Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia

Abstract

Summary• Shrub expansion in alpine and arctic areas is a process with possibly profound implications for ecosystem functioning. The recent shrub expansion has been mainly documented by remote sensing techniques, but the drivers for this process largely remain hypotheses.• Here, we outline a dendrochronological method, adapted to shrubs, to address these hypotheses and then present a mechanism for the current shrub expansion by linking recent climate change to shrub growth performance in northern Sweden.• A pronounced increase in radial and vertical growth during recent decades along an elevational gradient from treeline to shrubline indicates an ongoing shrub expansion. Age distribution of the shrub population indicates the new colonization of shrubs at high elevations.• Shrub growth is correlated with warm summers and winter snow cover and suggests the potential for large-scale ecosystem changes if climate change continues as projected.

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.…”

“…Earlier studies indicate, however, that the immediate response of snowbed species to changed snow-depth and growing season length is highly speciesspecific (Galen and Stanton 1995;Sandvik et al 2004). An alternative explanation is that graminoids have increased due to decreased lemming grazing (Virtanen 2000;Hentton and Wallgren 2001 Table 1 Further, we also found that the cover of juniper increased in agreement with findings by Hallinger et al (2010) in the same general location. Rundqvist et al (2011) described, a more complex pattern in which juniper increased in plots with low cover of mountain birch while it decreased in a plot with much higher cover of mountain birch.…”

Changes Versus Homeostasis in Alpine and Sub-Alpine Vegetation Over Three Decades in the Sub-Arctic

“…It was somewhat surprising that Salix reticulata was the only shrub that significantly increased overall, since other surveys have shown an extensive increase of shrubs (e.g. Tape et al 2006;Hallinger et al 2010;Hedenås et al 2011;Rundqvist et al 2011;Myers-Smith et al 2011). The decrease of the two Equisetum taxa may be related to a change in snow cover, e.g.…”

“…The most pronounced vegetation shift supported both by warming experiments (Walker et al 2006) and observations over the last few decades, is an increased cover, biomass and upward movement of deciduous trees and shrubs (Kullman 2002;Van Bogaert et al 2010;Hallinger et al 2010;Hedenås et al 2011;Rundqvist et al 2011). Warming experiments also suggest that graminoids will generally increase in abundance (Walker et al 2006).…”

The role of summer precipitation and summer temperature in establishment and growth of dwarf shrub Betula nana in northeast Siberian tundra

“…In our dataset, there was a positive relationship between shrub diameter at the soil surface and height (r = 0.43, p \ 0.01, n = 90, data not shown). Since the vertical and radial growth of woody species may be positively correlated (Hallinger et al 2010), the height limitation controlled by snow depth could largely control the radial growth as well. This may explain why the ring-width growth of B. nana was initially fast, but declined quickly and stayed at very low levels after the first 15 years.…”