Evidence for P limitation in eight northern hardwood stands: Foliar concentrations and resorption by three tree species in a factorial N by P addition experiment

Nitrogen and phosphorus, alone or in combination, are the nutrients most often limiting to plants. Resorption is one way plants conserve nutrients, thereby reducing dependence on nutrient uptake from soil. We investigated foliar nutrient concentrations, ratios, and resorption in three northern hardwood species growing in eight stands across three sites and in two age classes as part of a long-term N × P factorial fertilization experiment. We found that neither P nor N addition affected N resorption, but trees in plots receiving P addition exhibited lower P resorption. Foliar N:P ratios often indicated P limitation in the control and N plots, but co-limitation by N and P in plots where P was added, alone or with N. Green leaf N and P concentrations and P resorption were highest at the site with the highest N availability and intermediate P availability. Though these stands are in a region where trees are commonly assumed to be N limited, we found numerous indications of P limitation in these stands, as well as site and species differences in resorption proficiency and efficiency.