Phosphorus limitation of aboveground production in northern hardwood forests

Forest productivity on glacially derived soils with weatherable phosphorus (P) is expected to be limited by nitrogen (N), according to theories of long‐term ecosystem development. However, recent studies and model simulations based on resource optimization theory indicate that productivity can be co‐limited by N and P. We conducted a full factorial N × P fertilization experiment in 13 northern hardwood forest stands of three age classes in central New Hampshire, USA, to test the hypothesis that forest productivity is co‐limited by N and P. We also asked whether the response of productivity to N and P addition differs among species and whether differential species responses contribute to community‐level co‐limitation. Plots in each stand were fertilized with 30 kg N·ha⁻¹·yr⁻¹, 10 kg P·ha⁻¹·yr⁻¹, N + P, or neither nutrient (control) for four growing seasons. The productivity response to treatments was assessed using per‐tree annual relative basal area increment (RBAI) as an index of growth. RBAI responded significantly to P (P = 0.02) but not to N (P = 0.73). However, evidence for P limitation was not uniform among stands. RBAI responded to P fertilization in mid‐age (P = 0.02) and mature (P = 0.07) stands, each taken as a group, but was greatest in N‐fertilized plots of two stands in these age classes, and there was no significant effect of P in the young stands. Both white birch (Betula papyrifera Marsh.) and beech (Fagus grandifolia Ehrh.) responded significantly to P; no species responded significantly to N. We did not find evidence for N and P co‐limitation of tree growth. The response to N + P did not differ from that to P alone, and there was no significant N × P interaction (P = 0.68). Our P limitation results support neither the N limitation prediction of ecosystem theory nor the N and P co‐limitation prediction of resource optimization theory, but could be a consequence of long‐term anthropogenic N deposition in these forests. Inconsistencies in response to P suggest that successional status and variation in site conditions influence patterns of nutrient limitation and recycling across the northern hardwood forest landscape.