Acadia is not the Alps: Plasticity in leaf out phenology does not vary by elevation across a compressed environmental gradient in Acadia National Park, Maine

Background/Question/Methods:  Advancing plant phenology is recognized as an accessible and compelling indicator of the ecological effects of anthropogenic climate change. Variations in phenological response have implications for plant performance and conservation managers can use phenological plasticity as a trait in vulnerability assessments. Species-level rates of response (days/°C) are commonly reported, but phenological sensitivity often varies within species across altitude or latitude. In Acadia National Pak, Maine, we studied population-level adaptations in phenological plasticity in a compressed environmental gradient. We used observational transects and experimental gardens to investigate leaf out phenology from 2013-2016. Our transects covered the north and south aspects of three ridges; we divided each ridge into four elevation zones and deployed temperature loggers in these zones on each ridge. Phenology observations for thirty plant species were recorded twice a week on each ridge. Experimental transplant gardens were established for three widespread plant species at three elevations on one transect. We filled each garden with transplants from the summit, mid-elevation, and base of the mountain, and monitored for spring phenology for three growing seasons. We used the observational transects and experimental gardens to explore the relationship among spring temperatures, local adaptations, and plant phenology in a heterogenous environment.

Results/Conclusions:  We found consistent advancing phenology in response to warmer spring temperatures in both transects and transplant gardens. Along the transects, we modeled our focal species’ response to temperature, year, ridge, elevation, zone and aspect factors, and found that mean spring temperatures is the most consistent predictor of leaf out date. In the gardens, all three species displayed earlier phenology in response to warmer spring temperatures, while the source population was a less consistent and often insignificant factor. Using our transects, we calculated leaf out advances at 3.3, 4.3, and 2.3 days/°C for the three species in our gardens; in the gardens, these species matched the direction of phenological change but at a lower magnitude. Most common garden studies report some genetic variation, and in these cases the populations from colder source populations (higher altitude or higher latitude) were nearly always less sensitive to temperature cues than populations from warmer provenances. However, in Acadia the source populations did not display a clear pattern: populations sourced from warmer sites did not consistently display advanced or delayed leaf out phenology compared to cooler sites.