Understory trees and seedlings are often under-represented within analyses of ecosystem productivity in temperate forests. Further, the impacts of age-class and microclimate, and invasive/native categorization can complicate how we view carbon cycling physiology in understory trees. Here we examine leaf-level carbon cycling, fluorescence, leaf traits, and microclimatic differences in two woody mid-story species, Rhamnus cathartica (Buckthorn, an invasive shrub) and Prunus serotina (a native small tree) seedlings and trees at two points during the growing season in a temperate Upper Midwest US forest. Data collected from this study was used to address an open question as to the strong, persistent colonization abilities of R. catharica might be explained through adaptive and competitive growing season leaf-level physiology. Values of photosynthetic capacity, chlorophyll fluorescence, and leaf trait measurements were taken for each combination of species and age class at four plots within a temperate deciduous forest. Data was also collected on stomatal density and size, which lends a functional link from leaf traits to physiological processes.
Results/Conclusions
All measures of photosynthetic rate and efficiency (Vcmax, Jmax, Fv/Fm) were significantly greater for the tree age class of P. serotina, but did not differ significantly within R. cathartica age classes. The different photosynthetic responses across species may indicate an increased environmental or developmental sensitivity in P. serotina, which is not invasive, and a less sensitive regulation in R. cathartica, an invasive shrub. Further, the photosynthetic variables of Vcmax, Jmax, and Fv/Fm were similar across the mid-season for R. cathartica, but increased significantly from early-to-mid season for P. seritona. This may also indicate a response to shading as the canopy closes in that the invasive R. cathartica has adapted to be less sensitive towards in terms of optimal carbon cycling rates. Differences in developmental stage of the plant - seedling or tree - may matter less than environmental microclimate and access to limiting light in the understory, and may offer an advantage in the invasive R. cathartica in a closed canopy, staging the ground for expansion in temperate forests.