A previous study found that trees are advancing their leaf-out times faster than wildflowers in Concord, Massachusetts in response to a warming climate, based in part on the records of Henry David Thoreau. Tree species advanced their leaf-out times by 4.4 days per 1°C spring warming on average, while wildflower species advanced their leaf-out times by 2.2 days per 1°C. This mismatch between the understory and overstory may lead to a reduction of high light experienced in the early spring, which is a critical period when many spring-blooming wildflowers perform most of their photosynthesis. Some wildflowers may lose 10-48% of their carbon budget by 2080. In order to determine the geographical extent of this potential ecological mismatch, we assessed leaf-out and flowering times using over 2,000 herbarium specimens of six common, overstory tree species – Acer rubrum, Acer saccharum, Carya glabra, Fagus grandifolia, Quercus alba, and Quercus rubra – and six common, understory wildflower species – Anemone quinquefolia, Dicentra canadensis, Dicentra cucullaria, Erythronium americanum, Hepatica americana, and Sanguinaria canadensis – collected from 1829 to 2018 across eastern North America. We paired phenological data with historical climate data to quantify leaf-out differences between trees and wildflowers across time, temperature, and region.
Results/Conclusions
As a group, tree species were more responsive in their leaf-out times to spring temperatures than wildflower species, although the difference was not as great as seen in Concord. Tree species advanced their leaf-out times by 3.6 days per 1°C spring warming on average, while wildflower species advanced their flowering times by 3.1 days per 1°C. Trees in the southern part of their range advanced their leaf-out times faster than trees in the North or wildflowers across their range; wildflowers’ responses did not differ significantly across their range. This difference suggests that the mismatch between trees and wildflowers differs across large spatial scales, such that wildflower populations in the South may be more affected by phenological mismatch. In addition, phenological sensitivities to spring temperature substantially differed among species, suggesting that certain species are more likely to be “winners” or “losers” as climate warming progresses. This research also highlights the value of newly-available digitized museum collections in ecological research.