Pine (Pinus sylvestris L.) Penetration towards the Head of the Handölan Valley: Recent Reversal of Long-Term Retrogressional Trend –Contrasting Responses to Climate Change of Tree-and Forest Line

Abstract With a climate-change perspective, this study focuses on the recent history and performance of the much separated pine (Pinus sylvestris L.) tree and forest lines in a mountain valley in the southern Swedish Scandes. Historical records define quite accurately these “lines” by the early 20th century and mid-1970s. Their subsequent dynamic performances have been surveyed up to the present day. Both lines are currently positioned within the subalpine mountain birch forest belt (Betula pubescens ssp. czerepanovii). Consequently, their past, present and future changes rely on the evolution of the birch forest matrix. Between the early 20th century and the mid-1970s, the position of the pine forest- and tree lines remained fairly stable. Conspicuously, the forest line population densified in accord with early-20th century climate warming, although with a stagnant position. In great contrast, the tree line, i.e. scattered solitary, fast-growing and vigorous trees, has shifted up-valley 135 m through the birch forest belt to a position about 12 km south of the position by the mid-1970s. It is hypothesized that the vast separation of forest- and tree line relates to the presence of the subalpine birch forest belt. This contention is supported by vigorous growth of outlier pine trees, predominantly in birch forest gaps and a treeline gradually rising towards the south in the valley. Accordingly, it is hypothesized that, with present-day climatic conditions, the potential pine forest line is much south (and higher) of its present position within the competing birch forest belt. In the case of future climate warming, this forest range is anticipated to be realized. That would be a return to the situation during the early- and mid- Holocene, when pine dominated the upper tree line ecotone and the birch belt was poorly developed.

Keywords Tree line, forest line, Pinus sylvestris, climate change, subalpine birch forest.

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