Temperate forests have long been thought to be nitrogen (N) limited, but chronic N deposition from air pollution should reduce N limitation over time, and theory suggests that ecosystem productivity should be co-limited by multiple nutrients. Experimental tests of N vs. phosphorus (P) limitation in temperate forest systems are lacking. This study will combine modeling and field studies to explore processes mediating nutrient colimitation in relation to successional change in managed hardwood forest ecosystems. An improved understanding of ecosystem optimization of resource use, including the maintenance of co-limitation, is important to forest management, environmental protection, and basic scientific knowledge. Balanced nutrient cycling is essential to maintaining forest sustainability, and scientists and managers at the White Mountain National Forest will evaluate silvicultural practices in light of results from the proposed research. For example, their forest management plans require justifying intensive harvest removals in terms of the ability of forests to acquire nutrients for regrowth. Feedbacks such as the biological enhancement of mineral weathering when nutrients are limiting are clearly relevant to this appraisal. The Multi-Element Limitation (MEL) model represents co-limitation from the perspective of resource optimization theory and ecosystem biogeochemistry. We have extended the model to include P as well as N, carbon, light, and water, and applied it to simulate primary and secondary succession in northern hardwood forests. Using nutrient additions of N alone in some forest stands, P alone in some forest stands, N+P in some forest stands, in replicated stands of three ages (~20, 30, and > 100 years) at the Bartlett Experimental Forest in New Hampshire, we will test the patterns of resource limitation predicted by the model and multiple mechanisms of allocation of effort to acquire N and P. Specifically, the model predicts a greater response of aboveground productivity to N+P than N or P alone. In older stands, it predicts a greater response to N than to P addition, but in younger stands, the model suggests that the supply of N from detritus should be sufficient to create P limitation.
