Moisture availability versus grazing and burning as drivers of Holocene forest-grassland coexistence in Europe: A case study from open ecosystems of southeastern Romania

Southeastern Europe is home to remnants of highly diverse open ecosystems, including grasslands and forest-steppe. To understand the impacts of climate changes, fire disturbance, and herbivory on forest-grassland coexistence in this region, we integrated new and published paleoecological data from two sedimentary records in southeastern Romania with information on herbivore population dynamics and dietary habits. Our findings indicate that fluctuations in forest-grassland cover closely mirrored changes in regional growing season moisture availability in the Black Sea region. During periods with increased moisture availability (e.g. 6000–3800 cal yr BP), diverse broadleaved forest dominated by Quercus expanded. Conversely, more drought-tolerant herbaceous cover increased during drier intervals (7200-6800 and 3800-2000 cal yr BP). We identified a critical tree cover of 25–40% at Mangalia Herghelie and 25–55% at Lake Oltina where neither forest nor grassland dominated. Stable forest states emerged above 40% tree cover and 55% respectively, while grassland-dominated states had tree cover below 25%.

Disturbances by fire and herbivores fluctuated over time, and were further influenced by human activity, which along with deforestation, altered the composition and extent of steppe and forest-steppe vegetation. High fire severity occurred during intermediate moisture conditions and tree cover (6800–5500, 3800–2800 cal yr BP), while low fire severity was observed when herbaceous biomass dominated (7200–7000, 2800–2000 cal yr BP) or under greater tree cover (5500–3800 cal yr BP). Herbivore dynamics shifted from large-bodied grazers in the Neolithic and early Eneolithic (7650–6550 cal yr BP; Prehamangia and Hamangia cultures) to a more diverse array of feeding types involving smaller domestic and wild herbivores along with sedentary agriculture during the flourishing Eneolithic (6550-5850 cal yr BP; Gumelnita culture), before returning to larger domestic grazers as well as omnivores in the Bronze and Iron Ages (3500–2000 cal yr BP). Large-bodied herbivores with selective diets (primarily grazers) had a more substantial effect on grasses compared to mixed feeders with bulk diets, whereas domestic herbivory was associated with increased apophyte abundance and diversity. Our findings underscore the essential factors for sustainable management of forest-grassland mosaics under changing climate conditions with projected accelerating aridification. They also highlight the need for continued research to enhance our understanding of these biodiverse, sensitive open ecosystems to inform effective management strategies.