Nitrogen-Phosphorus Interactions in Northern Hardwoods Stands Indicate P Limitation: Foliar Concentrations and Resorption in a Factorial N by P Addition Experiment

Anthropogenic disruption of natural nutrient cycles is a driver of global environmental change but potential long-term effects are unclear. Little is known regarding the influence of relative nutrient availability on leaf nutrient concentrations and resorption, the process by which trees reabsorb foliar nutrients prior to leaf abscission. In the White Mountains of central New Hampshire, plots in eight natural forest stands of two age classes have been fertilized annually since 2011 with low doses of nitrogen (N; 30 kg N ha^-1 y^-1) and phosphorus (P; 10 kg P ha^-1 y^-1) in a full factorial design. We analyzed N and P concentrations of green leaves collected in August, 2015, and senesced leaves collected in October, 2015, from red maple (Acer rubrum), sugar maple (A. saccharum), and American beech (Fagus grandifolia) to compare foliar nutrition and resorption proficiency and efficiency within and among species, treatments, and forest age classes. Contrary to what would be predicted by ecological theory, control plots in these temperate forests had foliar N:P ratios in the P-limited range (20.0 ± 0.5). Foliar N:P ratios were 13% higher with N additions (p < 0.001) and 32% lower with P additions (p < 0.001). Resorption efficiencies of N and P were highly correlated with each other, with green leaf N concentrations, and with foliar N:P (p < 0.001). Thus, unlike most resorption studies, we found a greater proportion of N was resorbed with higher green leaf nutrient status. We also built upon previous studies in these stands by finding a positive correlation between green leaf N concentrations and P resorption efficiency, indicating multiple element control over resorption. Higher litter N and P concentrations with N additions (p < 0.03) suggest that higher nutrient availability lessens tree dependence on internal nutrient recycling and accelerates ecosystem turnover of N and P, with the potential for cascading effects on biogeochemistry and biodiversity.