Abstract
Research Highlights: Bottomland hardwood forests exhibit seasonal flooding, are species diverse, and provide numerous ecosystem services including floodwater storage, wildlife habitat and nutrient mitigation. However, data are needed to adequately predict the potential of individual species to achieve these services. Background and Objectives: In bottomland hardwood forests, increasing tree species richness may increase functional diversity unless species exhibit an overlap in physiological functioning. Therefore, the objectives of this study were to (1) compare physiological and anatomical leaf parameters across species, (2) determine if leaf anatomical and nutrient properties were correlated with physiological functioning, (3) determine intra-species variability in leaf stomatal properties and determine how whole crown metrics compare with leaves measured for gas exchange and (4) measure soil nitrogen for evidence of denitrification during inundation periods. Materials and Methods: We measured gas exchange, leaf nutrients and anatomical properties in eight bottomland hardwood species including Carya ovata, Fraxinus pennsylvanica, Quercus michauxii, Quercus nigra, Quercus pagoda, Quercus phellos, Ulmus alata and Ulmus americana. Additionally, we quantified soil ammonium and nitrate content during winter inundated conditions to compare with non-inundation periods. Results: We found that leaf-level water use parameters displayed greater variability and diversity across species than photosynthesis and leaf nitrogen parameters, but green ash and shagbark hickory exhibited generally high leaf N concentrations and similar physiological functioning. Elms and oaks displayed larger variability in leaf physiological functioning. Stomatal density was significantly correlated with photosynthetic capacity and tree-level water use and exhibited high intra-species variability. Conclusions: This bottomland hardwood forest contains more diversity in terms of water use strategies compared with nitrogen uptake, suggesting that differences in species composition will affect the hydrology of the system. Green ash and shagbark hickory exhibit higher leaf nitrogen concentrations and potential for nutrient mitigation. Finally, leaf anatomical parameters show some promise in terms of correlating with leaf physiological parameters across species.
Highlights
Bottomland hardwood forests are an important, but understudied, ecosystem relative to other temperate forests
More data are needed to confirm that our largely positive δ15 N values correspond to soil N losses. These findings highlight the degree of functional redundancy for leaf-level gas exchange in this bottomland hardwood forest both within and across genera, with most species displaying statistically similar leaf-level physiological parameters with the exception of green ash in most cases
This statistical similarity is the result of the high variability within species, with individuals in these genera located throughout PC1 which represented leaf physiological functioning
Summary
Bottomland hardwood forests are an important, but understudied, ecosystem relative to other temperate forests. Once a prevalent and continuous system, a majority of bottomland hardwood forests in the United States (US) have been lost or fragmented due to anthropogenic activities [1]. The current extent of bottomland hardwood forests is most prevalent on the Gulf and Atlantic. Bottomland hardwood forests are made up of over 70 different tree species, co-occurring to form numerous plant community types [3]. The development of bottomland hardwood plant communities is strongly influenced by relative flooding depth and duration, with the possible number of suitable community types increasing from hydric to mesic sites [3]. The restoration of bottomland hardwood forests can be an effective tool for removing excess nitrogen from systems [5], which will reduce the eutrophication of nearby waterways
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