Symbiotic nitrogen fixation strategies relate to phenotypic plasticity but not invasion risk

Many invasive plant species are symbiotic N-fixers that can have strong impacts on ecosystem processes. Nitrogen-fixing plants use a diversity of strategies to regulate the degree of N-fixation, each well suited for specific environmental conditions. However, little is known about whether fixation strategies are related to invasiveness. Weed risk assessment scores were used as an index of invasiveness for eight non-native N-fixing tree species (four high-risk and four low-risk for invasiveness) in Hawaiʻi. In a shade house experiment using an isotopic tracer, we found that species varied in their growth, biomass allocation, and N-fixing traits in response to three levels of nitrogen fertilization. Species sorted into distinct fixation strategies with three species displaying a facultative strategy, four species displaying an incomplete downregulation strategy, and one species displaying either a facultative or incomplete downregulation strategy. Fixation strategies were associated with the trait plasticity of each species, but not related to risk assessment scores for invasiveness in Hawaiʻi. Facultative fixers had the highest trait plasticity and were able to regulate symbiotic nitrogen fixation with the greatest magnitude. Collectively, our results suggest that species growth traits are better predictors of N fixation strategy than weed risk assessment scores, suggesting that the link between invasiveness and N fixation strategy is tenuous.