Toward a comprehensive understanding of the phenological responses of nonnative plants to climate warming: A review.

Background: Plants often shift their phenology in response to climate warming, with potentially important ecological consequences. Relative differences in the abilities of native and nonnative plants to track warming temperatures by adjusting their phenologies could have cascading consequences for ecosystems. Our general understanding of nonnative species leads us to believe these species may be more phenologically sensitive than native species, but evidence for this has been mixed, likely due, in part, to the myriad of diverse ecological contexts in which nonnatives have been studied.

Scope: Here, we review the current state of knowledge on nonnative plant phenological responses to climate warming. From observational and experimental studies, we synthesize: 1) the ways in which nonnative plant phenology shifts with increased temperature, 2) the relative differences between natives and nonnatives in phenological timing and sensitivity to warming, 3) the contingencies driving variable nonnative phenological responses to warming, and 4) the ecological consequences of warming-induced phenological shifts in nonnatives.

Conclusions: Early-season phenophases tend to advance with warming, sometimes (but not always) more so in nonnative species relative to native species. Late-season phenophases, on the other hand, tend to be more variable; advancing, delaying, or remaining unchanged. Similarly, relative differences in phenological sensitivity between native and nonnative plants were less consistent for late-season phenophases. However, our ability for inference is limited by the scope of studies done to date, which best represent temperate ecosystems in the Northern Hemisphere. We found phenological shifts in nonnative species to be driven by various factors including their evolutionary histories and the environmental context of the invaded system. Shifts in nonnative phenologies result in varied ecological consequences, from shifting demographics of the nonnative species themselves, to changes in ecosystem level processes such as carbon cycling. Additional study addressing key gaps is vital to improving understanding of nonnative phenological responses to warming.