The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps

Abstract

Bugmann, H. 2007. The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps. Silva Fennica 41(1): 55-70.Recent changes of climate and land-use are often regarded to affect the European Alpine region substantially and to trigger an increase in the elevation of the upper treeline. The patterns of tree invasion on a subalpine meadow at 1900 m a.s.l. in Sedrun, Canton Grisons, Switzerland, were studied in order 1) to reconstruct the process of tree establishment and tree-growth dynamics in space and time, and 2) to evaluate the influence of site properties, land-use change and climate on these processes. Dendroecological analysis of 105 Norway spruce combined with an assessment of 48 vegetation plots and 17 soil profiles revealed that the trees were established in one main period (1965-1980s), starting 15 years after the abandonment of the agricultural use of the meadow, and that there is a pronounced environmental gradient along the forest-meadow ecotone. Tree establishment and height growth were favoured close to the former forest edge, but all saplings irrespective of their distance to the forest edge and their age showed increased radial growth since 1990, coinciding with a period of higher summer temperatures in the region. Therefore, we conclude that the observed tree-line dynamics in Sedrun are the result of both land-use and climate change: Tree establishment was triggered by the abandonment of the agricultural use of the meadow, and strongly favoured by particularly good growing conditions in a warm decade, which illustrates the sensitivity of conifers near the alpine tree-line to temperature fluctuations.

Recent forest limit changes in south-east Norway: Effects of climate change or regrowth after abandoned utilisation?

“…The small (B 3%) and very recent forest expansion above the UPCEFL of 1961Á1990, the forest height growth and the temperature development after 1995 could indicate a preliminary effect of recent summer warming. Such effects and similar apportionment have recently been documented from the Swiss Alps in studies were the effect of forest regeneration was separated from recent climate-driven forest expansion (Bolli et al 2007;Gehrig-Fasel et al 2007). The three findings of the present study point to increased future forest growth and higher actual and UPCEFL.…”

“…Following the findings of Vajda & Venäläinen (2005), this is anticipated to be caused by a sheltering effect from cold winds and wind-transported ice crystals. Thus, forest regrowth below the UPCEFL may improve the local climate growth conditions and subsequently the rate of forest expansion in deforested areas (Bolli et al 2007), whereas it may lower the root-zone temperature at the UPCEFL and hence retard the forest expansion at higher elevations (Kö rner & Paulsen 2004).…”

“…The results of these studies are less contrasting, although the emphasis on the driving forces behind the forest expansion of sub-and low-alpine areas varies. Bolli et al (2007) found that higher summer temperatures since 1990 in the Swiss Alps coincided with increased Norway spruce radial growth and tree establishment, but also that land-use abandonment was important for spruce forest limit advance. Gehrig-Fasel et al (2007) found that recent climate warming in the Swiss Alps explained far less of the forest expansion than land-use abandonment.…”

“…Recently, however, a number of studies have attempted to separate the effects of recent climate change from those of regrowth following the abandonment of human encroachment on the upper forest limit expansion in regions of previously extensive human outfield land-use (e.g. Motta et al 2006;Bolli et al 2007;Gehrig-Fasel et al 2007;Albert et al 2008;Gellrich et al 2008;Rutherford et al 2008). The results of these studies are less contrasting, although the emphasis on the driving forces behind the forest expansion of sub-and low-alpine areas varies.…”

Spruce colonization at treeline: where do those seeds come from?

“…These factors promote a progressive upward shift of the treeline ecotone (Kullman, 2002;Walther, 2003;Gehrig-Fasel et al, 2007), the belt between the upper continuous forest limit and the upper altitude isolated trees (Kö rner 1998). In the Alps, the area formerly used for grazing has been slowly colonized by subalpine species such as Norway spruce, European larch and European stone pine (Stü tzer, 1999;Didier, 2001;Bolli et al, 2007).…”