The structure of tree stands and features of their phytomass accumulation on the steppified slopes of the Kraka Mountain massif (Southern Urals)

Against the general background of an increase in global temperature and spatiotemporal rearrangements in the distribution of precipitations in the 20th century, there are significant changes in the structure and properties of terrestrial ecosystems at all levels of biosphere organization, while their carbon-depositing role is widely discussed in connection with an increase in the concentration of carbon dioxide (CO2 ) in the last century. In the massif of the Kraka mountains (South Urals), extrazonal mountain (petrophytic) steppes are located on the southern, highly insolated slopes of the ridges, where peculiar harsh conditions are formed that restrict the growth of woody plants. Due to climate change, forests are expanding to mountain steppes and their area is increasing. The aim of our research is to study the contemporary structure of tree stands and the features of their phytomass accumulation in the transition zone between the forest and mountain steppes on the slopes of the Kraka mountains (53°15′ – 53°50′N, 57° 36′ – 58°12′E), as well as to assess their changes in the last decades (See Fig. 1). In the massifs of the North and South Kraka mountains on the forest-mountain steppe ecotone, 8 altitudinal transects were installed, with division into levels depending on the crown density: the upper (first) – at the upper border of the sparse tree stands (groups of trees with crown density 0.05–0.1), middle (second) – at the upper border of open forests (crown density 0.2–0.3), lower (third) – at the upper border of closed forests (crown density 0.4–0.5). At the lower and middle altitudinal levels, 3 plots with a size of 20 × 20 m were installed. At the upper level, polygons were set up in the form of a rectangle with a size of 1-3 hectares. On Bashart Mount, a polygon profile was installed, without division into levels due to the mosaic distribution of trees along the entire profile. For each tree, the following parameters were determined: height, trunk diameter at the base and at a height of 1.3 m, crown projection diameter in two directions, and vital state. To determine the age of trees with a diameter of more than 3 cm, a bored wood sample (core) was taken. The age was determined using standard dendrochronological methods. The data on the phytomass of 16 model pine trees and 12 larch trees were obtained with subdivision into the following fractions: phytomass of the trunk, branches, needles and generative organs. The trunk phytomass in the bark was determined by direct weighing in the field with an accuracy of 50 g, for which the trunk was cut into meter sections. The percentage of dry matter, both in the wood and in the bark, is determined by the discs cut from the ends of the sawn sections. Wood and bark from the discs were weighed with an accuracy of 0.1 g at the site, and then they were sent to the laboratory for further drying and determination of their weight in an absolutely dry state. When determining the phytomass of the crown and its structural parts, all branches from the trunk were chopped off. The total weight of the crown was determined by direct weighing at the site in the field, after which the crown was divided into parts covered with needles and without them, and they were weighed separately. A sample of up to 20–30% of the total weight was taken. The needles in the sample were separated from the branches and weighed. To determine the absolutely dry matter, a sample weighing 30-60 g was taken from each crown fraction for subsequent drying to an absolutely dry state in laboratory conditions. The assessment of the scale of changes in the area of treeless territories on the slopes of the Kraka mountain ranges was carried out using the functionality of the SAS.Planet 160707 program. Using the “tools” from the “Tags” tab, on the basis of the images in the layers of modern (2015) satellite images of sub-meter resolution and maps (~ 1986) of the GosGisCentre (M 1: 25000), all non-forested areas were identified where the forest stands below 35-40%. For each treeless territory, its area was estimated in 1986 and 2016. Comparative characteristics of the habitat within the ecotone between closed forests and the mountain steppe and the patterns of changes in the morphometric parameters of tree stands at different altitude levels on the slopes of the South Kraka massif showed that the trunk diameter, height and age of trees increase from the lower to the upper level. At the upper level, single, old-growth, strongly intermittent trees are most often found. Down the slope, the number of trees at the middle level increases 15 times, and 40 times at the lower level. At the same time, the average age and diameter of trees decrease, while the height increases. In the North Kraka, the average values of the diameter, height and age, on the contrary, decrease from the lower to the upper level. Here the number and the average age of trees are significantly lower than in the South Kraka, especially at the upper levels (See Table 2). On the basis of the set of model trees of Scots pine and Sukachev larch, we studied the dependences of the aboveground phytomass and fractional structure (trunk, crown, needles) between different parameters of trees. We developed regression equations for the total aboveground phytomass of trees, as well as of the trunk, crown and needles from the diameter at the base and the height of the trunk (See Fig. 2 and 3; Table 4). On the basis of the equations, the reserves of aboveground phytomass were calculated for all altitudinal levels of the studied transects, and separately for the South and North Kraka mountains. The revealed features of the accumulation of phytomass of various fractions of trees and their ratio depend on the altitudinal position in the ecotone, the age and tree species. We found out that the total aboveground phytomass of stands at the upper border of closed forests on average for all profiles in the South Kraka is 72.9 ton /ha and ranges from 45.4 to 106 ton /ha. When moving to the upper border of open forests, it decreases 1.8 times and averages 40.1 ton / ha (37.7–42.4 ton / ha). Towards the border of sparse tree stands, the aboveground phytomass of stands is reduced by almost 16 times - to 4.4 ton/ ha (2.4–7.4 ton / ha). In the North Kraka mountain massif, the aboveground phytomass of stands at the border of closed forest and open woodlands is significantly higher than in the Southern Kraka, averaging 98.7 and 42.1 ton / ha, respectively, with a 2.3-time decrease in phytomass. In the sparse tree stands, on the contrary, the phytomass of stands is lower than in the South Kraka, averaging 1.69 ton / ha across all transects, which is almost 60 times less than at the third altitudinal level (Table 5). Analysis of climate change over the past 80 years showed that in the study area there was a 1.3°C increase in air temperature and an increase in precipitations by 41 mm, which are most pronounced during the cold season. Climate change had a favorable effect on the growth of woody plants, which led to an active overgrowth of mountain steppes with forests, as well as an increase in the density of woodlands. Over the past 30 years, there has been a decrease in the total area of treeless territories on the slopes of the mountains in the Kraka massif by more than 2 thousand hectares (17.6%). The overgrowth of mountain steppes with woody vegetation in the Kraka mountains led to an increase in the area of forests and the stock of phytomass concentrated in them equals to 92.3 thousand tons. This is equivalent to the long-term binding of appoximately 42.6 thousand tons of free carbon of the Earth’s atmosphere. The paper contains 3 Figures, 5 Tables, and 42 References.