Siberian Federal University

In Southern Siberia, where forests meet steppes, lack of water affects tree growth. During hot summers, droughts can lead to more extreme events such as forest fires. Therefore, dendroclimatologists (scientists who study the relationship of weather conditions and tree rings) pay special attention to such regions.

“Although the effect of climatic fluctuations on vegetation is a widely studied problem, researchers still work on filling the gaps. One of the challenges is getting the climate data. We can use information from loggersensors installed on the studied forest site, though such measurements are of short duration. Most often, we use data from state meteorological stations, but in the large territory of Siberia, there might be hundreds of kilometres from the studied forest stand to the nearest station. There are also geographically dispersed data on climate, averaged over a geographic coordinate grid with a spatial resolution of tens of kilometres, but one cell can include terrain that is heterogeneous in relief and vegetation. Then again, the microclimate under the forest canopy differs from the climate of the open area. By default, it is assumed that fluctuations in climatic factors, especially temperature, occur quite synchronously at such distances. From this perspective, we can apply methods of mathematical statistics and calculate the relationship between climatic factors and tree growth,” said Liliana Belokopytova, author of the idea of research, senior researcher at the Dendroecological Laboratory of Khakass Technical School, a branch of Siberian Federal University.

Normally, dendroclimatologists consider only atmospheric and terrestrial weather conditions and extreme events. But the vital part of the tree – the root system – is underground and remains behind the scenes.At the same time, it is absolutely obvious that seasonal fluctuations of moisture and temperature of soil also affect the tree growth, since the roots are the nourishing and supporting part of the tree.
The temperature regime of the soil at several depths is determined by its heat exchange with the atmosphere and therefore depends on air temperature and sunlight. But the soil temperature range has a more mosaic structure.Its heterogeneity is increased by many factors, such as the composition and structure of the soil, uneven terrain and groundwater, snow cover, permafrost, shading by the plant canopy, etc. Not every weather station measures soil temperature and not during the whole period of its functioning, which limits the interest of the scientific community in studying the influence of the underground climate on plants. The SibFU research team tried to reveal whether it is possible to use data from weather stations on soil temperature within long distances for dendroclimatic studies.

“For several years, our laboratory has been studying the growth of trees in the Khakass-Minusinsk depression. We compared the daily series on soil temperature at several depths, above-ground temperature and precipitation at two weather stations with the tree-ring width of Siberian larch in three forest-steppe areas located within 100 km from the stations. It turned out that in spring and early summer, soil temperature has an even stronger effect on larch growth than the air temperature. We discovered that the maximum effect occurs at depths of 20-80 cm (the root zone). When these layers are heated, the water from the soil evaporates quickly, and the plants experience stress. From the point of view of the suitability of the series of remote weather stations, we foundthat the similarity of the soil structure in the studied forest area and at the weather station is as important for the analysis of the influence of underground climate as the distance between these sites,” shared Dina Zhirnova, head of the Dendroecological Laboratory (Khakass Technical School, SibFU).

The scholars also studied the heterogeneity of the climatic influence on the growth of larch on the scale of individual trees and revealed that the sensitivity to the above-ground and underground climate correlations depends approximately to the same degree on the micro-conditions of the landscape, forest density, age and other features of individual trees. According to the research results, the scientists confirmed that despite the mosaic nature of the soil cover, the soil temperature measurements at weather stations are as essential for dendroclimatic analysis as commonly used atmospheric climatic data.

The research was supported by Russian Science Foundation project 19-14-00120 Study of the Genetic Adaptation of Trees to Stressful Environmental Factors based on Genome-Wide and Dendrochronological Analysis Within Global Climate Change, carried out under the supervision of Konstantin Krutovsky, honorary professor of Siberian Federal University.