Carbon gain phenologies of spring-flowering perennials in a deciduous forest indicate a novel niche for a widespread invader.

Abstract

Strategies of herbaceous species in deciduous forests are often characterized by the timing of life history phases (e.g. emergence, flowering, leaf senescence) relative to overstory tree canopy closure. Although springtime photosynthesis is assumed to account for the majority of their annual carbon budgets, the 12-month photosynthetic trajectories of forest herbs have not been quantified. We measured the temporal dynamics of carbon assimilation for seven native herbaceous perennials and the biennial Alliaria petiolata, a widespread invader in eastern North American forests. We assessed the relative importance of spring, summer, and autumn to species-level annual carbon budgets. Spring-emerging species showed significant variation in carbon assimilation patterns. High spring irradiance before canopy closure accounted for 39-100% of species-level annual carbon assimilation, but summer and autumn accounted for large proportions of some species' carbon budgets (up to 58% and 19%, respectively). Alliaria was phenologically unique, taking advantage both autumn and spring irradiance. Although spring-emerging understory species are often expected to rely on early-season irradiance, our results highlight interspecific differences and the importance of mid-late season carbon gain. Phenological strategies of forest herbs are a continuum rather than discrete categories, and invasive species may follow strategies that are underrepresented in the native flora.