, Ventenata dubia, along a soil moisture gradient in eastern Oregon, USA. To measure invasion resistance, we compared V. dubia biomass in seeded subplots with and without above-ground vegetation removal and with varying trait compositions across three distinct vegetation types (scab-flat, low sage-steppe, and wet meadow) using linear mixed models.
Results/Conclusions
Resistance to invasion increased with increasing community biomass and increasing trait similarity to V. dubia, in support of the productivity-resistance and competition trait-similarly hypotheses. Of the seven above- and below-ground traits we measured, community trait-resistance relationships were strongest for specific leaf area, leaf nitrogen, and ratio of fine-to-total root volume. Overall, trait-resistance relationships were strongest in wet meadows where environmental stress was lowest, suggesting that competitive interactions were weaker in areas with higher abiotic stress as posited by the stress-gradient hypothesis. Compared to most native neighboring species, V. dubia had slightly higher specific leaf area and fine-to-total root volume and lower root-to-shoot ratio and root diameter, indicating higher competitive potential and lower stress tolerance. Unlike V. dubia which heavily invaded all three vegetation types, native and introduced neighboring species with similar trait values, such as Bromus tectorum, were not abundant throughout the study area demonstrating V. dubia’s unique ability to take advantage of a seemingly vacant niche. Our results illustrate how abiotic condition and community traits interact to influence community resistance to invasion. Restoration aimed at improving community resistance to V. dubia invasion may consider seeding species with high biomass and functional similarity to V. dubia and focusing efforts in wet meadows where biotic interactions are strongest.