Short‐ and long‐term effects of changes in reindeer grazing pressure on tundra heath vegetation

Abstract

Summary1 Previous studies of plant-herbivore interactions have typically focused either on short-term or long-term effects. By directly comparing effects at different temporal scales, I studied whether short-term experiments provide good indications of the long-term effects of herbivory. 2 I reciprocally transplanted turfs of tundra heath vegetation within areas covered by a long-term (40 years) reindeer manipulation experiment, in order to examine the effects of all possible permutations of light, moderate and heavy grazing pre-and post-transplantation. 3 The effects of a short-term (three growing seasons) increase in reindeer grazing pressure gave good qualitative indications of most of the long-term effects of grazing on plant biomass, species richness, moss cover and lichen cover. In contrast, reducing grazing pressure on previously heavily grazed vegetation had no significant effects on any of these variables over the same time-scale. 4 Although few effects on individual species were recorded from the short-term manipulations, all those that were significant were qualitatively similar to the long-term responses of the respective species to herbivory. 5 The grass-dominated vegetation from the heavily grazed area changed little when grazing and trampling pressure were reduced for 3 years. In contrast, the dwarf shrubdominated vegetation in the lightly grazed area changed rapidly into grasslands when the grazing pressure was enhanced. 6 Re-establishment of dwarf shrubs appears to be both seed and microsite limited, but it appears that dwarf shrubs may be able to re-establish in previously heavily grazed vegetation in the absence of reindeer. 7 Transitions from moss-or dwarf shrub-dominated ecosystems to grass-dominated ecosystems are currently occurring at different locations in boreal, arctic and alpine regions due to atmospheric nitrogen deposition and livestock grazing. This loss of habitat may be reversible, given a relatively long time frame.

The Impact of Climate Change on Recent Vegetation Changes on Dovrefjell, Norway

“…Vistnes and Nellemann [65] also conclude that heavy grazing may be beneficial to a range of colonizing species in the short term, while in the long term the vegetation cover might be destroyed by erosion. However, this is not supported by Olofsson [66] who reports that the short-term effects from increase in reindeer grazing can give good indications of most of the long-term effects on species richness.…”

“…Exclusion of grazing reindeer has been shown to lead to an increase in cover for Betula nana, as well as an increase in the cover of dwarf shrubs, forbs and grasses [67]. Dwarf shrub dominated vegetation on tundra heaths in areas with light grazing pressure from reindeer will often rapidly change into grasslands when the grazing pressure increased [66]. The cover of Betula nana has been shown to increase under moderate grazing of sheep but decrease under a high grazing pressure [68].…”

The response of plant diversity to grazing varies along an elevational gradient

“…Plant diversity does not, however, always change in response to grazing; particularly, when environmental variation has more impact on vegetation composition than grazing (Stohlgren, Schell & Vanden Heuvel 1999). Furthermore, the response rates of plant communities to increased and decreased herbivory may not be symmetric, with a slower response observed when herbivory is decreased than increased (Olofsson 2006).…”

Multiple Feedbacks Contribute to a Centennial Legacy of Reindeer on Tundra Vegetation

“…A transplantation study between high and low reindeer density areas showed that the heath vegetation can change into graminoid-dominated vegetation in just a few years, although such graminoid-dominated vegetation is fairly stable even in the absence of active grazing during the first decade after herbivore removal. This was interpreted as a priority effect linked to the dense grass swards being hard to invade (Olofsson 2006). However, like many other systems where strong historical contingency, priority effects and alternative stable states are discussed (Fukami 2015), and knowledge of conditions that deliver such effects, as well as empirical data on the stability of states, are lacking (Beisner and others 2003).…”