Phenological effects from climate change have been widely reported around the world, including in Concord, Massachusetts, home of Henry David Thoreau and Walden Pond. Concord is emerging as a living laboratory for climate change research because of the existence of unique records of phenological observations collected by Thoreau in the mid 19th century. Since that time mean annual temperatures in the area have risen approximately 2.4°C, and previous studies have shown a significant advancement in spring plant flowering and the arrival of several bird species. In this study we used contemporary observations and previously unanalyzed historical records collected by Thoreau between 1853 and 1860 to determine whether leaf-out dates of trees and shrubs have shifted in response to warming temperatures. We compared the first leaf dates of 43 tree and shrub species in 2009 and 2010 with the first leaf dates of the same species for the years 1853-1860. Vegetative phenology changes in response to temperature have been largely linear thus far, but it is unreasonable to assume that this relationship will continue indefinitely. In order to better understand the precise relationships among temperature and phenology, we selected a subgroup of our focal species to subject to experimental warming at the Boston Area Climate Experiment (BACE) facility. We planted seedlings of Acer saccharum, Betula populifolia, Quercus alba, and Vaccinium corymbosum at ambient temperature, as well as in plots that were set to be heated to ambient T+2°C, and ambient T+4°C, conditions simulating growing conditions in the future as temperatures continue to rise as a result of anthropogenic climate change.
We found that on average, plants in Concord are leafing out 18 days earlier now than they were during Thoreau’s time. Experimental warming also advanced leafing, the seedlings at BACE leafed significantly earlier in experimental plots than in control plots. This significant advancement in spring onset, and the implications that advancement is likely to continue, has important implications for ecosystem processes, from how much carbon a forest is sequestering to possibilities for trophic mismatches if pollinators or herbivores are not responding to changing temperature in the same way as their food or host plants. Further experimental work or modeling simulations would be needed to determine the responses of mature individuals and responses to more severe warming, which may lead to unmet chilling requirements.