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

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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.