Photosynthetic induction and sunfleck responses of three understory tree species in forests of the Mid-Atlantic U.S

Key message

Photosynthetic induction was rapid in three shade-tolerant understory saplings, and surprisingly, native and non-native Acer species gained induction more quickly when exposed to sequential lightflecks, relative to constant saturating light.

Abstract

Growing regeneration mismatch between forest canopy and understory environments is evident in broadleaf forests of the eastern U.S., leading to novel understory forest composition. Three co-occurring shade-tolerant tree species in forest understories of southeastern Pennsylvania are the natives Acer rubrum and Fagus grandifolia and the non-native Acer platanoides. Physiological attributes contributing to these species’ relative success in the understory’s dynamic light environment are poorly characterized. Three saplings per species from three sites were sampled to examine photosynthetic induction and lightfleck responses. Predictors of photosynthesis (A) and stomatal conductance (g_s) during lightflecks were also examined. The rate and magnitude of photosynthetic induction were similar among species, with all gaining induction rapidly, as expected for shade-tolerant species. However, physiological strategies supporting induction responses varied by species, as A. rubrum had more responsive stomata and significantly lower stomatal limitation during induction, as well as significantly higher A/g_s when fully induced. Surprisingly, lightfleck responses differed from induction responses for the two Acer species, both of which exhibited significantly greater cumulative carbon assimilation than when exposed to constant saturating light, particularly during short-duration lightflecks. Leaf nitrogen and g_s were significant predictors of lightfleck responses, while species identity was not. Although induction gain under saturating light was rapid for all species, variable lightfleck responses favoring the Acer species provide an advantage given the intermittent nature of understory light availability. As eastern broadleaf forest composition continues to change in response to diverse pressures, the Acer species are well-positioned to persist.