Soil erosion rates during the Holocene continuity in a forest‐steppe landscape

Abstract

Despite a long‐term human impact, Central and Eastern Europe exhibit patches of steppe ecosystems having the highest plant species diversity worldwide. These unique ecosystems have persisted over millennia even though the local climatic conditions would support the formation of a closed forest. Several sources of disturbances have contributed to the persistence of the forest‐steppe landscape such as grazing, fire events and human impact. These disturbances have been recorded in local erosion rates. To gain a deeper understanding of the soil dynamics we aimed at deciphering the long‐ and short‐term erosion rates and the age of the soil mantle. The steppes in Transylvania, Romania, were studied to find evidence of a Holocene continuity of grasslands. Long‐term (millennia) average erosion rates were determined using meteoric 10Be in soils and in situ 10Be of rock outcrops (scarp). Long‐term rates were also estimated by the percolation theory. Short‐term (last few decades) erosion rates were obtained from 239+240Pu in soils. The soils started to form prior to the Last Glacial Maximum, probably during the Eemian Interglacial. The average, long‐term erosion rates varied between 0.18 and 0.63 t ha−1 yr−1. These rates are slightly elevated compared to expected soil erosion rates. The soils of the Transylvanian Plain formed over a long period and reached a quasi‐steady state (soil production equals denudation) that contributed to the maintenance of a biodiversity‐rich forest‐steppe landscape. The slightly elevated erosion rates are an effect of factors that contributed to the Holocene continuity (fire, grazing) and indicate open rather than a forested character of the landscape during soil development. During the last few decades, the erosion rates increased by a factor of 5–10, with values in the range of 1.31–4.05 t ha−1 yr−1. These large differences are caused by changes in human management of the soils. The biodiversity‐rich forest‐steppe landscapes are now under threat.

Relationship between extreme species richness and Holocene persistence of forest-steppe grasslands in Transylvania, Romania

“…However, we accepted and assumed colluviation and the presence of creep. These processes were confirmed radiometrically in the profiles (Püntener et al, 2023). Moreover, the radiometry revealed a limited occurrence of past mechanical disturbance at some sites, e.g.…”

“…On the other hand, forest-steppe soils, Phaeozems, were in an initial stage of (e.g. clay) translocation: clay coatings were practically absent in the soils of the Fânaţele Clujului I and II sites (Püntener et al 2023). This supports the idea of a long-term local presence of grasslands, as grasses may slow down the progressive development of soils (pH decrease, leaching of carbonates, translocation of nutrients and clay; Eckmeier, et al 2007; Marković, et al 2018).…”

“…However, the most species-rich grasslands occur on the periphery of the Transylvanian Basin (Dengler et al, 2012; Roleček et al 2021), in an area that is climatically favourable for mesophilous forests; such forests do indeed occur near the well-preserved grassland sites (Bădărău, 2005). Also local soils show signs of historical occurrence of forests or at least woody species (Püntener et al, 2023). They are no longer Chernozems as in the central part of the Transylvanian Basin, but Phaeozems and even Luvisols, i.e.…”

“…They are no longer Chernozems as in the central part of the Transylvanian Basin, but Phaeozems and even Luvisols, i.e. soils dominated by the vertical translocation of substances in the environment of the transition between Ustic and Udic moisture regime (Eckmeier et al, 2007; Püntener et al, 2023).…”

“…The dominant soil-forming process is the gradual translocation of clay and organic matter (Püntener et al, 2023; own data). The bedrock is in all cases Tertiary sediments, mainly calcareous claystones and marlstones, with a higher proportion of sand at Sălicea.…”