The evidence for shrub expansion in Northern Alaska and the Pan‐Arctic

Abstract

One expected response to climate warming in the Arctic is an increase in the abundance and extent of shrubs in tundra areas. Repeat photography shows that there has been an increase in shrub cover over the past 50 years in northern Alaska. Using 202 pairs of old and new oblique aerial photographs, we have found that across this region spanning 620 km east to west and 350 km north to south, alder, willow, and dwarf birch have been increasing, with the change most easily detected on hill slopes and valley bottoms. Plot and remote sensing studies from the same region using the normalized difference vegetation index are consistent with the photographic results and indicate that the smaller shrubs between valleys are also increasing. In Canada, Scandinavia, and parts of Russia, there is both plot and remote sensing evidence for shrub expansion. Combined with the Alaskan results, the evidence suggests that a pan-Arctic vegetation transition is underway. If continued, this transition will alter the fundamental architecture and function of this ecosystem with important ramifications for the climate, the biota, and humans.

Changes in Tree Growth, Biomass and Vegetation Over a 13-Year Period in the Swedish Sub-Arctic

“…There were significant transitions in the cover of vegetation types; the cover of ''Meadow with low herbs M(lh)'' and ''Birch forest of heath type with mosses BFo(m)'' increased significantly, while the cover of ''Moderate snowbed vegetation SB(mod)'' decreased significantly. Our study concurs with the results of other studies which suggest that there has been a general increase in cover and biomass of trees and shrubs in sub-Arctic and Arctic areas (e.g., Sturm et al 2001;Tape et al 2006;Danby and Hik 2007;Tømmervik et al 2009;Forbes et al 2010;Hallinger et al 2010;Van Bogaert et al 2011;Rundqvist et al 2011, this issue). Tree biomass increased on average 1.5% per year from 3.5 t ha -1 in 1997 to 4.2 t ha -1 in 2010.…”

“…However, it could also be suspected that heaths dominated by dwarf shrubs may be converted to heaths dominated by graminoids or larger shrubs, as experiments suggest that shrubs and graminoids may increase following warming (e.g., Walker et al 2006). Indeed, comparison of old and new photographs has revealed an expansion of large shrubs throughout the Arctic (Sturm et al 2001;Tape et al 2006;Forbes et al 2010).…”

Modelling Tundra Vegetation Response to Recent Arctic Warming

“…The underlying mechanism in the model is an increase in light attenuation, favouring taller plants to the detriment of the ground vegetation they shade. Shrub expansion and densification has been reported in numerous studies and related to recent warming trends (Kullman 2002;Jia et al 2003;Tømmervik et al 2004;Tape et al 2006;Hedenås et al 2011;Rundqvist et al 2011). A popular hypothesis is that increased microbial activity in warmer soils enhances the availability of nutrients, particularly nitrogen, and that this lends a competitive advantage to shrubs relative to other types of tundra plants (Chapin et al 1995;Tape et al 2006).…”

“…Shrub expansion and densification has been reported in numerous studies and related to recent warming trends (Kullman 2002;Jia et al 2003;Tømmervik et al 2004;Tape et al 2006;Hedenås et al 2011;Rundqvist et al 2011). A popular hypothesis is that increased microbial activity in warmer soils enhances the availability of nutrients, particularly nitrogen, and that this lends a competitive advantage to shrubs relative to other types of tundra plants (Chapin et al 1995;Tape et al 2006). This temperature-nutrient effect cannot explain the simulated increase in shrub abundance in LPJ-GUESS, as the model does not include nutrient cycling; community changes in our study are primarily mediated by increased competition for light as the productivity and density of vegetation increases.…”

“…Changes in Arctic plant communities may already be taking place in response to warming over recent decades. Important lines of evidence include positive trends in surface greenness and photosynthetic activity inferred from satellite data (Tucker et al 2001;Bunn and Goetz 2006;Bhatt et al 2010;Beck and Goetz 2011), advancement of elevational and latitudinal treelines (Sonesson and Hoogesteger 1983;Kullman 2002;Harsch et al 2009;Van Bogaert et al 2010, 2011, and an increased cover, abundance and stature of shrubs in tundra areas (Kullman 2002;Jia et al 2003;Tømmervik et al 2004;Tape et al 2006;Hedenås et al 2011;Rundqvist et al 2011). Despite numerous local exceptions, the weight of evidence from observational studies suggests that, in general, Arctic vegetation is responding to rising temperatures through increases in productivity, density, cover and stature of vegetation and, in many areas, an increase in woody biomass and the representation of trees and shrubs (Post et al 2009;Callaghan et al 2011;Elmendorf et al 2012).…”