In deciduous forests, the short window of high light in early spring before tree canopies shade the understory is critical for many spring-blooming wildflowers that acquire much of their annual carbon gain before trees leaf out. The duration of this high light period is determined by the timing of tree canopy leaf out, and differences in phenological responses to changing spring temperatures might reduce (or expand) this period for understory wildflowers, creating a phenological mismatch under climate change. Further, non-native, invasive species often exhibit novel phenological strategies, leafing out earlier in spring and shading out wildflowers. The regional patterns, ecological consequences, and cueing mechanisms behind these potential mismatches are only partly understood. To address this, we first analyzed local trends of leaf out responses to climate using observations initiated by Henry David Thoreau (1850s); we then used herbarium specimens to study these trends across the eastern US. Second, we quantified potential impacts of phenological mismatch on wildflowers through a simulated analysis of daily carbon gain data of seven species throughout the year. Last, we quantified phenological cues determining leaf out in trees, wildflowers, and invasive shrubs through a growth chamber study.
Results/Conclusions
In Thoreau’s woods, we found that overstory tree leaf out was more responsive to increased spring temperature than understory wildflower phenology. The critical period of high light before tree leaf out is now reduced by nearly one week in Concord, Massachusetts, compared to Thoreau’s era (1850s). Herbarium specimen data indicated a qualitatively similar pattern, but tree populations in the southeastern US exhibited greater phenological sensitivities compared to trees in the northeastern US. Based on local phenological trends and photosynthetic measurements, we projected that wildflower spring carbon budgets may decrease 10-48% during this century. Growth chamber studies demonstrated that native species have greater chilling requirements than invasive species, and that trees and shrubs break leaf bud dormancy faster than wildflowers with experimental warming. Further exacerbating these differences, we found that belowground temperatures experienced by dormant wildflower leaf buds warm more slowly than the air experienced by tree buds in canopy. Collectively, these results suggest that phenological disruptions caused by climate change and species invasions may be a widespread phenomenon that will affect the future of deciduous forests.