"In many regions of the Arctic, shrub patches are increasing in size and abundance," Isla Myers-Smith of the University of Alberta, Canada, told environmentalresearchweb. "Increases in shrub growth could be a response to climate warming in tundra ecosystems; however, other factors such as changes in snow cover, nutrient availability and interactions with other plant species may also be important."

Tundra in northern Alaska is typically seeing more alder shrubs, while the western Canadian Arctic is experiencing increases in alder and willow, the Canadian High Arctic is becoming richer in dwarf willow and evergreen shrubs, northern Quebec is seeing more birch, and in arctic Russia willow cover is increasing. On mountain slopes, willow and alder are moving uphill in Alaska and the Yukon, juniper is heading higher in subarctic Sweden, and a variety of shrub species are growing at higher altitudes in the Alps.

But increases in shrub growth are not found everywhere; Myers-Smith reckons factors such as herbivory by reindeer, sheep, muskoxen, lemmings, ptarmigan, moose or hare may be limiting the rates of shrub expansion in some regions of Scandinavia, Greenland and Alaska.

Shrubs can shade out other vegetation growing beneath, leading to reductions in sun-loving species, such as lichen and moss.

"An increase in the height and density of shrub canopies could lead to a decline in important forage species for reindeer," said Myers-Smith. "Reindeer herders, hunters and others who depend on tundra ecosystems for their livelihoods may be adversely impacted."

What's more, the darkening of the surface associated with shrubs growing above snow could accelerate snowmelt and cause regional warming.

"Shrubs trap snow, which insulates soils in winter and shades soils in summer," said Myers-Smith. Winter soil temperatures beneath shrubs can be up to 30 °C warmer than air temperatures, although soil in shrub-free sites nearby may be almost the same temperature as the air. "These insulating and cooling influences of shrub canopies could alter soil temperatures, nutrient cycling and permafrost thaw. Feedbacks between increases in shrub growth and abundance, soil carbon and disturbance could increase future rates of shrub expansion and climate warming."

But it's not clear which way feedbacks will go – in some circumstances increased shrub growth may act to counteract temperature rises.

As well as warming and its associated effects such as permafrost thaw and more frequent fires, tundra regions are often experiencing increased disturbance from man.

"Fire, permafrost degradation, stream channels, animal trampling or human activities create and maintain microsites where shrubs can establish and grow," said Myers-Smith. "In the short-term, landscape and soil disturbances are likely to stimulate more recruitment of shrub species than expected by warming alone."

According to Myers-Smith, future changes in the abundance and dominance of shrubs in tundra ecosystems will influence energy exchange, carbon balance, nutrient cycling, permafrost thaw, habitat for wildlife species and biodiversity. "However, increased shrub abundance will likely not occur uniformly and will vary among species and regions," she said. "We outline research priorities for this field, so that we can better understand and project future shrub expansion in tundra ecosystems."

These priorities include: studying the response of different shrub species to change as more is known about birch than willow, alder or other species; examining the effect of landscape topography factors, such as hydrology and wind protection on shrub expansion; and looking at feedbacks.

Now the researchers are combining their data on the growth of shrub species from sites around the circumpolar Arctic. By looking at annual growth rings they'll be able to "see how responsive shrub growth is to changes in temperature and precipitation, to better understand the relationship between the shrub increases in tundra ecosystems and the recent high-latitude climatic changes".

The scientists reported their work in Environmental Research Letters (ERL).