In temperate deciduous forests, understory light environments vary dramatically throughout spring, summer, and autumn due to tree canopy leaf display. This poses a physiological challenge for understory species, particularly for plants that produce sun-adapted leaves in the spring before being shaded by the tree canopy and for those that display leaves late into autumn after tree canopy senescence. Failure to adjust to light level changes may result in lost carbon assimilation opportunities. Many understory plants with early and late season leaf display are also non-native invaders in Eastern North American forests. We measured leaf traits and photosynthetic capacity of 30 native and non-native understory shrub species growing in a common garden in Syracuse, New York, USA during spring, summer, and autumn of 2015. Seasonal plasticity photosynthetic parameters and leaf traits (quantum efficiency, photosynthetic capacity, rates of electron transport, carboxylation, and daytime dark respiration) were estimated using hierarchical Bayesian modeling techniques. We assessed the relative importance of mean and variance in trait values seasonally for predicting average net photosynthetic rate and annual leaf production rate. We also analyzed the extent to which leaf adjustments depended on an indeterminate leafing habit because new leaf tissue functions differently than old leaf tissue.
Results/Conclusions
Both native and non-native species leaf traits varied seasonally, but plasticity varied in magnitude and contribution to average photosynthetic rate and/or leaf production. Non-native invaders had the highest seasonal plasticity, but contrary to our hypotheses, seasonal plasticity did not contribute to average photosynthetic rate or leaf production for non-native species. Seasonal plasticity did, however, increase average photosynthetic rate for native indeterminate species, primarily through plasticity of a photosynthetic trait: maximum electron transport rate. Non-native invaders instead maximized average photosynthetic rates by maintaining higher photosynthetic trait values throughout the growing season.