A moose stands on a hill, partially occluded by shrubbery. Pine trees rise in the background.
Moose prefer munching the thin branches and nutritious leaves of deciduous trees, but acidic pine will do in a pinch. Credit: Bryan Rodriguez, Unsplash

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Source: Journal of Geophysical Research: Biogeosciences

Moose can cool boreal forests, according to a new study. Salisbury et al. show that these large grazers expose gaps between trees, revealing a snowy forest floor that reflects light and keeps temperatures down.

Boreal forests are found at high latitudes in North America, Europe, and Asia where temperatures are at or below freezing for at least 6 months of each year. They cover about 17% of Earth’s land surface, and their trees sequester carbon, shelter animals, and supply humans with about 25% of our lumber.

After timber harvests, the clear-cut trees grow back, and herbivorous moose make meals out of new growth. As the forests’ largest herbivores, the plants that moose eat and trample determine, in part, how forests recover—and where the carbon goes.

In the new study, over the course of a decade, researchers tracked how clear-cut forest sites throughout Norway grew back when moose both could and could not access them. They measured how moose change the structure, composition, carbon emissions, and albedo of boreal forests that have been clear-cut.

Sites with moose grew back more sparsely than sites that excluded them. Sparse trees resulted in a thinner canopy, which allowed sunlight to reach the forest floor. The overall reflectivity—the albedo—of a thin canopy and a bright forest floor is higher than that of a dense canopy, especially when snow coats the ground. That bounces sunlight back to space and cools the forest for several years after clear-cutting. The effect fades over time, as the forest canopy grows denser and darker.

Clearing trees affects carbon storage, too, and that might not be great news. The moose consume a lot of plant biomass, which removes carbon from vegetation and cycles it back through soil and feces. The researchers estimate that for sites with high moose density (about one moose per square kilometer), carbon emissions and lost carbon storage from moose activity accounted for 40% of regional carbon emissions. Still, the cooling effect from increased albedo offsets about two thirds of the warming the emissions would cause, the researchers found.

The findings, the authors say, will help forest managers regulate moose species after timber harvests, control forest growth, and ultimately minimize emissions and warming from forested regions. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2022JG007279, 2023)

—Rebecca Dzombak, Science Writer

Citation: Dzombak, R. (2023), Munching moose cool forest floors, Eos, 104, https://doi.org/10.1029/2023EO230084. Published on 7 March 2023.
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