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。氮是一种养分，已被证明在许多生态系统中会限制森林生长。近几十年来，许多研究都集中在人类活动如何增加了雨、雪和尘埃中的大气氮对森林的输入。然而，这些大气中的氮源现在正在北美的大片地区减少。在一些温带森林中，树木的氮素利用率显著下降，这不能仅用大气输入的下降来解释。有些人担心，氮素有效性的减少可能会降低森林生产率。氮素限制可能会降低森林生态系统应对干扰和环境变化的能力。该项目将研究美国北部森林中氮素的供需情况。该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，指导未被充分代表的少数民族本科生进行研究。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推广和科学传播计划。研究项目将通过这个项目为几名本科生、研究生和一名博士后研究员提供培训。研究项目将测试这样一种想法，即相对于植物需求的氮素可获得性的减少，即氮素少营养，是由秋季和春季的季节性变化驱动的。这一想法需要对温带森林氮素循环进行重大重新评估。具体地说，美国北部阔叶林中的氮素营养不足可能是由于气温上升、生长季节延长和其他环境变化导致植物氮素需求增加所致。反过来，这些过程导致树木在秋季吸收更多的氮，增加凋落物中的碳/氮比例，并在第二年春天增加土壤微生物对氮的净固定。然而，春季融雪和土壤结冰的时间可能会进一步影响植物的净矿化和氮素有效性。这些假设将在新汉普郡的哈伯德布鲁克实验森林采用观察、实验和建模相结合的方法进行测试。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.