Local environment, not local adaptation, drives leaf-out phenology in common gardens along an elevational gradient in Acadia National Park, Maine.

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Climate-driven changes in phenology are substantially affecting ecological relationships and ecosystem processes. The role of variation among species has received particular attention; for example, variation among species' phenological responses to climate can disrupt trophic interactions and can influence plant performance. Variation within species in phenological responses to climate, however, has received much less attention, despite its potential role in ecological interactions and local adaptation to climate change. We constructed three common gardens across an elevation gradient on Cadillac Mountain in Acadia National Park, Maine, to test population-level responses in leaf-out phenology in a reciprocal transplant experiment. The experiment included three native species: low bush blueberry (Vaccinium angustifolium), sheep's laurel (Kalmia angustifolia), and three-toothed cinquefoil (Sibbaldiopsis tridentata). Evidence for local adaptation of phenological response to temperature varied among the species, but was weak for all three. Rather, variation in phenological response to temperature appeared to be driven by local microclimate at each garden site and year-to-year variation in temperature. Population-level adaptations in leaf-out phenology appear to be relatively unimportant for these species in Acadia National Park, perhaps a reflection of strong genetic mixing across elevations, or weak differences in selection on phenological response to spring temperatures at different elevations. These results concur with other observational data in Acadia and highlight the utility of experimental approaches to understand the importance of annual and local site variation in affecting phenology both among and within plant species.