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Harvard Forest Symposium Abstract 2006

• Title: Nitrogen isotope evidence for ecological changes and adaptations relating to agricultural land-use legacies in the foliage of four New England plant species
• Primary Author: David Diaz (Harvard University)
• Additional Authors: Noreen Tuross (Harvard University)
• Abstract:

  Agricultural land-use has been demonstrated to result in distinct ecological legacies which may endure for more than a century following agricultural abandonment and natural reforestation. Of particular interest to this study, historical land-use has been found to significantly contribute to persistent changes in forest biogeochemistry and biogeography. The extensive documentation of historical land-use at Harvard Forest provides an ideal location for the evaluation of agricultural land-use legacies.
  
  
  
  
  
  The application of stable isotope methodologies within the field of ecology has offered an exceptional opportunity for the untangling of complex ecological processes which remain poorly understood. The application of stable isotope methods to land-use history is relatively new, but has provided some promising results for identifying and characterizing land-use legacies. Despite these promising developments, in terms of their contribution to plant ecology, earlier studies showing unique land-use signatures in foliar and soil N isotope concentrations have either focused predominantly on soils or utilized only a single plant species for foliar analysis. In order to incorporate a larger scope for approximating plant responses to changes in soil N transformations induced by historical agricultural land-use, we carried out a survey of four plant species across different historical land-use types on the Prospect Hill tract of Harvard Forest. Utilizing the four land-use categories of woodlot, unimproved pasture, improved pasture, and cultivated, we collected and analyzed the stable isotope concentrations of foliage from the club moss Lycopodium obscurum, eastern hemlock (Tsuga canadensis), wintergreen (Gaultheria procumbens), and cinnamon fern (Osmunda cinnamonea).
  
  
  
  
  
  Three of the four plants sampled demonstrated statistically significant trends across land-use types although the direction of the trends varied with plant species. These differences in the direction of land-use related trends significantly complicate the diagnostic capabilities of foliar isotope analysis. Revision of existing models for interpreting foliar δ15N suggests that the differences in observed trends among sampled plants relate to specific preferences for different forms of available nitrogen. Based on observed trends in foliar δ15N of the four plant species, it is likely that a combination of nitrogen source availability, the baseline isotopic values of the nitrogen sources and the uptake/metabolic fractionation of the plant are likely sources for the observed trends. The evidence for specialization in the acquisition of soil nutrients is consistent with recent findings of N partitioning in arctic and grassland ecosystems. The results of this survey also suggest that these plants may actively respond to changes in the concentration and species of soil inorganic N driven by changes in soil nutrient transformations related to agricultural land-use legacies.
  
  
  

• Research Category: Historical and Retrospective Studies
  Physiological Ecology, Population Dynamics, and Species Interactions
  Soil Carbon and Nitrogen Dynamics