One key findings is that forests with high levels of nitrogen in their leaf canopies have a direct relationship with cooling the earth by reflecting more sunlight and absorbing more carbon dioxide than their low-nitrogen counterparts. The study will be published next week in the journal Proceedings of the National Academy of Sciences

While scientists have long known that individual nitrogen-rich leaves are usually more efficient at pulling carbon dioxide out of the atmosphere, it was previously unclear whether this applied to entire ecosystems. The University of Minnesota scientists and their team found that in fact, the photosynthetic capacity of a forest is proportional to the concentration of nitrogen in the canopy.

The researchers also discovered that variation in forest nitrogen can be accurately assessed from satellites. As a result, it is possible to continuously track these global-scale effects of forests on the earth's climate system.

The discoveries add a twist to scientists' understanding of the climate system and raise questions about the underlying nature of ecosystem-climate interactions. Changes in climate, rising atmospheric carbon-dioxide levels, air pollution, land use and species composition all influence nitrogen levels in foliage, and all of these may be part of climate feedback mechanisms that climate models have not yet examined.

Department of forest resources professors Peter Reich and Paul Bolstad, along with research associate Bruce Cook, say they stumbled upon the discoveries while poring over six years' worth of data collected from research sites across North America. The study involved a novel combination of NASA satellite- and aircraft-based instruments, along with meteorological towers from the AmeriFlux network and leaf-level measurements, to analyze various aspects of forest canopies. When the scientists noticed that the overall reflectivity of forest canopies (also known as albedo) rose and fell in conjunction with leaf nitrogen, they had a eureka moment.

The Minnesota researchers were part of a larger team led by Scott Ollinger of the University of New Hampshire. The team's next study will examine how the relationships between nitrogen, carbon dioxide and other factors affect climate; they'll also work with climate modelers to determine how the nitrogen-albedo and nitrogen-carbon uptake mechanisms will influence predictions of climate change.

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From University News Service, November 21, 2008