The importance of terrain factors in the spatial variability of plant cover diversity in a steppe gully

Abstract

Information about the slope angle and its exposure is often given when describing plant communities. However, the slope angle should be noted to affect a complex of different phenomena and processes. The slope angle affects the albedo and thermal regime of the soil. The slope angle affects the redistribution of moisture and determines the moisture availability of a particular area. The slope angle also determines the risks of erosion processes. In turn, erosion processes determine the depth of the soil layer and the content of organic matter in it, which greatly affects the conditions for plant life and their dynamics. Therefore, the slope angle of the relief is a complex environmental indicator, which is its most important weakness. The information on the slope angle of the relief surface does not indicate which environmental factor is limiting and determines the characteristics of the vegetation cover. Similarly, the quantitative orientation of a slope using rhumb lines is a rather crude way to indicate the role of terrain in the redistribution of solar energy. With a certain degree of accuracy, rhumbs characterize incoming solar radiation, but are not sensitive to estimating the amount of diffuse radiation. Modern geoinformation technologies make it possible to determine the amount of solar radiation reaching the terrain surface using a digital elevation model. The aim of the article was to identify the role of relief factors in terms of topographic wetness index, erosion index, and solar radiation in the spatial variability of the vegetation diversity of a steppe gully. There was no linear correlation between the other predictors. Some correlations can be interpreted as the result of certain nonlinear patterns. In this sense, the geomorphological predictors are mostly linearly independent and thus each of them carries independent information for characterizing environmental conditions. The vegetation cover of the gully system was represented by 263 plant species. The analysis of the synoptic phytosociological table allowed us to find out that the vegetation cover of the studied gully system is represented by six classes of vegetation. The geomorphological predictors allowed for a classification of vegetation types with an accuracy of 23.8% to 100%. The topographic wetness index was the most important for classification. Altitude and insolation were also important for classification (94 and 95 respectively). The topographic wetness index provided an accurate identification of wetland vegetation of the Phragmito-Magnocaricetea class. Naturally, this type of vegetation preferred biotopes with the highest level of soil moisture. High insolation accurately labeled the locations where Festuco-Brometea steppe vegetation was most likely to be found. Some of the artificial tree plantations that occurred in the thalweg of the gully can be clearly identified by the high level of the topographic wetness index. Another group of artificial tree plantations is located on relatively high relief areas and should be differentiated from Galio-Urticetea communities, which prefer locations with higher light levels, and from Molinio-Arrhenatheretea, which prefer locations with a higher risk of erosion. The most arid locations are preferred by Agropyretalia intermedio-repentis vegetation. Steppe and meadow vegetation located at altitudes less than 116 meters also differ in their preferred height. Steppe vegetation is usually located at a level higher than 95 meters. Natural steppe and meadow vegetation at elevations below 116 meters differs from semi-natural Galio-Urticetea vegetation in that the latter usually prefers more well lit locations. Steppe communities are misclassified as meadow in 15.3% of cases, and meadow is misclassified as steppe in 18.5% of cases. Artificial tree plantations are misclassified as steppe in 42.9% of cases and 14.3% are misclassified as Agropyretalia intermedio-repentis. In its turn, Agropyretalia intermedio-repentis is misclassified as Festuco-Brometea in 6.7% of cases, as Molinio-Arrhenatheretea or Onopordetalia acanthii in 3.3% of cases, and as Galio-Urticetea in 10.0% of cases. The Onopordetalia acanthii community was misclassified as Festuco-Brometea, Molinio-Arrhenatheretea and Galio-Urticetea in 6.7% of cases, respectively.