Abstract
Climate warming impacts on alpine treeline dynamics. However, we still lack robust assessments of the long-term impacts of climate on tree recruitment at the treeline, particularly in remote areas such as the subarctic regions of Russia subjected to different climate influences. We expected that the treelines in two regions may have different features and dynamics patterns. We analyzed climate variables and assessed treeline dynamics by quantifying recruitment using the tree rings of ca. 7000 trees of four species (Betula pubescens Ehrh. ssp. tortuosa, Pinus sylvestris L., Picea abies Ledeb. ssp. obovata, Larix gmelinii Rupr.) along 14 altitudinal transects (series of study plots). We compared the Khibiny Massif (Kola Peninsula) and the western Putorana Plateau, subjected to oceanic and continental influences, respectively. In both regions, summers became warmer, and winters became snowier during the past century. At the low part of the treeline ecotone, tree recruitment has slowly increased since the mid-18th century at the Putorana Plateau and the mid-19th century at the Khibiny but accelerated in the early 20th century at both regions and reached a maximum peak in the second half of the past century. Treeline encroachment intensified in the 1930s at the Khibiny and the 1950s at the Putorana Plateau. Trees encroached in the tundra leading to upward treeline shifts in the late 20th century. The slope exposure affected the rates of treeline shift with higher upward advances on southern-oriented slopes. Tree recruitment and early-winter precipitation were positively correlated. The differences in species composition, treeline altitude and influences of slope orientation on treeline dynamics can be explained primarily by differences in the degree of continentality. The abundance of saplings in both regions allows the future encroachment of trees into tundra and further treeline upward shifts to be forecast.
Highlights
The warming of the climate system, especially since the 1950s, is unprecedented on a scale of decades to millennia and has impacted cold regions across the Arctic [1]
The results obtained confirm our hypotheses: (1) in two regions of subarctic Russia, differing in terms of continentality and tree composition, upward treeline shifts were observed; (2) treeline dynamics were driven by changes in heat supply modulated by topography and microrelief; the most favorable conditions for tree settlement were found on southern-exposed slopes and in the upper and middle parts of the treeline ecotone; (3) the upward tree expansion was closely related to summer warming and increased winter precipitation, and depended on snow accumulation
The results of this study are important for understanding the patterns and dynamics of subarctic forests and treeline ecotones under cold-limiting conditions
Summary
The warming of the climate system, especially since the 1950s, is unprecedented on a scale of decades to millennia and has impacted cold regions across the Arctic [1]. Boundaries between different plant communities such as treeline ecotones are subjected to constant changes in space and time [3,7]. This is due to the fact that the altitudinal and latitudinal position of the treeline depends primarily on temperature and, its position is highly sensitive to such climate variables [4]. It is known that repeated fluctuations of the alpine and polar treelines and forest limits occurred during the Holocene in response to temperature fluctuations [8,9,10,11]
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