Temperate forests are traditionally assumed to be N-limited. However, changes in the N cycle, such as anthropogenically caused increases in atmospheric N deposition, are affecting the biogeochemistry of these forests. Foliar nutrient concentrations and N:P ratios can be used as an indicator of limitation status. We collected green and senesced leaves of five dominant species (American beech, pin cherry, red maple, white birch, and yellow birch) in early successional stands in the White Mountains of central New Hampshire in which N and P had been added for 4 growing seasons in a full factorial design. We analyzed leaves for N and P concentrations and calculated the N:P ratio. We also calculated N and P resorption efficiency, the proportion of nutrients withdrawn from senescing leaves before abscission.
Results/Conclusions
In unmanipulated controls, foliar N:P ratios ranged from 17 - 24 and litter N:P ratios ranged from 18 - 42, depending on the species. These values are indicative of P limitation, although this forest type has been assumed to be limited by N. In addition, N:P resorption ratios in the control plots were <1, suggesting proportionately more conservation of P through resorption than N. Nitrogen and P additions increased N and P concentrations in leaves; more importantly, P addition reduced N concentration, possibly indicating alleviation of growth limitation by P. Resorption proficiency (indicated by the concentration of an element in leaf litter) of N increased with P addition, suggesting increased demand for N with alleviation of P limitation. Both resorption proficiency and efficiency (the proportion of leaf nutrients resorbed) of P were higher with N addition, consistent with exacerbated P limitation. These results provide evidence for P limitation in these temperate forests. Red maple had the lowest foliar N and P concentrations, and pin cherry had the highest, but both species had low green leaf N:P ratios, suggesting a possible competitive advantage in forests experiencing a shift to anthropogenically induced P limitation.