"The effect reindeer grazing can have on albedo and energy balances is potentially large enough to be regionally important," said Mariska te Beest of Umeå University, Sweden. "It also points towards herbivore management being a possible tool to combat future warming. Most of the Arctic tundra is grazed by either domesticated or wild reindeer, so this is an important finding."
Two implications are, according to te Beest, the understanding that herbivorous mammals can be important for the Earth system beyond their local grazing impact, and the knowledge that changes in growing-season albedo, as well as changes in albedo during snow-melt, have implications for local climate.
"The polar regions are important for global climate," te Beest added. "They cool the climate by reflecting a large part of the incoming solar radiation. However, due to climate warming the periods of snow cover are becoming shorter and the vegetation is becoming more shrub-dominated." This reduction in snow cover and "Arctic greening" lead to less reflection of solar radiation and so more absorption, te Beest explained.
Scientists already knew that reindeer have the capacity to reduce the abundance of shrubs in Arctic tundra. "We therefore wanted to investigate if this would affect the reflectivity of the surface, i.e. the albedo, as well," te Beest told environmentalresearchweb.
To find out, te Beest and colleagues studied sites around 100 metres above the tree line that were grazed by reindeer in Reisadalen, Norway, on either side of a fence built in the 1960s. Reindeer mainly forage here in mid-August, the beginning of the autumn. The team measured reindeer activity by collecting dung and by assessing whether nails in the ground had been trampled on.
The team assessed four types of vegetation, classified according to its state on the lightly grazed side of the fence; on the heavily grazed side grasses and forbs dominated. The researchers measured plant abundance and species composition, shrub canopy height, leaf-area index and normalized difference vegetation index, as well as albedo, soil moisture and soil temperature. They also used the JULES land surface model to assess how the vegetation’s properties altered the energy balance.
When reindeer reduced shrub height and abundance, summer albedo increased in both dwarf birch (Betula nana)-dominated heath vegetation and gray willow (Salix glauca)-dominated willow depressions. Models of heavily grazed shrub-dominated sites had lower net radiation, and reduced latent and sensible heat fluxes.
The differences in net radiation between highly grazed and low-grazed sites were equal to or higher per unit area than the global atmospheric heating of 4.4 W per square metre associated with a doubling of atmospheric carbon dioxide, the team found.
"By preventing the encroachment of shrubs, reindeer can affect the local climate," said te Beest. "A novel result is that it was not only tall shrubs that were important, but also reducing the abundance of dwarf shrubs increased the albedo substantially."
Were reindeer to remove light-coloured lichens, however, this could decrease surface albedo.
"Of course, the impact the reindeer have will vary according to their densities and the subsequent effects on the vegetation levels across the whole tundra," said te Beest.
Now the team would like to look at areas with lower reindeer densities than the current study site to evaluate the importance of grazing across the whole range of reindeer densities found in the Arctic tundra. "The strong effects of reindeer on albedo are probably restricted to areas with high reindeer densities, since a dramatic vegetation change is essential," wrote the scientists in Environmental Research Letters (ERL).
Related links
- Reindeer grazing increases summer albedo by reducing shrub abundance in Arctic tundra te Beest M et al 2016 Environ. Res. Lett. 11 125013
- ERL
- Mariska te Beest, Umeå University