Convergence and divergence of flowering times across the Arctic in response to warmer temperatures
The phenology of plants in Arctic regions is strongly affected by temperature, and thus is predicted to be particularly sensitive to climate change. Plants located at higher latitudes or from colder climates may shift phenological events more than those from warmer regions. This could lead to a convergence of flowering times across latitudes, and increased gene flow between tundra plant populations.
We examined up to 20 years of phenology measurements of 22 plant species from 13 Arctic sites to test the hypothesis that there is a convergence of flowering times across latitudes in response to warmer temperatures. We compared the phenological responses of plants to the historical summer climate at each site to determine if species from colder climates advanced flowering dates more in response to warmer temperatures than plants from warmer sites. To measure phenological responses, we compared the day of year of a phenological event (flowering or flower senescence) to growing degree days from snowmelt until the average day that the species flowered for each species by site combination. Additionally, we examined species-level data for the most common tundra species occurring at three or more sites: Cassiope tetragona, Dryas integrifolia, Dryas octopetala, Luzula confusa and Polygonum viviparum.
Overall, species at sites with historically colder climates advanced flowering and flower senescence earlier per growing degree day than species from warmer locations in the Arctic. However, individual species responded idiosyncratically to warmer temperatures at different sites. Phenological responses of Dryas integrifolia and Luzula confusa supported our hypothesis; plants from sites with colder climates flowered earlier per degree warming than those from warmer sites. Phenological responses of Cassiope tetragona and Polygonum viviparum did not differ between high and low Arctic sites, and interestingly, phenological responses of Dryas octopetala were greater at sites with warmer historical climates than plants from colder sites. The results highlight the importance of examining phenological responses of multiple species over large spatial scales to understand plant responses to climate change. As the climate warms in the future, we may see a convergence in flowering times between species across latitudes, and this convergence could lead to increased potential for gene flow between populations of Arctic plant species. However, phenological shifts will vary between plant species and as a result of local site conditions, indicating that gene flow may increase for some species, and in some areas, more than others.