Trait convergence under warming: drought tolerance, not thermal limits, mediates native–invader dynamics in alpine ecosystems

Abstract

Climate warming and plant invasions jointly alter alpine ecosystems, but the mechanisms underlying these interactions remain unclear. A key to predicting ecosystem vulnerability is whether native and invasive species can sustain performance as conditions warm. While invaders are often more plastic and competitive, it is uncertain whether these advantages hold in cold, resource‐limited alpine environments. We used a factorial warming × invader‐removal field experiment at two invaded alpine sites (one shrub‐ and one forb‐invaded site) to assess how warming and invasion influence intraspecific trait variability and the community‐weighted means of physiological traits. We measured traits linked to water use (δ 13 C, leaf thickness, relative water content) and thermal tolerance (hot/cold limits). We also examined trait coordination among carbon‐flux (A max , R d , F v /F m ), water use and thermal traits. Trait differentiation among natives and invaders was generally low, particularly for the invasive forb, indicating that growth form rather than plant status better predicted trait values. However, the invasive shrub displayed twice as much variation in A max , significantly lower leaf thickness and ~ 20% less water content than native species. Invader removal had greater effects than warming on the traits of native species, increasing coordination among leaf carbon fluxes (A max , R d ) and water use (RWC) traits. Invasive shrub cover reduced community‐level drought tolerance, while invasive forb cover had minimal effects. Overall, our results show that water use‐related traits, more than thermal tolerance, mediated species performance and community resistance under warming. Similar water use trait values among natives and invaders suggests strong environmental filtering in these alpine ecosystems, with the potential for drought to constrain invasion under future climate change.