Background/Question/Methods Over the past several decades, climate change has driven and continues to drive temperate tree phenology earlier in spring. Although this can be beneficial for canopy trees due to longer growing seasons and increased carbon assimilation, this dynamic also has the potential to harm temperate tree seedling performance for species that rely on phenological escape for survival. Seedlings of many deciduous species depend on this strategy (i.e., expanding their leaves days-weeks before the canopy closes) to assimilate 50-80% of their annual foliar carbon budget in the first few weeks of the growing season. This period of high assimilation should help them to withstand high summer respiration costs associated with drought and warmer temperatures and thus enable them to grow, survive, and successfully recruit. In this study, we used a transplant experiment to investigate the relationships between phenological escape, annual carbon assimilation, and performance of two species of temperate tree seedlings common across eastern North America (Acer saccharum and Quercus rubra). We then used this information, alongside models of seedling and canopy phenology shifts in response to temperature, to forecast how climate change will affect seedling performance by the end of the century. Results/Conclusions We found that individual seedlings assimilated an average of 84% and 53% (A. saccharum and Q. rubra, respectively) of their annual carbon in spring, consistent with the literature. Furthermore, there were significant correlations between duration of spring phenological escape and annual carbon assimilation for both species, suggesting that this strategy is the driving force behind annual assimilation dynamics. Our results indicate that annual carbon assimilation is significantly associated with survival of both species and with growth of A. saccharum, directly linking phenological escape to seedling performance and recruitment. Our phenology models showed that seedling leaf-out is likely to outpace shifts to earlier canopy closure, indicating that seedlings will gain access to spring light under climate change conditions. Simultaneously accounting for shifts in phenology and changes in assimilation and respiration rates in response to climate change, our results indicate that spring carbon assimilation will increase by 71-176% under an extreme climate change scenario. In contrast, however, annual carbon assimilation will decrease by -10 to -210% because of increased respiration costs. Overall, we project that seedling phenological escape will help offset negative effects of climate change, but that performance will still be reduced, likely reducing recruitment success.
