Reindeer trampling promotes vegetation changes in tundra heathlands: results from a simulation experiment

By Dagmar D. Egelkraut, Hélène S. A. Barthelemy & Johan Olofsson

The photo above shows a beautiful rolling hillside in the north of Norway. It also shows a fence; those who look more closely might notice a distinct difference between the vegetation on one side of the fence compared to the other.

This fence, installed in the 1960s, separates grazing ground for reindeer (at the far side of the fence) from land that reindeer have access to only sparingly. The influence on the vegetation composition is striking. It illustrates the general effect that grazers can have on vegetation: without reindeer, typical slow-growing arctic species such as dwarf birch (Betula nana) and crowberry (Empetrum nigrum ssp. hermaphroditum) dominate. With reindeer present, however, the vegetation can rapidly shift towards a state dominated by grasses and herbs, which are resistant to, and can benefit from, the grazing pressure.

But what is grazing exactly? This general term summarizes the combined impact of several activities of grazing animals. Taking reindeer as an example, they actually do many different things. They eat plant material (defoliate), they defecate (add nutrients), they walk around and trample on the vegetation. This trampling also results in a substantial reduction of the moss layer, which is especially important in cold areas where moss forms an important insulating layer on the soil.

We wanted to find out how each of these activities separately brings about vegetation change. This is important, because grazers don’t always perform all grazing activities at the same time and location. Reindeer migration routes, for example, are more affected by trampling than by defoliation.

We set out to test the importance of the separate activities on vegetation composition using an experiment. Our study was set up right alongside that fence shown in the picture above – on the side where reindeer have no access. We chose this location because we knew that here, a transition towards grass-dominated vegetation could occur in just a few years.

Because it takes a lot of effort to train reindeer to only eat, trample, or defecate in one spot, we mimicked the activities ourselves. We used scissors to cut leaves of plants preferred by reindeer, we collected fresh reindeer faeces (true story, see the photo) to deposit on our plots, removed moss, and simulated trampling by dropping a heavy, pointed pole 100 times from knee height in the plot. These treatments were applied individually, and in various combinations, on the vegetation.

After five summers of applying these treatments, we found that the vegetation had changed: especially when we had applied all treatments simultaneously, the community was different from controls. Interestingly, these changes were mostly caused by a decrease in the woody species, which did not recover well from the repeated trampling impact. What surprised us was that even after adding fertilizer (reindeer dung) for five years, we did not see the increase in grass cover that we expected. Why? Maybe the grasses just take a little longer to respond and establish. But we also suspect that reindeer urine, which was not included in this study, might have an essential role in promoting grass growth. The follow-up experiment should be interesting…

This is a plain language summary for the paper of Egelkraut, Barthelemy & Olofsson published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12871). The post was prepared by Dagmar Egelkraut.