Forests advancements to grasslands and their influence on soil formation: Forest Steppe of the Central Russian Upland

Abstract

Natural replacement of grasslands by forests during the Late Holocene and in the modern period was observed. Soil changes as reaction to this phenomena were analyzed. Long-term transformation of steppe Chernozems into forest Phaeozems and Luvisols has been studied in archaeological landscapes in 5 key sites by comparison of paleosols buried under Early Iron Age defensive ramparts, with modern background soils of broad-leaved natural environments. Short-term changes of Chernozems covered by tree cover were studied in 3 key sites, presented by 55 yrs broad-leaved shelterbelts, planted on initially open spaces with Chernozems in agroforestry landscapes. Long-term (during the last 2800-2000 yrs) steppe Chernozems transformation into the forest categories of soils leaded to decrease of humus pools in the soil profiles more than 2 times. After 55 years of steppe Chernozems existence under a tree canopy (in shelterbelts) soil profiles exhibited an increase of humus stocks to depths 1 m in average on 13% at initial values. Stages of Chernozem evolution in response to changing land cover from forest to grassland is discussed. Natural advancement of forest over grasslands continues in the contemporary period. From 1970-2014 rates of climatically determined frontal advancement of forests edges onto grasslands, studied in 6 key sites in different regions of forest steppe, have linear connection with the hydrothermal coefficient (HTC). According to this linear trend, rate of forests advancement to grasslands in modern period declines to zero at HTC 0.75 (the index of the center of steppe zone).