Herbivory influences tree lines

Abstract

Summary1 Transitions between major vegetation types, such as the tree line, are useful systems for monitoring the response of vegetation to climate change. Tree lines have, however, shown equivocal responses to such change. 2 Tree lines are considered to be primarily thermally controlled, although recent work has highlighted the importance of biotic factors. Dispersal limitation and the invasibility of the tundra matrix have been implicated and here we propose herbivory as an additional control at some tree lines. 3 We propose a conceptual model in which differing relative impacts of foliage consumption, availability of establishment sites, trampling, dispersal and seed predation can lead to very different tree-line responses. 4 The presence of large numbers of small trees above the current tree line at a site in northern Sweden that experiences limited reindeer ( Rangifer tarandus ) herbivory suggests range expansion. Other locations in the same region with higher reindeer populations have considerably fewer small trees, suggesting that range expansion is occurring much more slowly, if at all. 5 The use of tree lines as indicators of climate change is confounded by the activity of herbivores, which may either strengthen or nullify the impacts of a changed climate. Similar arguments are likely to be applicable to other ecotones.

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.…”

“…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).…”

Human land-use, forest dynamics and tree growth at the treeline in the Western Italian Alps

“…In the studied plots browsing of domestic ungulates has been the main driving force in controlling the forest dynamics for many centuries [6] and only in the recent decades the tree establishment has not been strongly affected by indirect (grazing) or direct (stone pine uprooting) human influences.…”

“…The hypothesis attempting to explain these processes are focused on changes in climate, CO 2 concentration, fire regimes and, in the mountains where humans have social or economical interests, land-use change and, particularly, the grazing of domestic animals [6,7,15,34,57,65].…”