The impact of herbivory within the treeline ecotone

Background/Question/Methods

Climate warming has been linked to elevational advance of treelines, species and communities and an increase in species richness in mountain vegetation.  However, grazing can also have substantial impacts on mountain vegetation, and alpine areas of Norway have a long history of livestock grazing.  Here we use a long-term and large-scale sheep grazing experiment in the mountains of southern Norway to investigate the influence of livestock grazing on treeline dynamics and plant communities. The distribution, height and radial growth of treeline birch were assessed and a dendroecological approach was used to disentangle the interactive influences of grazing and climate on annual growth of birch. Plant community dynamics were analysed using repeat vegetation surveys of permanent quadrats.

Results/Conclusions

Where sheep were removed, we observed recruitment of mountain birch far above the current treeline.  In comparison, in grazed areas recruitment of birch above the treeline was suppressed at even low densities of sheep. Sheep grazing also reduced both height and radial growth of birch within the treeline ecotone. The tree ring record showed that the effects of sheep grazing outweighed the effects of inter-annual climatic variation (covering a 4°C range). Plant community change within the treeline ecotone was minimal where sheep density was maintained at previous levels.  Where grazing ceased, there was a decrease in species richness at low elevations and an increase in species richness at higher elevations.  This occurred in conjunction with an elevational advance of grassland plant community composition of 3 m over 8 years.  Contrastingly, where sheep density was increased there was a decrease in species richness and down-slope shift of 4 m in grassland community composition. This study shows how herbivore-vegetation interactions can vary along an environmental gradient, and that grazing herbivores can buffer changes in treelines and treeline ecotone vegetation caused by other drivers.  This is highly important for the management of biodiversity and ecosystem services as well as for grazing management practice.