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

• 1 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 shrub-dominated 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.

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