Climate change evidence in tree growth and stand productivity at the upper treeline ecotone in the Polar Ural Mountains

Abstract

Background: Recent warming is affecting species composition and species areal distribution of many regions. However, although most treeline studies have estimated the rates of forest expansion into tundra, still little is known about the long-term dynamic of stand productivity at the forest-tundra intersection. Here, we make use of tree-ring data from 350 larch (Larix sibirica Ledeb.) and spruce (Picea obovata Ledeb.) sampled along the singular altitudinal treeline ecotone at the Polar Urals to assess the dynamic of stand establishment and productivity, and link the results with meteorological observations to identify the main environmental drivers. Results: The analysis of stand instalment indicated that more than 90% of the living trees appeared after 1900. During this period, the stand became denser and moved 50 m upward, while in recent decades the trees of both species grew faster. The maximum afforestation occurred in the last decades of the twentieth century, and the large number of encountered saplings indicates that the forest is still expanding. The upward shift coincided with a slight increase of May-August and nearly doubling of September-April precipitation while the increase in growth matched with an early growth season warming (June + 0.27°C per decade since 1901). This increase in radial growth combined with the stand densification led to a 6-90 times increase of biomass since 1950. Conclusion: Tree-ring based twentieth century reconstruction at the treeline ecotone shows an ongoing forest densification and expansion accompanied by an increased growth. These changes are driven by climate change mechanism, whereby the leading factors are the significant increase in May-June temperatures and precipitation during the dormant period. Exploring of phytomass accumulation mechanisms within treeline ecotone is valuable for improving our understanding of carbon dynamics and the overall climate balance in current treeline ecosystems and for predicting how these will be altered by global change.

Relationship between Species Richness, Biomass and Structure of Vegetation and Mycobiota along an Altitudinal Transect in the Polar Urals

“…The response of the vegetation to climate warming is well studied in the Polar Urals [ 2 , 9 , 20 , 21 , 22 ]. Over the 60 years that have passed since the beginning of these long-term monitoring studies, the climate has changed significantly in the Slantsevaya mountain are: the average annual summer temperature has increased by 1.8 °C, and the winter temperature by 2.0 °C, and, as a consequence, the length of the growing season has increased by 7 days.…”

“…from 28.34 to 97.60 t/ha. The crown density of the trees has also increased by 20% [ 9 , 22 ]. Photographs taken in 1962 and in 2020 on the slopes of Slantsevaya mountain show a drastic transformation of the vegetation in the river valley and on the mountain slopes from forest-tundra to north boreal forest [ 2 ], and impressively document the rising of the upper forest boundary by 50 m over 58 years ( Figure 3 ).…”

“…The morphometric parameters were measured for a total of 694 trees and undergrowth units, and the plant age was calculated for each of them. The trees on the lower levels (I and II) were studied in 2020, but the data from the upper levels (III-V) were taken from our previous studies [ 9 , 22 ].…”

“…The aboveground tree biomass was estimated for the two most widespread tree species in the study area: Siberian larch ( Larix sibirica ) and common spruce ( Picea obovata ) according to methods described before [ 21 , 22 , 29 ]. For the upper forest boundary and the upper enclosed forest, the tree biomass estimates were conducted at the border of the trial-plots.…”

“…Then, the masses of all these biomass fractions were summarized. Allometric functions between the total tree biomass (dry-weight) and the tree diameters at base height were calculated [ 22 ]. Using these data together with the tree ages, the annual biomass accumulation could be calculated for each tree.…”