Cloud Cover Homogenizes Arctic Vegetation

Street, L., University of Edinburgh

Surface Properties

Cloud-Aerosol-Precipitation Interactions

Street LE, GR Shaver, EB Rastetter, MT van Wijk, BA Kaye, and M Williams. 2012. "Incident radiation and the allocation of nitrogen within Arctic plant canopies: implications for predicting gross primary productivity." Global Change Biology, 18(9), 10.1111/j.1365-2486.2012.02754.x.

Few places on Earth are as vulnerable to even slight changes in environmental conditions as the Arctic tundra—home to plant communities that thrive under cold conditions, under conditions of weak sunlight, and on permanently frozen soil (called permafrost). The tundra, however, with frozen soil and niche biology, acts as a vast reservoir of carbon—the building block of life—and nitrogen, a critical nutrient stored in plants and released back into to the environment upon decay.

A study published last year in the journal Global Change Biology shows that plant communities in the Arctic tundra may adapt their ability to store nitrogen, depending on how the clouds over the region scatter sunlight.

Lorna Street, an ecologist in the University of Edinburgh, made theoretical calculations about how cloud fraction (and associated solar energy reaching the surface) at five places across Sweden, Greenland, Alaska, and Svalbard can affect the rate of nitrogen and carbon exchange rates within plant communities.

According to Street’s calculation, under cloudier conditions, the nitrogen stored in plant leaves is spread more evenly throughout the plant canopy. This means that if a region becomes cloudier in the future, the total amount of nitrogen stored in plant leaves could increase, even double.

Street and her colleagues used data from the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility to verify the effects of diffuse versus direct solar radiation on the allocation of leaf nitrogen in Arctic plant communities.

Clouds scatter incoming sunlight; under cloudy conditions, the amount of sunlight reaching the Earth is more even, or diffuse. In areas without clouds or other light-scattering elements in the atmosphere, sunlight reaches the surface like a beam of light or direct radiation.

“We argued that some site-level [regional] differences in nitrogen allocation could be explained theoretically by the amount of diffuse radiation, i.e., where radiation is more diffuse, leaf N [Nitrogen] allocation is more uniform due to greater light penetration into the canopy. Our field measurements, together with radiation data from ARM, supported this argument,” explained Street in an email.

“This is an interesting relationship. It is important to realize that because of the generally low sun angle in the Arctic, it is true that most of the vegetation could be 'in the shade' from 'direct' radiation. The increased diffuse radiation could remove the sun angle effect,” said Raymond McCord, a former terrestrial ecologist currently working as a data manager at the Oak Ridge National Laboratory. McCord was not involved in the study.

Changes in nitrogen content affect the rate of decomposition and hence the amount of nitrogen and carbon that is released back to the atmosphere, as well as below the surface of the soil, where plants use nitrogen as a nutrient. The impact of this study, however, goes even further —in improving ecological models that attempt to simulate how Arctic vegetation will change in response to changing environmental conditions.