Eco‐evolutionary context modifies a destructive plant invader's response to climate

Summary Understanding the relationship between climate and fitness will be important when predicting how plant populations respond to climate change. We conducted a replicated common garden experiment (4 sites × 2 yr) with 96 genotypes ( n = 22 492 individuals) of the invasive annual grass Bromus tectorum (cheatgrass) to understand how eco‐evolutionary context mediates this important relationship. We grew genotypes in varying soil microclimate conditions by manipulating surface albedo, and at two planting densities. We calculated a ‘climate mismatch’ for each genotype within each microclimate to characterize the role of local adaptation to climate in explaining variation in fitness. Genotype‐by‐environment interactions increased the predictive accuracy of our statistical model of cheatgrass fitness with strong evidence for local adaptation to source climate. Survival responses to soil microclimate were density‐dependent and there was stronger evidence for local adaptation at lower compared with higher planting density. Our model generally predicts increases in fitness with an increase in temperature across source populations. Important eco‐evolutionary context (e.g. genotype‐by‐environment interactions, density dependence) should be considered when predicting plant fitness in an era of rapid environmental change.