Arctic Shrubs Expand Their Turf
Shrubs are transforming the face of Arctic tundra.
Alder, willow and dwarf birch have recently moved into areas where they never used to grow. Patches of shrub have as well become denser and the bushes themselves are growing bigger than before.
A comparison of 155 detailed photos taken over 50 years ago of land lying north of Alaska's Brooks Range with ones taken there recently finds that the amount of shrub cover increased in 87% of the photo pairs. The largest extent of change occurs on terrain below 400 m elevation in the deeper valleys.
Above the Arctic Circle, encroaching vegetation has renovated floodplains. Rivers used to be a network of barren gravel and sand bars through which water channels often shifted their course. The bars are now covered in bushes that bound the rivers in narrow, stable channels.
The broad terraces of sedge tussock that flank the floodplains have also experienced a surge in shrub cover. Some that were devoid of shrubs 50 years ago now sport vegetation that's two meters tall.
On floodplains and terraces, the area growing shrubs increased by 160% over a half century. Shrubs covering hill slopes above the valley bottoms expanded by 33%. Altogether in this part of northern Alaska, deciduous bushes spread an average of 0.4% a year during the last 50 years of the 20th century.
The ability to compare high-quality Arctic landscape photos taken 50 years apart is a rather unique opportunity for documenting vegetation changes, as older photos with sufficient detail are a rare find. Elsewhere though, evidence from satellite data, field studies and local observations suggest that shrubs have recently extended their bounds throughout the Arctic, particularly in Canada, but also in Scandinavia and Russia.
Scientists state that warmer temperatures in the Arctic have propelled the invasion of shrubs across the tundra. The warmer weather, along with deeper snow in winter, has encouraged microbes which generate the nutrients that plants need. Valley bottoms are where the nutrients have mainly accumulated, and where shrubs have expanded most. Shrubs particularly benefit from the influx as they are more efficient than other tundra plants at using nutrients.
Calculations show however, that the shrubs began moving into new territory long before the Arctic's current warming trend began in 1970. Expansion of tundra shrubs seems to have started 85 or 100 years ago, probably in response to the climatic warming that followed the Little Ice Age. The study's authors note that the spreading shrub cover will generate its own influences on the Arctic's climate.
Ken Tape, Matthew Sturm and Charles Racine. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology. 12(4): 686-702.