Species diversity and competition influence nitrogen resorption efficiency in mixed hardwood plantations

Nutrient dynamics substantially influences forest productivity, yet the influence of competition on these processes in forests is not well understood. Nitrogen is an important limiting nutrient for tree growth, but how canopy nitrogen dynamics are influenced by tree diversity and competition is unclear. Nitrogen-use efficiency is one mechanism suggested to support niche complementarity, promoting increased productivity as diversity increases, and is influenced by the resorption of foliar nutrients during senescence. We tested how diversity and competition affect canopy nitrogen dynamics by examining nitrogen resorption efficiency (NRE) in over 200 trees from three tree species in a 12-year-old mixed-species hardwood planting that included a full-factorial design of three levels of species diversity, representing intra- and interspecific competition, and two levels of planting density, representing competition intensity. Midseason and senescent foliar samples were collected from black cherry (Prunus serotina), American chestnut (Castanea dentata), and northern red oak (Quercus rubra) and we determined midseason canopy and foliar litter nitrogen concentrations and calculated NRE. Higher midseason canopy nitrogen concentrations were observed in more diverse, low-density plantings, but the response magnitude varied among species. NRE increased as diversity increased; the response, however, varied among species, ranging from a statistically significant increase to no response, but was not influenced by planting density. These findings provide evidence that while the influence of diversity on tree physiological responses depends on the type of competitive interaction (i.e., diversity or intensity), different combinations of species will likely affect processes related with niche complementarity and, ultimately, a positive biodiversity-productivity relationship.