Tue, Aug 03, 2021:On Demand
Background/Question/Methods
In Eastern deciduous forests, changes in dominant tree species composition between the canopy and understory that are consistent with forest mesophication suggest the development of novel forests in the near future. Although this pattern is widely documented, there is a limited understanding of physiological performance in emerging understory dominants; a group that includes invasive species. Therefore, assessing species-specific photosynthetic induction and responses to sunflecks will help elucidate mechanisms underlying the success of emerging understory forest dominants. This study sought to quantify the understory photosynthetic responses of the mesophytic species Acer rubrum and Fagus grandifolia, as well as the increasingly common non-native, invasive Acer platanoides. Three individuals per species (mean dbh: 6.4 cm, mean height: 6.3 m), each located at three forest understory sites in southeastern Pennsylvania, were selected for study. Leaves on each tree were used to measure photosynthetic induction curves, as well as photosynthetic responses to artificially generated sunflecks with a LI-6800. Leaf nitrogen per unit area (Narea) was determined for each leaf. Species-specific responses were analyzed with linear mixed-effects models where the random effect was site, and ANOVAs were performed for each model.
Results/Conclusions Photosynthetic rates in dark-adapted leaves did not vary among species at low light (understory conditions), nor during photosynthetic induction at 50, 90, or 100% of full induction. Similar results were observed for stomatal conductance (gs), though at full induction, gs was significantly depressed for A. rubrum (p < 0.05). Significant differences among species emerged when five consecutive 30 or 60 s sunflecks were applied to leaves. Although all species exhibited a significantly increased photosynthetic rate during each sunfleck time series (p < 0.0001), significantly lower photosynthetic rates were observed for F. grandifolia (p ≤ 0.001). This corresponded to a trend toward lower gs in F. grandifolia. Acer platanoides’ photosynthetic performance was indistinguishable from A. rubrum in all three experiments despite significantly lower Narea in the former species (p < 0.05). Variation in the potential for photosynthetic carbon gain exists among these species, such that the two Acer species are better adapted to exploit intermittent light availability in the forest understory. In particular, A. platanoides’ robust performance contributes to a growing body of research warning of the species’ strong competitive potential. Keeping this species in check will be important in supporting native tree diversity during this time of tremendous forest change.
Results/Conclusions Photosynthetic rates in dark-adapted leaves did not vary among species at low light (understory conditions), nor during photosynthetic induction at 50, 90, or 100% of full induction. Similar results were observed for stomatal conductance (gs), though at full induction, gs was significantly depressed for A. rubrum (p < 0.05). Significant differences among species emerged when five consecutive 30 or 60 s sunflecks were applied to leaves. Although all species exhibited a significantly increased photosynthetic rate during each sunfleck time series (p < 0.0001), significantly lower photosynthetic rates were observed for F. grandifolia (p ≤ 0.001). This corresponded to a trend toward lower gs in F. grandifolia. Acer platanoides’ photosynthetic performance was indistinguishable from A. rubrum in all three experiments despite significantly lower Narea in the former species (p < 0.05). Variation in the potential for photosynthetic carbon gain exists among these species, such that the two Acer species are better adapted to exploit intermittent light availability in the forest understory. In particular, A. platanoides’ robust performance contributes to a growing body of research warning of the species’ strong competitive potential. Keeping this species in check will be important in supporting native tree diversity during this time of tremendous forest change.