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

在大多数温带森林中，养分的供应限制了树木的生长速度。氮是一种在许多生态系统中限制森林生长的营养物质。近几十年来，许多研究都集中在人类活动如何增加雨、雪和尘埃中大气氮对森林的输入。然而，北美大部分地区的这些大气氮源正在减少。在一些温带森林中，树木可获得的氮显著减少，这不能仅仅用大气输入的减少来解释。有些人担心，氮素供应的减少可能会降低森林生产力。氮的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，指导代表性不足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。本项目将为本科生、研究生和博士后提供培训。该研究项目将检验这样一种观点，即氮素供应相对于植物需求的减少，被称为氮少营养化，是由秋春季的季节变化引起的。这个想法需要对温带森林氮循环进行重大的重新评估。具体来说，美国北部阔叶林的氮少营养化可能是由温度上升、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所驱动的。反过来，这些过程导致秋季树木对氮的吸收增加，凋落物中碳氮比增加，土壤微生物在来年春季对氮的净固定化增加。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有效性。这些假设将在新罕布什尔的哈伯德布鲁克实验森林用观察、实验和建模的方法进行验证。该奖项反映了美国国家科学基金会的法定使命，并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，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.