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.

Phenological mismatch with trees reduces wildflower carbon budgets

“…This seasonal period of high light availability before overstorey canopy closure has been shown to be critical for spring wildflowers (Ida & Kudo ; Heberling et al . ). We show this critical period has decreased by nearly 1 week in Concord since Thoreau initiated this study over 160 years ago.…”

“…The length of spring was adjusted proportionally by species, as described in Eqn 1–3 below:where for each species, sp, Daily C gain is estimated from photosynthetic measurements (Methods ; Heberling et al . ), DOY emerge is the median day of year for leaf emergence in 2016, 143 is the day of year of canopy closure in 2016 (first day of ‘summer’), and Δ spring length is the net change between tree and wildflower phenology compared to 2016.…”

“…To model the effects of phenological mismatch on spring carbon gain in the understorey, we used detailed data in an ecologically similar set of herbaceous species from a separate companion study (see Heberling et al (2019) for complete data set and results).…”

“…; Heberling et al . ) – a common shade avoidance strategy known as ‘phenological escape’ (Jacques et al . ).…”

“…Plant growth in deciduous forest understories is controlled by this dramatic and predictable seasonal event, with full sunlight in the spring followed by low light levels soon after trees leaf out (Hutchison & Matt 1977). Many forest herbs (hereafter 'wildflowers') emerge and flower in the early spring to exploit this critical period of high light (Lapointe 2001;Kudo et al 2008;Augspurger & Salk 2017), reaching maximum photosynthetic rates that abruptly decline as the overstorey trees leaf out and the canopy closes (Rothstein & Zak 2001;Kudo et al 2008;Heberling et al 2019)a common shade avoidance strategy known as 'phenological escape' (Jacques et al 2015). This phenomenon is most dramatic among spring ephemerals, which go dormant after overstorey leaves appear.…”