Spring ephemerals advance their spring phenology at a higher rate than trees in response to warming in northeastern North American hardwood forests

Evelyn F Alecrim and Jessica Forrest, Department of Biology, University of Ottawa, Ottawa, ON, Canada, Risa Sargent, Applied Biology, The University of British Columbia, Vancouver, BC, Canada

Background/Question/Methods
The spring phenology of co-occurring trees and understory herbaceous plants (spring ephemerals) in northeastern hardwood forests are thought to be regulated by different environmental drivers. Previous work at a single site has suggested that global climate change could reduce photosynthetic rates and reproductive success of spring ephemerals through advancements in the timing of canopy leaf-out relative to timing of understory growth. However, phenological responses to warming vary interspecifically and geographically, making generalizations difficult. To answer more generally whether tree leaf-out and flowering of spring ephemerals have advanced their phenologies at different rates with respect to increasing temperature, we examined the phenological responses to warming of 22 species of trees and spring ephemerals (26,972 observations from 1,270 sites) in the Northeastern US using data from the USA-National Phenology Network for the past 12 years. We fitted a hierarchical Bayesian model of the first day of the year at a given site when “breaking leaf buds” (trees) or “flowers or flower buds” (spring ephemerals) were reported as a function of temperature. To account for latitudinal variation, we included a varying slope for each combination of functional group and latitudinal band (day of the year = α + β[FG, Latitude] x temperature).
Results/Conclusions
Overall, phenology of trees and of spring ephemerals occurred later with increasing latitude, and earlier with increasing temperature. Contrary to expectation, however, phenology of spring ephemerals varied more strongly with latitude than did tree phenology, with a mean change rate of 9.0 days/decimal degree of latitude for spring ephemerals vs. 2.9 days/ degree of latitude for trees. Similarly, at all latitudes, spring ephemerals advanced their phenology at a faster rate than trees in response to warmer temperatures. Contrary to what was previously suggested, our results show that since phenology of spring ephemerals respond more strongly to temperature than do trees, their growing season and carbon uptake could increase as spring temperatures rise. Still, an earlier leaf-out is also associated with a higher risk of late frost damage. Additionally, if pollinators of spring ephemerals do not respond to warming at the same rate as the plants, a mismatch could occur, decreasing fitness of plants or pollinators. Thus, we need more studies to better understand the consequences of warming temperatures for spring ephemerals. Our study is the first to show spring ephemerals advancing their phenology at a faster rate than trees in response to warming.