| Summary: | In the northern larch forests of Siberia growing on permafrost soils the top drying phenomenon is widely spread. Its causes remain unclear. We suggest that an acute water deficiency in continuous climate warming could trigger the process of top drying in larch trees. In order to validate this hypothesis, dendroclimatic and wood anatomy approaches were used. A comparative analysis of the base-to-tip radial growth dynamics and wood anatomical structure in healthy and stag-headed Gmelin larch trees (Larix gmelinii (Rupr.) Rupr.), growing in the even-aged forest on the permafrost soil of the north-facing slope (64°19′23″ N, 100°13′28″ E) was made. The tree ring width, as well as lumen radial size and wall thickness of tracheids were measured at 1/4, 1/2 and 3/4 of the stem height and 20-25 cm below the top, in 15 healthy and 12 stag-headed trees. Decreasing trends of the aforementioned parameters from tree base to top were found in all the trees, which was especially evident in the stag-headed trees. Wood anatomical structure in the upper part of the stag-headed stems underwent modifications over the last 20 years: there occurred tree ring boundaries became indistinct, disturbance of the tracheid rows, thinning of early- and latewood tracheid walls. Using sliding climate correlations with the indexed radial increments it was found that the trees on the north-facing slope could suffer from water deficiency from the end of May until the late June. The presence of both stag-headed and neighboring healthy trees on the north-facing slope can be explained by high variability of soil hydrothermal growth conditions due to very high spatial mosaic moss-lichen cover, common to the north-facing slopes. The trees, growing in these unfavorable local hydrothermal conditions under continuous climate warming could experience an extremely acute water deficiency, leading to top drying out.
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