Collaborative Research: Changing Seasonality and Nitrogen Reductions in the Northern Hardwood Forest

在大多数温带森林中，养分的可用性限制了树木的生长速度。氮是一种营养素，已被证明限制许多生态系统中的森林生长。近几十年来，许多研究都集中在人类活动如何增加大气氮的输入，雨，雪和灰尘到森林。然而，在北美的大部分地区，这些大气氮源现在正在减少。在一些温带森林中，树木所能获得的氮显著减少，这不能仅仅用大气输入量的减少来解释。有人担心，氮供应减少可能会降低森林生产率。氮限制可能会降低森林生态系统对干扰和环境变化的反应能力。本项目将研究美国北方森林中氮的供应和需求。本项目还将开展一系列教育和推广活动。这些活动包括制定一项新战略，指导代表性不足的少数民族大学生进行研究。外联活动包括公众参与气候科学和生态系统生态学。该项目的研究成果将被纳入当地的环境宣传和科学传播计划。通过该项目将为几名本科生、研究生和一名博士后研究员提供培训。该研究项目将测试氮供应量相对于植物需求量的减少（称为氮贫营养化）是由秋季和春季的季节变化驱动的这一想法。这一想法需要对温带森林氮循环进行重大重新评估。具体而言，美国北方硬木林中的氮贫营养化可能是由于温度升高、生长季节延长和其他环境变化引起的植物对氮的需求增加而导致的。反过来，这些过程导致更大的氮吸收树木在秋季，增加的碳，氮在凋落物中的比例，更大的净固定氮的土壤微生物在第二年春天。然而，春季融雪和土壤冻结的时间可能会进一步影响植物的净矿化和氮的有效性。这些假设将在新罕布什尔州的哈伯德布鲁克实验森林中使用观察、实验和建模方法进行测试。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

In most temperate forests, the availability of nutrients limits rates of tree growth. Nitrogen is a nutrient that has been shown to limit forest growth in many ecosystems. In recent decades, much research has focused on how human activities have increased inputs of atmospheric nitrogen in rain, snow, and dust to forests. However, these atmospheric sources of nitrogen are now declining over large areas of North America. There have been remarkable declines in the availability of nitrogen for trees in some temperate forests that cannot be explained by declines in atmospheric inputs alone. There are some concerns that diminished nitrogen availability could reduce rates of forest productivity. Potentially, nitrogen limitation could reduce the capacity of forest ecosystems to respond to disturbances and environmental change. This project will study the supply and demand of nitrogen in a forest in the northern U.S. This project will also develop a set of education and outreach activities. These activities include development of a new strategy for mentoring underrepresented minority undergraduate students in conducting research. Outreach activities include public engagement with climate science and ecosystem ecology. The research results from this project will be integrated into local environmental outreach and science communication programs. Training will be provided through this project to several undergraduate students, graduate students, and a postdoctoral researcher.The research project will test the idea that reductions in nitrogen availability relative to demand by plants, called nitrogen oligotrophication, are driven by seasonal changes during autumn and spring. This idea requires a major reevaluation of temperate forest nitrogen cycling. Specifically, nitrogen oligotrophication in hardwood forests in the northern U.S. may be driven by increasing nitrogen demand by plants, caused by rising temperatures, longer growing seasons, and other environmental changes. In turn, these processes lead to greater nitrogen resorption by trees in autumn, increased ratios of carbon to nitrogen in litter, and greater net immobilization of nitrogen by soil microbes in the following spring. However, the timing of snowmelt and soil freezing in spring may further affect net mineralization and nitrogen availability for plants. These hypotheses will be tested using a combination of observational, experimental, and modeling approaches at Hubbard Brook Experimental Forest in New Hampshire.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.