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Kristin Odden Nystue and Øystein Opedal carry heavy wire exclosure cages into the Dovre mountains in central Norway. The cages were used to exclude sheep from grazing in areas three different communities. Credit: Photo: Mia Vedel Sørensen
Kristin Odden Nystue and Øystein Opedal carry heavy wire exclosure cages into the Dovre mountains in central Norway. The cages were used to exclude sheep from grazing in areas three different communities.

Sheep grazing may have little effect on alpine vegetation

Grazers appear to have little effect on low alpine shrub communities.

When animals graze, they affect the environment by keeping meadows open and fertilizing the soil with their feces, but some questions include what effect grazing has on overall biomass, if grazing affects carbon capture and how many plants -- and what type -- can survive.

A group of researchers at the Norwegian University of Science & Technology (NTNU) decided to find answers to these questions.

"This can tell us something about carbon sequestration in mountain plants and how grazers affect it," said Mia Vedel Sørensen in the NTNU department of biology. She recently earned her doctorate on the topic.

In 2013, the researchers set up 48 wire cages at various locations near Hjerkinn village in Norway's Dovre mountains. These were grazing areas where sheep dominate and have the occasional company of moose, lemmings, field mice, ptarmigan and a few reindeer passing through, NTNU said.

Researchers placed the cages in a meadow, a heath and a Salix shrub landscape with several different willow species.

The exclosures were designed to keep out resident herbivores so the researchers could eventually compare the vegetation inside and outside them, NTNU explained.

The following year, in 2014, the researchers planted willow cuttings (Salix) to simulate shrub expansion.

For the past few decades, shrub vegetation has taken over parts of the mountain where heather and meadows were more common 30 years ago, NTNU noted. However, grazing can slow down, and potentially even reverse, this expansion.

The researchers had to wait until the summer of 2015 to measure the different effects of their planting and cages. They recently published their results in BMC Ecology.

"We only found significant effects of the treatments in the heath community. The biomass there increased where the herbivores were kept out," Vedel Sørensen said.

In other words, they found that on the heath, growth was better inside the protected cages than outside, where the sheep could graze. They did not see the same effects in the meadow or shrub landscapes. "That surprised us to begin with," she said, noting that heath is the least attractive pastureland for sheep.

While the assumption would be that the animals would have a greater impact on grassy meadows or the area with shrubs, that assumption "actually did turn out to make sense," Vedel Sørensen said.

Perhaps the grazing was better elsewhere, but the sheep still liked being on the heath, because it was their favorite place to relax and chew their cud.

Where the sheep relax the most, they also defecate the most, which nourishes the plants and increases growth. In addition, the animals have an impact by trampling the vegetation. This gave some of the plants inside the cages a huge advantage and resulted in a greater difference between the protected and unprotected plant growth, NTNU said.

Shrub expansion may have effect

The researchers found that planting the shrubs showed no effects on the ecosystem, but that doesn't mean that the shrub expansion doesn't matter. Shrubs provide more shade, which, in turn, can affect other plants.

"We figure it's premature to judge these effects and that this area will provide some interesting results later," Vedel Sørensen said. She hopes that either she or others can follow up on these trials in a few years.

In an earlier study, Vedel Sørensen showed that meadows sequester the most carbon and heath the least of the three landscape types investigated. This project is part of a larger project called ECOSHRUB, which is led by professor Bente Jessen Graae and associate professor Richard Strimbeck from NTNU.

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