ALBERT

Description: Northern Africa’s past climate is characterized by a prolonged humid period known as the African Humid Period (AHP), giving origin to the “Green Sahara” and supporting human settlements into areas that are now desert. The spatial and temporal extent of climate change associated with the AHP is, however, subject to ongoing debate. Uncertainties arise from the complex nature of African climate, which is controlled by the strength and interactions of different monsoonal systems, resulting in meridional shifts in rainfall belts and zonal movements of the Congo Air Boundary. Here, we examine a ∼12,500-years record of hydroclimate variability from Lake Dendi located in the Ethiopian highlands based on a combination of plant-wax-specific hydrogen (δD) and carbon (δ13C) isotopes. In addition, pollen data from the same sediment core are used to investigate the response of the regional vegetation to changing climate. Our δD record indicates high precipitation during peak AHP (ca. 10 to 8 ka BP) followed by a gradual transition toward a drier late Holocene climate. Likewise, vegetation cover changed from predominant grassland toward an arid montane forest dominated by Juniperus and Podocarpus accompanied by a general reduction of understory grasses. This trend is corroborated by δ13C values pointing to an increased contribution of C3 plants during the mid-to late Holocene. Peak aridity occurred around 2 ka BP, followed by a return to a generally wetter climate possibly linked to enhanced Indian Ocean Monsoon strength. During the last millennium, increased anthropogenic activity, i.e., deforestation and agriculture is indicated by the pollen data, in agreement with intensified human impact recorded for the region. The magnitude of δD change (40‰) between peak wet conditions and late Holocene aridity is in line with other regional δD records of East Africa influenced by the CAB. The timing and pace of aridification parallels those of African and Indian monsoon records indicating a gradual response to local insolation change. Our new record combining plant-wax δD and δ13C values with pollen highlights the sensitive responses of the regional vegetation to precipitation changes in the Ethiopian highlands.

Description: The Mongolian steppes with a long history of nomadic pastoralism cover a large area of the Palaearctic steppe biome and are still relatively intact. As livestock number has increased over the last two decades, grazing has been considered as the main reason of pasture degradation. However, the impact of grazing on vegetation dynamics, and its interaction with climate, is still not clear. We reviewed 44 publications in Mongolian language, covering 109 sites in five main steppe types, i.e., desert, dry, meadow, mountain, and high mountain steppe, with a mean annual precipitation and temperature range from 120 to 370 mm and from −6 to +5°C, respectively. We calculated relative changes in vegetation cover, species richness, and aboveground biomass from heavily grazed with respect to lightly/non-grazed conditions. Multiple linear regression models were used to test the impact of environmental factors, i.e., mean annual precipitation, coefficient of variation for precipitation, mean annual temperature and elevation. Grazing had a stronger effect on the vegetation of dry, desert and high mountain steppes, whereas its effect was less pronounced in the meadow and mountain steppes with mesic climate and high productivity. Vegetation cover, species richness and aboveground biomass were reduced by heavy grazing in the dry, desert and high mountain steppes. In the meadow steppes, grazing reduced vegetation cover, but increased richness and had nearly no effect on biomass. In the mountain steppe, richness and cover were not affected, but biomass was reduced by heavy grazing. Additionally, grazing effects on biomass tended to be more pronounced at sites with higher amounts of annual precipitation, and effects on cover changed from negative to positive as elevation increased. In conclusion, grazing effects in Mongolian steppes are overall negative in desert, dry and high mountain steppes, but no or even positive effects are found in meadow and mountain steppes. Especially, heavy grazing showed a detrimental effect on all vegetation variables in desert steppes, indicating the existence of combined pressure of climate and grazing in arid habitats, making them potentially sensitive to overgrazing and climate change. Grassland conservation and management should consider characteristics of different steppe types and give importance to local environmental conditions.