Abstract
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.
Highlights
By tradition, alpine treelines are considered to be controlled basically by heat deficiency and may in principle serve as sensitive indicators of past and recent climate-driven landscape ecological transformation at the interface between upper boreal forest and alpine tundra. [1, 2, 3, 4, 5, 6]
The soil temperatures usually peaked in mid-August, with record-breaking high values in 2020 (Fig. 3), when annual terminal shoots averaged about 30 cm, which is more than double the common value of 10-15 cm during the observation period
The obtained growing season soil temperature of 7.1 ± 0.7 °C recorded during a 21-year period complies with analogous records worldwide, consistent with the idea of a common lower thermal threshold for tree growth at the fringe of the alpine tundra [38, 6]
Summary
Alpine treelines are considered to be controlled basically by heat deficiency and may in principle serve as sensitive indicators of past and recent climate-driven landscape ecological transformation at the interface between upper boreal forest and alpine tundra. [1, 2, 3, 4, 5, 6]. At smaller spatial scales, treeline positions and dynamics are modulated, to various degree, by complex interactions and feedbacks of local-scale factors, more or less decoupled from the regional climate, e.g. geomorphology, soil depth and wind as well as various disturbances, e.g. human and animal impacts [7, 18, 19] These constraints are spatially idiosyncratic, a circumstance which is even more influential at the upper limit of closed forest and its often diffuse character with relatively delayed responses to climate change [20, 21, 22, 23, 24, 25].
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