Bottomland hardwood forests once covered vast areas of the southeastern United States, but have been diminished due to agricultural land conversion and changes in hydrology. These forests are typically species rich and can form numerous plant community types depending on flooding regimes and soil quality. Despite their limited extent, they remain important in terms of flooding mitigation, water quality, carbon sequestration and habitat provision and numerous afforestation efforts have been employed throughout the region. In order for these projects to be successful and to better understand the ecosystem services provided by individual species, physiological parameters of photosynthetic carbon uptake, transpiration and leaf nitrogen content need to be assessed across species. In addition, determining if leaf anatomical and nutrient parameters are correlated with physiological functioning may allow for better predictions of photosynthetic parameters important for ecosystem models in these forest systems. Therefore, we measured leaf gas exchange including A/Ci curves, stomatal properties, carbon (C) and nitrogen (N) isotopes, and C and N concentrations in mid-canopy and overstory trees growing in a seasonally inundated hardwood forest in central Mississippi. Measured species included Carya ovata, Fraxinus pennsylvanica, Quercus pagoda, Q. phellos, Q. nigra, Q. michauxii, Ulmus americana, and U. alata.
Results/Conclusions
For photosynthetic gas exchange parameters, species were similar within genera, but genera differed with F. pennsylvanica exhibiting significantly greater net photosynthetic rates, Rubisco-limited carboxylation rates (Vcmax), transpiration rates and stomatal conductances. Stomatal lengths and densities differed significantly across all measured species with Fraxinus and Ulmus generally having larger, fewer stomata and Quercus and Carya having smaller, more frequent stomata. Leaf nitrogen concentration also differed by species and was highest in Q. pagoda, Fraxinus and Carya and lowest in the other Quercus species and Ulmus. In this bottomland forest across the measured species, Vcmax was significantly and negatively correlated with leaf mass per unit area, vein density, stomatal density and leaf C/N ratio and positively correlated with photosynthetic nitrogen use efficiency (PNUE). PNUE was significantly and positively correlated with stomatal conductance and transpiration and negatively correlated with intrinsic water use efficiency. Overall, these data suggest that, despite species differences in leaf anatomy and N concentration, leaf-level photosynthetic parameters were statistically similar suggesting a convergence in photosynthetic functional diversity across genera. There was also a tradeoff between using water or nitrogen efficiently and greater leaf nitrogen and nitrogen use efficiency were more important for increased photosynthetic capacity than increased water uptake capacity.