The tall shrub tundra represents a biodiversity hot spot, with open tundra grasslands holding abundant nutrient rich food plants and tall shrubs providing food and shelter for a range of herbivores. The warming climate causes the shrubs to encroach the grasslands, and is part of the “greening of the Arctic”. How will the biodiversity of the tall shrub tundra change and to what extent will herbivores counteract the shrub expansion?
Grassland and tall shrubs
In the low arctic tundra landscape of Varanger, the tall shrub tundra exists in riparian and other moist habitats as extensive grasslands with tall shrub patches of varying size. A range of forb and grass species are abundant in the grasslands and constitute nutrient rich food for a range of herbivores. The tall shrubs are essential as habitat to a range of birds as well as to other herbivores. The mosaic of grasslands and tall shrubs in the tall shrub tundra thus provide ample resources for herbivores in an otherwise barren landscape.
Species richness of birds is dependent on the presence of tall life stages of tall shrubs (also termed thickets). (Read article).
However, such mosaics are not stable ecosystem states. Natural plant succession continuously causes the tall shrubs to overgrow the grassland. Yet, depending on the browsing pressure on the emerging shrubs, the succession may be decelerated or prevented from happening. Furthermore, small rodents, especially at their peak densities, cut down the small life stages of the tall shrubs and open up the grassland. Hence there is a transition between grasslands and shrublands that is continuously modified and where the mosaic of a given tall shrub tundra state is likely to be regulated by the herbivores.
Small life stages of tall shrubs are held in a browse trap at densities above approximately 5 animals per km2. (Read article).
Expected climate impact
Climate warming is causing tall shrubs to grow faster and encroach into their habitats more rapidly, in turn causing a lasting transition from grasslands to tall shrublands. Through the accompanied changes to the availability of food plants and shelter plants, this transition is expected to cascade through the food webs and manifest as an ecosystem state shift.
The climate warming induced transition from grasslands rich in forb and grass species to tall shrub dominance is likely to be mediated by the species inhabiting the tall shrub tundra. The abundance of small life stages of the tall shrub species indicates how fast a meadow might be overgrown. Yet the plant species constituting the grasslands may differ in their susceptibility to be overgrown. Moreover, the presence and abundance of small rodents and ungulates are likely to modify both the extent and the speed of the transition.
Expected effects of climate change and ungulate management on tall shrub tundra, along with effects of small rodents. The direct impact of a warmer summer climate on the expansion of tall shrubs can be counteracted by ungulates when these are at densities higher than approx. 5 animals per km2 and also by small rodents when they are at their peak densities. The distribution of tall shrubs is in turn expected to impact the bird community as well as having potential climatic feedback through modified albedo. The expansion of tall shrubs is at the expense of grasslands (in the model stated as meadows), whereas the species composition of the grasslands is likely to modify the extent to which herbivores graze and hence the speed at which the shrubs encroach.
- Information about tall shrub encroachment in response to climate change is an early warning of a changing ecosystem state and important when deciding on management actions to counteract tall shrub encroachment through e.g. altered ungulate densities.
- The role of herbivores in changing the palatability state of the grasslands is identified through exclosure experiments and more. This information can be used by management to alter herbivory in order to maintain a high palatability state. Whereas management actions are possible through altered ungulate herbivory, the palatability state has consequence to the whole herbivore guild.
- Changes to the extent of tall shrubs is important for the herbivores dependent on the shrubs for food and habitat structures during winter, and hence important information to the management of moose, ptarmigan and hare populations.
Plants: Yearly abundance estimates of functional group and dominant species in grassland communities since 2005. From 2018 this will be accompanied by nutrient content assessment of the plants (as described in this article).
Birds: Sound records since 2005.
Herbivore guild: Animal occupancy (small rodents) and fecal counts (ungulates, hares and ptarmigan) since 2005.