Judith Sitters, Mehdi Cherif, Dagmar Egelkraut, Reiner Giesler and Johan Olofsson
Large mammalian herbivores occur in many terrestrial ecosystems and have large impacts on plant communities. They can for example change the availability of growth-limiting nutrients for plants, such as nitrogen (N) and phosphorus (P). Through this, herbivores might change nutrient limitation of plant growth (between N and P limitation) and thereby impact plant productivity.
Reindeer (or caribou) are important herbivores in arctic tundra and they are known to affect soil nutrient availabilities. In this study we examined the effect of long-term light and heavy reindeer grazing on nutrient limitation of plant growth in a Scandinavian arctic tundra. We performed a fertilization experiment, whereby we added N and P in a full-factorial manner, across the two grazing regimes in heath and meadow vegetation.
The productivity of the plant communities showed contrasting responses to our fertilization treatments under light and heavy reindeer grazing. Under light grazing, productivity increased with N and P additions in both the heath and meadow vegetation. Under heavy grazing, productivity increased when N was added in the heath, and increased even more when N was added together with P. In the meadow, productivity increased when P was added, either alone or in combination with N.
These results clearly show that heavy and long-term grazing by reindeer promoted P limitation of plant communities in both the heath and meadow vegetation. The mechanisms behind the shift towards these conditions were not the same in both vegetation types. In the N-poor heath, reindeer increased soil N availability due to a shift towards more N-rich graminoids, while in the meadow reindeer decreased soil P availability. It remains unclear how reindeer decrease soil P, but they may simply export more P from the system than N due to their large P demand for the production of their antlers.
In this study, we have experimentally confirmed that heavy grazing by large mammalian herbivores for a long time has the potential to change nutrient limitation of plant growth. Such a change can have significant ecological impacts, including changes in plant species composition and success of alien plant species.