Variability, contingency and rapid change in recent subarctic alpine tree line dynamics

Abstract

Summary1 Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. 2 We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce ( Picea glauca ), and to construct static age distributions of willow ( Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. 3 Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65-85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40-65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. 4 Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30-50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. 5 The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species' biology.

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.…”

Four Decades of Plant Community Change in the Alpine Tundra of Southwest Yukon, Canada

“…All sections were sanded to a fine polish and annual rings were counted under a stereo microscope. Cross-dating was conducted using iteratively identified marker years (Danby and Hik 2007).…”

“…Signs of warming are also increasingly apparent in these northern alpine ecosystems. For example, in southwest Yukon, where temperatures have increased significantly over the last 40 years, there is compelling evidence linking variation in climate and weather to changes in alpine treeline and the survival, reproductive success and growth of several species of mammalian herbivores (Danby and Hik 2007;Morrison and Hik 2007). Observed changes in tundra plant communities have been related to herbivory (McIntire and Hik 2005) and interactions between plant species and the local environment (e.g., Mitchell et al 2009), but long-term changes associated with warming are difficult to infer from short-term studies.…”

Transition from Connected to Fragmented Vegetation across an Environmental Gradient: Scaling Laws in Ecotone Geometry

“…Interest in this topic increased further after reports of significant shifts in vegetation boundaries (e.g., tree lines) due to recent climate changes (Allen and Breshears 1998;Danby and Hik 2007). Detecting boundary shifts has been suggested for biomonitoring climate change (Kimball and Weihrauch 2000; but see Zeng and Malanson 2006 and references therein).…”