Comparisons in the native and introduced ranges reveal little evidence of climatic adaptation in germination traits

Plant invasions are increasing due to globalization and environmental change, including through anthropogenic climate change. Yet we lack an understanding of how some species become widespread invaders while others do not. Two competing mechanisms have been posited: post-introduction rapid evolution to the novel environments of the introduced range and broad environmental tolerance in the native population that makes invaders tolerant of diverse introduced environments. Each mechanism has implications for how invaders respond to climate change: either by evolving with future climates, or already being tolerant of diverse current/future climates. Disentangling these mechanisms requires investigating how evolution versus tolerance drive invasion traits (germination success and timing; growth rate). Here, we tested for evidence of rapid evolution in these traits by using growth chambers to provide common climates for seven herbaceous plant species sampled from multiple populations in their native (European) and introduced (North American) ranges. Chambers provided two levels of stratification—to simulate different winter lengths—and four temperature levels post-stratification—to simulate different spring conditions. We used Bayesian multilevel models to examine responses, while controlling for population and seed family. Across all species, trait responses were largely similar between native and introduced populations, except in response to particular climates representing cold winters and warm springs where introduced populations germinated later and grew faster. Our results suggest that broad environmental tolerance, not rapid evolution, likely underlies invasion success for these invaders—and may sustain their spread with continued warming—but species may evolve in response to specific combinations of winter and spring climatic regimes.