In greening Arctic, caribou and muskoxen play key role

The study, published today in the journal PNAS Nexus, highlights the importance of large herbivores to the Arctic ecosystem, linking grazing with plant phenology and abundance in the Arctic tundra.

Phenology is the study of the timing and cyclical patterns in nature, such as when birds migrate, or when a plant first sprouts or blooms. Understanding such patterns is critically important in the Arctic, which is warming faster than anywhere on Earth.

"Caribou and muskoxen play a key role in how soon plants emerge and this translates to how abundant they become," said lead author Eric Post, a professor and arctic ecologist in the UC Davis Department of Wildlife, Fish and Conservation Biology. "This is an important, and overlooked, factor we need to consider as we seek to more fully understand climate change impacts on tundra vegetation in the Arctic."

Exclusion experiment

The research was conducted at a long-term study site near Kangerlussuaq, Greenland, where Post has been studying plant and animal interactions for 22 years. To understand how herbivores affect the timing of plant growth and abundance, the scientists excluded caribou and muskoxen from some study areas. From 2009 to 2017, they compared the timing of spring green-up of nine plant species with and without the grazing animals.

In general, plants where caribou or muskoxen were present experienced earlier green-up and greater abundance later in the growing season. About two-thirds of plants greened up earlier, and three-quarters were more abundant later in the season compared to plots without grazing. These include arctic draba and gray willow.

Some species, including dwarf birch and harebell, emerged later with grazers present. Dwarf birch was the only species studied that did not increase in abundance under grazing. In all cases, the presence or absence of large grazing wildlife influenced how the plants responded.

Grazing awareness

Post said it is not yet clear why the plants respond in this way, but it is important to understand that there is a connection.

"We're used to thinking of the timing of plant availability as impacting the productivity of grazing animals, but not the reverse," Post said. "The absence or presence of herbivores can also impact the timing of plant growth and their productivity."

This is especially important considering thatmany caribou populations in the Arctic are in decline. Migratory tundra caribou (Rangifer tarandus) are listed as vulnerable by the IUCN Red List and have lost more than half of their total abundance since the 1990s.

A separate study coauthored by Post and published Nov. 7, noted that supporting sustainable populations of herbivores in the Arctic could be a more effective nature-based solution to climate change in the region than planting trees there.

Co-authors for the PNAS Nexus study include UC Davis alum Conor Higgins of the Yolo County Resource Conservation District, Pernille Bøving of UC Davis, Christian John of UC Santa Barbara, Mason Post of the University of Washington, and Jeffrey Kerby of the Scott Polar Research Institute at Cambridge University. The authors give special thanks to the late Mads Forchhammer for his critical input and inspiring the study.

The study was funded by the U.S. National Science Foundation, National Geographic Society, European Union's Horizon 2020 research program, and Aarhus University Research Foundation.