Ecosystem theory suggests that productivity in northern hardwood temperate forests on relatively young soils is expected to be nitrogen (N) limited. However, recent studies and modeling simulations based on resource optimization theory suggest that northern hardwoods are co-limited by N and phosphorus (P). We tested nutrient limitation of forest productivity in a full factorial NxP fertilization experiment in 13 northern hardwood forest stands of central New Hampshire. In this study we asked- (i) is forest productivity limited by the availability of N or P or is it co-limited by N and P, and (ii) are individual species limited by different nutrients, contributing to community-level co-limitation? Plots in each stand had been fertilized with either 30 kg N ha-1 yr-1, 10 kg P ha-1yr-1, the same amounts of N+P, or nothing (control) since 2011. The productivity response to treatments was tested using relative basal area increments, for each species individually and for all species together.
Results/Conclusions
Forest growth responded to P fertilization in the successional (p=0.02) and mature (p=0.07) stands suggesting that northern hardwood forest productivity is P limited. Only two species (white birch, p=0.00 and beech, p=0.03) responded significantly to P, driving the overall P effect. We found no evidence for nutrient co-limitation of productivity, at the ecosystem or at the community-level. Longer response time may reveal growth responses by other species. Our P limitation results were unexpected since they support neither the N limitation prediction of ecosystem theory, nor the N and P co-limitation prediction of resource optimization theory. The positive growth response to P additions could be a consequence of long-term anthropogenic N deposition in these forests, suggesting a potential N-induced secondary P-limitation.