Recent treeline shift in the kebnekaise mountains, northern sweden – A climate change case

Elevational treeline change for different subalpine species over the past 100 years was assessed in the Kebnekaise-area of northern Swedish Lapland. The concerned species were mountain birch (Betula pubescens ssp. czerepanovii), rowan (Sorbus aucuparia), aspen (Populus tremula) and bird cherry (Prunus padus). The methodology relied on older published field measurements, checked for reliability by tree ring analyses, and compared with present-day assessments of uppermost tree growth at the same locations. All species showed various degrees of up shifts. Betula and Sorbus displayed maximum advances by 200 m in elevation, with smaller displacement for other tree species. A common pattern for all the concerned species was that treeline advance was achieved by phenotypic responses (rapid height growth) of old-established individuals, which had prevailed as krummholz prior to the temperature rise in the 1920s and 1930s. Age structure analyses evidenced a striking correspondence between summer temperature and the initiation of birch stems, which have gradually attained tree stature up to the present day. As a consequence, substantial densification of the upper birch forest took place, although without perceivable advance of the forest-limit. Overall, the results comply in detail with the outcome of analogous studies of the same and other species, further south in the Swedish Scandes. This inter-regional conformity contrasts with earlier claims of differential response between north and south, and support the contention that the ultimate reason is secular climate warming, common for the entire Swedish Scandes. There is little reason (as sometimes claimed) to invoke strictly local factors, e. g. reindeer grazing as superior driver of mountain birch population dynamics at the treeline. The finding that a subalpine clonal population of Populus tremula has existed for more than 6500 year, casts some doubt on recent claims that this species has recently expanded its elevational range in these mountains.