Soil temperatures at the birch treeline (Betula pubsescens ssp. czerepanovii) - a 21-year record in the Swedish Scandes and a contribution to general treeline theory

This study addresses the issue of climate control of the elevational treeline, foremost the role of soil temperatures. During the period 1999 to 2020, soil temperatures were recorded over the year at a depth of 10 cm in a sparse stand (Betula pubescens ssp. czerepanovii) within the upper treeline ecotone of the Swedish Scandes. Over the years 2010 to 2020, the birch stand was repeatedly photographed. This endeavor, in combination with measurements of the tree heights provided an apprehension of individual responses to recent climate variability. This view was taken a step further by analyzing tree-ring patterns more than 100 years back in time. A main result was that the obtained growing season soil temperature of 7.1±0.7 degrees Celsius (°C) is well in accordance with earlier estimates of a global minimum threshold for tree growth at the treeline. Soil temperature was 2.7 °C lower than ambient air temperature. The tree-ring chronology displayed steadily increasing growth between 1880 and the late 1930s. It may be inferred that up to the latter date, the concerned birches were climatically suppressed specimens, entirely snow-covered during the winter. Thereafter, growth progression towards tree-size was initiated from the early 1940s and onwards, in response to climate warming.  This process appears to be still in progress as temperatures remain fairly high.