Nitrogen–phosphorous interactions in young northern hardwoods indicate P limitation: foliar concentrations and resorption in a factorial N by P addition experiment

Resorption, the process of withdrawing foliar nutrients prior to leaf abscission, is one of the most important nutrient conservation mechanisms in trees. Along with foliar nutrient concentrations, foliar resorption can be used to infer nutrient limitation. We collected green and senesced leaves of five species in early successional stands in the White Mountains of New Hampshire. In unmanipulated controls, foliar N:P ratios ranged from 20 to 31 and litter N:P ratios ranged from 19 to 36. These values suggest P limitation, although this forest type has been assumed to be N-limited. Additionally, N:P resorption ratios in control plots were < 1, reflecting proportionately more conservation of P through resorption than N. Four years into a full-factorial N × P fertilization experiment, N and P additions had increased N and P concentrations in leaves; more importantly, P addition reduced N concentration, possibly indicating alleviation of growth limitation by P. Resorption of P was less proficient (indicated by the concentration of an element in leaf litter) with P addition, as expected. Resorption proficiency and efficiency (the proportion of leaf nutrients resorbed) of N increased with P addition, suggesting increased demand for N with alleviation of P limitation. Resorption of P was more proficient and efficient with N addition, consistent with exacerbated P limitation. Temperate forests on glaciated soils are generally thought to be N-limited, but long-term N × P manipulations in this biome are lacking. Our results suggest that decades of anthropogenic N deposition may have tipped the balance to P limitation in these forests.