Warming leads to divergent responses but similarly improved performance of two invasive thistles

Plants' responses to climate change are complex. Even the same net performance changes may involve different responses of multiple life history traits. Here we show that two congeneric thistles, Carduus nutans and Carduus acanthoides, both grew taller under increased temperature, albeit following divergent response patterns. For C. nutans, warming advanced bolting more than flowering, leading to a longer growing period before flowering and ultimately taller plant height at the end of the growing season. Carduus acanthoides maintained the same length of growing period because of equally shifted events in the phenological sequence, however, post-flowering growth rate was increased, which also led to enhanced final plant height. As seeds from taller plants disperse farther, their responses imply that future invasion spread rates of these two species will increase. Similar consequences due to divergent responses in life history traits, as demonstrated in this study, suggest that considering only ultimate performance outcomes, and not the underlying processes generating such outcomes, is not enough to understand the impacts of climate change.