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

This study presents the latest history and output of widely spaced pine (Pinus sylvestris L.) tree- and forest lines in a mountain valley in the southern Swedish Scandes, with a focus on climate change. The prior geriatric structure and physiognomy of the treeline ecotone have become substantially rejuvenated during the past two decades. Historical records define quite accurately these “lines” by the early 20th century and mid-1970s. Their complex performances after that have been studied up to the present day. Currently, both lines are located within the subalpine mountain birch forest belt (Betula pubescens ssp. czerepanovii).As a consequence, the evolution of the birch forest matrix affects their past, present, and future changes to some extent.The pine forest- and treeline positions remained relatively constant between the early twentieth century and the mid-1970s.

Forest line populations grew in number and density in response to 20th century climate warming, though their positions remained unchanged. In great contrast, the treeline, i.e. scattered solitary, fast-growing and vigorous trees, has shifted up-valley 135 m through the birch forest belt to a site 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 and proliferous reproduction of outlier old-established pine trees, imposing a radical generation shift in the treeline ecotone. This process predominantly manifests in birch forest gaps. As a result, it is assumed that, under current climatic conditions, the potential pine forest line is much south (and higher) of its present position within the competing birch forest belt. Given the subalpine birch forest's demise due to drought, this forest range is expected to be realised in the case of potential climate warming. 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.