Herbivores inhibit climate‐driven shrub expansion on the tundra

Abstract

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana, has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (40.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (o0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.

Changes in Tree Growth, Biomass and Vegetation Over a 13-Year Period in the Swedish Sub-Arctic

“…The latest in 2004 severely defoliated birches in our study area, both at the north and south side of the lake (Babst et al 2010). Thus, the biomass of shrubs and trees could potentially have been even higher without reindeers or geometrid moths (Cairns and Moen 2004;Karlsson et al 2005;Olofsson et al 2009;Babst et al 2010;Van Bogaert et al 2011). Further, we cannot exclude the possibility that the observed increased biomass and cover of shrubs and trees was caused by past anthropogenic activities associated with railway construction.…”

“…The observed increase in shrub and tree cover and tree biomass may, however, be related to several factors other than climate, such as reduction in herbivory pressure (e.g., Olofsson et al 2009) or changed land management (Emanuelsson 1987;Karlsson et al 2007). It is well known that shrubs and trees increase in abundance and biomass with decreased herbivory (Cairns and Moen 2004;Olofsson et al 2009;Van Boagaert et al 2011).…”

“…It is well known that shrubs and trees increase in abundance and biomass with decreased herbivory (Cairns and Moen 2004;Olofsson et al 2009;Van Boagaert et al 2011). However, the browsing pressure has probably increased rather than decreased over the last 13 years as the population of the major herbivore, reindeer, has increased in the study area since 1995 (Van Boagaert et al 2011).…”

“…Plant community composition and the distribution of mountain birch forest and alpine vegetation are, however, not solely determined by climate. Experiments and observations have revealed that herbivores may have a substantial impact on the distribution and abundance of plants in Arctic and sub-Arctic areas (Tenow et al 2001;Cairns and Moen 2004;Olofsson et al 2009;Van Bogaert et al 2009Babst et al 2010). Human activities have also depressed the tree line in many areas of Scandinavia (Emanuelsson 1987;Karlsson et al 2007).…”

Long-Term Effects of Grazing and Global Warming on the Composition and Carrying Capacity of Graminoid Marshes for Moulting Geese in East Greenland

“…Even though the ecological responses to climate change may be complex and non-linear (Post et al 2009), it may be expected that increased vegetation productivity will provide herbivores with richer food supplies, resulting in increased population densities. In turn, increasing densities of herbivores may have a structuring role on the vegetation which may lead to changes in the vegetation composition (van der Wal 2006;Olofsson et al 2009) and, in extreme cases, degradation of the ecosystem (Jefferies et al 2006).…”