Recent Local Birch Forest (Betula pubescens ssp. czerepanovii) Evolution in the Treeline Ecotone of the Swedish Scandes-response to Earlier Snow-melt

In a context of recent climate change, the conversion of a restricted area of treeless alpine/subalpine tundra to mountain birch forest (Betula pubescens ssp. czerepanovii) was studied by repeat photography, demographic analysis in combination with tree growth surveillance in permanent plots. In addition, flora change was recorded within the emerging birch forest stand. The study was initiated in 1980, when a large snow bank still covered the site well into mid-July. Climate warming and associated enhanced snow melt since the early 20th century has made the snow disappear earlier during most summers. In response, a fairly dense population of seed-regenerated low-growing birch saplings gradually emerged. The population stagnated in average height until the early 1980s, when height growth and recruitment accelerated. Thereafter, a dense stand of tree-sized birches emerged. Concurrently, the character of the ground cover transformed from alpine snow bed to subalpine birch forest, as the snow has melted increasingly early, in combination with the presence of a shading tree layer. Earlier snow melt facilitated the establishment of scattered saplings of Pinus sylvestris. Possibly, an analogous course of patchy elevational subalpine forest expansion may be suggested as a reasonable option in a hypothetical case of further climate warming. The establishment of this forest stand may bear some resemblance to the evolution if the first Holocene mountain birch forests.