A transition of arctic tundra into a shrubland state is currently driven by temperature increase and with feedback to the climate system. Here we study how land use by reindeer husbandry are mitigating this transition in riparian areas, where palatable willows (Salix) are dominant tall shrubs and highly responsive to climate change. For the state transition to take place, small life stages of Salix must become tall and abundant. Therefore we predicted that the performance of small life stages (potential recruits) of the tall shrubs were instrumental to the focal transition, where reindeer managed at high population density would keep the small-stage shrubs in a “browse trap” independent of summer temperature. We used a large-scale quasi-experimental study design that included real management units that spanned a wide range of reindeer population densities and summer temperatures in order to assess the relative importance of these two driving variables. Ground-surveys provided data on density and height of the small shrub life stages, while the distributional limit (shrubline) of established shrublands (the tall shrub life stage) was derived from aerial photographs.
Results/Conclusions
Where reindeer densities were above a threshold of approximately 5 animals km-2 we found, in accordance with the expectation of a “browse trap”, that the small life stages of shrubs in grasslands were at low height and low abundance. At reindeer densities below this threshold the small life stages of shrubs were taller and more abundant indicating reindeer was not in control of the grassland state. For the established shrubland state, we found that the shrubline was at a 100 m lower elevation in the management units where reindeer have been browsing in summer as opposed to the migratory ranges with no browsing in summer. In both seasonal ranges the shrubline increased 100 m per 1°C increment in temperature. Our study shows how land use management can mitigate the transition from grassland to shrubland in a warming arctic.