The importance of phenological tracking to plant community structure under stationary versus nonstationary environments

Recent work has found links between how a species' phenology shifts with climate change and its changes in performance with climate change. Species that appear to 'track' climate change, for example North American temperate plant species that shift leafing or flowering earlier with warmer springs, also tend to increase in abundance or other metrics of plant performance, while the reverse is generally seen for species that delay phenology with warming. This correlation between phenological tracking and performance has been implicated in plant invasions and appears to be an important factor in predicting future plant populations and communities under climate change scenarios. We have, however, little theory on why--or why not--species may phenologically track the start of spring. Early-season phenology gives priority access to light and soil resources and thus, in a simplistic model where the start of season varies between years, all species should strongly track the start of season, yet communities are always a mix of phenological tracking strategies (from species that do not track the start of season to strong trackers). Using a stochastic community assembly model we examined how phenological tracking influences species abundances across time under two different environmental scenarios: one with a stationary environment and one with a nonstationary environment where the start of season shifts earlier over time. Our model shows that trade-offs are required between phenological tracking and parameters related to species-specific resource use to maintain a community with a diversity of phenological tracking strategies and that nonstationary environments strongly favor species that track the start of season. Our results suggest that trade-offs established under a stationary climate that allow a diversity of phenological tracking strategies across species in a community may not hold under non-stationary environments to maintain the same diversity of species. Instead, nonstationary environments may lead to abundance increases in species that strongly track climate and thus may explain recent findings linking phenological tracking to performance.