Structural Changes of Stands on Their Upper Limit at the Polar Urals over the Last 60 Years and Factors Effecting on Tree Establishment Rate

Climate has altered rapidly since the end of XIX century and species’ ranges advanced worldwide to higher latitudes and altitudes in response to changes of the environmental conditions. The most facts of treeline advance in response to global warming in the last century were obtained in studies of age structure of tree stands along altitudinal gradients, but long-term (40–70 years) monitoring of changes through time-expensive tree morphometry and age estimation on the same study plots was conducted only in the several locations. We evaluated dynamics of larch tree stands (Larix sibirica Ledeb.) on their upper distributional limit at the eastern macroslope of the Polar Urals Mountains over the last 60 years based on information of single trees on a permanent altitudinal transect 860 m long and 40–80 m wide obtained in result of mapping and morphometry measurements in 1961, 1999, 2011, 2020. Additionally, we analyzed how stand structure and dynamics depend on local conditions. We revealed by comparing tree characteristics at different time stamps that since 1960 up to present day sums of crown areas of adult trees (>1.5 m) have increased noticeably, but in the upper part of the studied transect by 2.7–2.8 times, and in the lower part by 5.5–16.2 times. In the same period, tree stands density and height increased at most of the sites by 2.3–5 times. At snowdrift locations, where snowpack depth decreased, stand density has grown by 22.2 times. However, at locations, where the depth increased, the density declined in 2.5 times. Based on analysis of local situations, changes of snow accumulation and stand structure, we assumed that the best preconditions for tree establishment within treeline ecotone existed on the locations with medium soil wetness and snowpack depth close to 1.5 m. A deviation from such conditions constrained the fast stands occupation of the sites above closed forest line. Our results highlight the necessity to consider the above mentioned factors when predicting climate-induced tree distributional responses at the single tree level.