Subalpine ”pinification”: Early signs of a pending and distinct biogeographic shift in the Swedish Scandes: Review and updates

This study focuses, by in situ records and long-term observations, on recent (post-Litte Ice Age), arboreal change in a mountain birch dominated (Betula pubescens ssp. czerepanovii) valley in the Swedish Scandes. During the early Holocene thermal optimum and up to the onset of the mid-Holocene Neoglaciation, Scots pine (Pinus sylvestris) dominated the tree cover of this valley and formed the treeline ecotone adjacent to the early alpine tundra. Subsequently, and consistent with progressive cooling until the late 19th century, prevailing pine stands demised and opened for landscape-level expansion of birch and spruce (Picea abies). A short and distinct break in that process took place by the Medieval Climate warming phase, about 1000-800 years before present. Subsequently, during pre-industrial time, temperatures reached their lowest levels of the entire post-glacial period. This was the so-called Little Ice Age, which ended the long-term Holocene cooling. That cold-climate epoque was broken by the late-19th and early 20th century. Thereafter and up to the present-day, temperatures in the study region (summer and winter) have increased by slightly less than 2 °C. As a consequence, treelines of all species have advanced by a maximum of more than 200 altitudinal meters. Pine displays the most persistent expansion, particularly over the past few decades. “Falangist” occurrences appear in the pure birch forest, tens of kilometers and hundreds of meters, respectively, beyond and above the outposts by the early 20th century. Occasionally, scattered young pine trees now grow close to the birch forest limit and somewhat above. In particular, at sites where pine stands demised during the Little Ice Age, c. AD. 1300-1850, prolific regeneration and insignificant winter mortality are recorded over the past 15 years. In comparison, birch and spruce provide no analogous signs of recent expansion. It may be hypothesized that unabated climate warming and corresponding arboreal progression will profoundly transform the plant cover of this valley, and others alike, into the same pine-dominated state that prevailed during the early Holocene. In other words, a new biogeographic zonation pattern may be on the rise, with pine back as the dominant subalpine species. This option is focused by continued monitoring.