Rangifer management controls a climate‐sensitive tundra state transition

Abstract

Rangifer (caribou/reindeer) management has been suggested to mitigate the temperature-driven transition of Arctic tundra into a shrubland state, yet how this happens is uncertain. Here we study this much focused ecosystem state 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 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 Rangifer 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 Rangifer 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. Where Rangifer densities were above a threshold of approximately 5 animals/km , 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 Rangifer densities below this threshold, the small life stages of shrubs were taller and more abundant indicating Rangifer were no longer 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 Rangifer had 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 supports the proposal that Rangifer management within a sustainable range of animal densities can mitigate the much-focused transition from grassland to shrubland in a warming Arctic.

Holocene floristic diversity and richness in northeast Norway revealed by sedimentary ancient DNA (sedaDNA) and pollen

“…; Bråthen et al . ). The headwaters of the Komagelva River originate from the north to northwest of the main Komagelva channel, which flows eastwards to Varangerfjord (Fig.…”

Tactical departures and strategic arrivals: Divergent effects of climate and weather on caribou spring migrations

“…, Bråthen et al. , Schmitz et al. ), their importance as prey species for predators and scavengers (Reynolds et al.…”

“…A central challenge in arctic ecology is understanding the potential effects of a changing climate on caribou and reindeer (Rangifer tarandus), the most widespread terrestrial herbivore in the Arctic (Hummel and Ray 2008). The centrality of caribou to high latitude ecosystems is reflected in their high circumpolar abundance, their strong impacts on tundra vegetation (V€ ais€ anen et al 2014, Rickbeil et al 2015, Bichet et al 2016, Br athen et al 2017, Schmitz et al 2018, their importance as prey species for predators and scavengers (Reynolds et al 1987, Bergerud 1996, Young and McCabe 1997, Legagneux et al 2014, and their importance to the subsistence and culture of human groups throughout the Arctic (Burch 1972, Wolfe and Walker 1987, Hummel and Ray 2008. In North America, the most numerically abundant ecotype of caribou are the migratory barren-ground caribou, which undertake long-distance movements of several hundred kilometers between wintering, calving, and post-calving summer ranges (Fig.…”

Removal of grazers alters the response of tundra soil carbon to warming and enhanced nitrogen availability

“…, , Bråthen et al. ) that otherwise may respond positively to warmer temperatures and enhanced nutrient availability and induce soil carbon losses (Mack et al. , Parker et al.…”

“…Yet, they can also mediate how the ecosystem responds to other environmental changes (Tylianakis et al 2008, Callaghan et al 2013. The role of herbivores could be pronounced in locations where grazers suppress the abundance and growth of deciduous shrubs (Olofsson et al 2004, Br athen et al 2017) that otherwise may respond positively to warmer temperatures and enhanced nutrient availability and induce soil carbon losses (Mack et al 2004, Parker et al 2015. This warming and fertilization-induced shift in shrub abundance, and thereby in the shrub-mediated carbon sequestration processes, is unlikely to take place in high herbivore densities.…”