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Abstract: Bush encroachment is a significant phenomenon in savanna environments as it affects wildlife and local livelihoods by preventing new pasture generation. In this article we present a 2000-year record of vegetation change in the Dara range of the Mago National Park, southwestern Ethiopia, an area inhabited by Mursi agro-pastoralists. We use an interdisciplinary approach to understand whether bush encroachment in this area is a recent event or a transitional state of the savanna and describe the local understanding of encroachment as a species-specific process. The vegetation record was obtained from a fossil hyrax midden, a type of sediment already used in Southern Africa but never before in East Africa. Six encroaching phases, led by Capparaceae and Grewia, were found over the last two millennia. The system proved to be resilient, with alternating open and encroached phases, and showed a non-linear response to environmental change, thereby fitting the control theory hypothesis for hysteresis loops. Determining the thresholds conditioning the system''s resilience could help to improve savanna management for both local people and National Park authorities. [Copyright &y& Elsevier]

A Holocene pollen record from Co. Clare, western Ireland, shows unusually high Ulmus pollen frequencies until about 2500 cal. BP, interrupted by four brief episodes of reduced Ulmus pollen between 6000 and 3000 cal. BP. Localized anthropogenic disturbance of the vegetation during this interval is inferred from higher magnetic susceptibility of the sediments, indicating soil erosion, and from increased Poaceae and Plantago pollen. The continued abundance of elm at several sites in Ireland, despite human impact, suggests that disease was a less significant factor in the reduction of Irish elm populations than elsewhere in northwestern Europe. It may be that the pathogen, its dispersal agent, or susceptible elm ecotypes were locally absent near the western range margin of elm, reinforcing the argument that the combined effects of human exploitation and a pathogen were necessary to cause the permanent elm decline at 6000 cal. BP in most of northwestern Europe. [ABSTRACT FROM AUTHOR]

Pollen and charcoal analysis of sediment cores from two lakes in the highlands of northern Ethiopia provide evidence that the vegetation has changed in response to human impact during the last 3000 years. The natural, pre-disturbance vegetation of the area was Podocarpus-Juniperus forest. At about 500 BC, following Semitic immigration to northern Ethiopia, the forests were cleared and replaced by a secondary vegetation of Dodonaea scrub and grassland that persisted for 1800 years. Grasslands were dominant from about AD 1200 to 1400, probably as a result of further intensification of grazing, perhaps exacerbated by drought. Juniperus forest, with Olea and Celtis, then expanded from AD 1400 to 1700, possibly because of drought-induced depopulation followed by increased rainfall. Deforestation and soil erosion has again intensified during the last three centuries. Since forest regrowth was possible after 1800 years of human impact, northern Ethiopia should again be capable of supporting forest under appropriate land management. [ABSTRACT FROM AUTHOR]

Pollen data from a 625 cm sediment core from Crescent Island Crater, a subbasin of Lake Naivasha, Kenya, provide an 1100-year record of vegetation change at a mean time resolution of 15 years. Earlier data from the same core show a record of lake depth (and thus inferred rainfall variation) in the form of changing sedimentary facies, supported by salinity inferences based on diatom and chironomid assemblages. Stratigraphic variation in the abundances of aquatic and semi-aquatic plants and chlorophyte algae generally supports the reconstructed lake depths. Woody Afromontane and woodland plant taxa decreased relative to Poaceae (grasses) during low rainfall periods, and increased during high rainfall periods. A decrease in the extent of lower montane forest coincident with favourable climatic conditions after about AD 1700 was probably caused by Kikuyu immigration and population expansion around that time, as indicated by simultaneous appearance of the food crop Zea mays (maize). Increases in ruderal herbaceous and exotic tree pollen during the twentieth century indicate increased local landscape disturbance during the colonial period. There is little direct pollen evidence for accelerating clearance of the montane forest during recent decades. [ABSTRACT FROM AUTHOR]

Pollen data from a radiocarbon-dated lake-sediment core from the Middle Atlas of Morocco show that forests of evergreen and winter-deciduous oaks (Quercus rotundifolia, Q. canariensis), present from the start of the Holocene, were invaded at 6200 BP by Cedrus atlantica in low numbers. Two thousand years later, after a short period characterized by higher deciduous oak frequency, Cedrus increased to its present abundance. These changes may have been in response to increasing effective moisture as summer temperatures decreased with declining northern-hemisphere seasonality, under the influence of precessional forcing. Palaeolimnological data from the same core show that the lake level fell sharply in five 200-400 yr-long arid intervals, but the pollen data show little or no evidence of a vegetational response. This suggests that summer (growing-season) rainfall remained adequate during the dry intervals, whereas depletion of the groundwater aquifer was the result of reduced winter precipitation. Anthropogenic exploitation since c. 1300 BP has had a greater effect on the forest ecosystem than any of the Holocene arid intervals. [ABSTRACT FROM PUBLISHER]

Leads, lags, or synchronies in climatic events among different regions are key to understanding mechanisms of climate change, as they provide insights into the causal linkages among components of the climate system. The well-studied transition from the Lateglacial to early Holocene (ca. 16–10 ka) contains several abrupt climatic shifts, making this period ideal for assessing the spatio-temporal structure of climate change. However, comparisons of timings of past climatic events among regions often remain hypothetical because site-specific age scales are not necessarily synchronised to each other. Here we present new pollen data (n = 510) and mean annual temperature reconstruction from the annually laminated sediments of Lake Suigetsu, Japan. Suigetsu's 14C dataset is an integral component of the IntCal20 radiocarbon calibration model, in which the absolute age scale is established to the highest standard. Its exceptionally high-precision chronology, along with recent advances in cosmogenic isotope studies of ice cores, enables temporally coherent comparisons among Suigetsu, Greenland, and other key proxy records across regions. We show that the onsets of the Lateglacial cold reversal (equivalent to GS-1/Younger Dryas) and the Holocene were synchronous between East Asia and the North Atlantic, whereas the Lateglacial interstadial (equivalent to GI-1/Bølling-Allerød) started ca. two centuries earlier in East Asia than in the North Atlantic. Bimodal migration (or 'jump') of the westerly jet between north and south of the Tibetan plateau and Himalayas may have operated as a threshold system responsible for the abruptness of the change in East and South (and possibly also West) Asia. That threshold in Asia and another major threshold in the North Atlantic, associated with switching on/off of the Atlantic meridional overturning circulation (AMOC), were crossed at different times, producing a multi-centennial asynchrony of abrupt changes, as well as a disparity of climatic modes among regions during the transitional phases. Such disparity may have disturbed zonal circulation and generated unstable climate during transitions. The intervening periods with stable climate, on the other hand, coincided with the beginnings of sedentary life and agriculture, implying that these new lifestyles and technologies were not rational unless climate was stable and thus, to a certain extent, predictable. [Display omitted] • The Lateglacial cold reversals in Lake Suigetsu (Japan) and NGRIP were synchronous. • The Lateglacial interstadial in Lake Suigetsu began earlier than in Greenland. • 'Postglacial' in Asia started already at ca. 15.0 kyr BP. • S–N migration of the westerly jet generated abrupt deglacial warming in Asia. • Climatic stability was the key for humans to commence agriculture. [ABSTRACT FROM AUTHOR]