Collaborative Research: Nutrient co-limitation in young and mature northern hardwood forests

北方幼龄和成熟阔叶林的养分共限制长期以来，温度林一直被认为是氮(N)受限的，但随着时间的推移，空气污染造成的长期N沉降应该会减少N的限制，理论认为生态系统的生产力应该受到多种养分的共同限制。在温带森林系统中缺乏N-P限制的实验测试。这项研究将结合建模和实地研究，探索调节与被管理的阔叶林生态系统的演替变化有关的养分共生的过程。更好地理解生态系统对资源利用的优化，包括维持共同限制，对于森林管理、环境保护和基本科学知识非常重要。平衡的养分循环对维持森林的可持续性至关重要，白山国家森林的科学家和管理人员将根据拟议研究的结果评估营林实践。例如，他们的森林管理计划要求根据森林获取养分以重新生长的能力来证明密集的收获转移是合理的。养分受限时矿物风化的生物增强等反馈显然与这一评价相关。从资源最优化理论和生态系统生物地球化学的角度来看，多元素限制(MEL)模型代表了共同限制。我们将模型扩展到包括P和N、碳、光和水，并将其应用于模拟北方阔叶林的初级和次生演替。在新罕布夏州巴特利特实验林三年生(~20年、30年和100年)的重复林分中，利用部分林分单独添加N、P和N+P的养分添加，我们将检验该模型预测的资源限制模式和获取N、P的多种分配机制。在年龄较大的林分中，模型预测对N的反应大于对P添加的反应，但在较年轻的林分中，该模型表明碎屑中的N供应应该足以产生P限制。

Nutrient Co-limitation in Young and Mature Northern Hardwood ForestsTemperate 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.