Mid-Holocene and glacial-maximum vegetation geography of the northern continents and Africa

BIOME 6000 is an international project to map vegetation globally at mid-Ho locene (6000 C-14 yr sp) and last glacial maximum (LGM, 18,000 C-14 yr sp), with a view to evaluating coupled climate-biosphere model results. Primary palaeoecological data are assigned to biomes using an explicit algorithm b ased on plant functional types. This paper introduces the second Special Fe ature on BIOME 6000. Site-based global biome maps are shown with data from North America, Eurasia (except South and Southeast Asia) and Africa at both time periods. A map based on surface samples shows the method's skill in r econstructing present-day biomes. Cold and dry conditions at LGM favoured extensive tundra and steppe. These biomes intergraded in northern Eurasia. Northern hemisphere forest biomes w ere displaced southward. Boreal evergreen forests (taiga) and temperate dec iduous forests were fragmented, while European and East Asian steppes were greatly extended. Tropical moist forests (i.e. tropical rain forest and tro pical seasonal forest) in Africa were reduced. In south-western North Ameri ca, desert and steppe were replaced by open conifer woodland, opposite to t he general arid trend but consistent with modelled southward displacement o f the jet stream. The Arctic forest limit was shifted slighly north at 6000 C-14 yr sr in som e sectors, but not in all. Northern temperate forest zones were generally s hifted greater distances north. Warmer winters as well as summers in severa l regions are required to explain these shifts. Temperate deciduous forests in Europe were greatly extended, into the Mediterranean region as well as to the north. Steppe encroached on forest biomes in interior North America, but not in central Asia. Enhanced monsoons extended forest biomes in China inland and Sahelian vegetation into the Sahara while the African tropical rain forest was also reduced, consistent with a modelled northward shift of the ITCZ and a more seasonal climate in the equatorial zone. Palaeobiome maps show the outcome of separate, independent migrations of pl ant taxa in response to climate change. The average composition of biomes a t LGM was often markedly different from today. Refugia for the temperate de ciduous and tropical rain forest biomes may have existed offshore at LGM, b ut their characteristic taxa also persisted as components of other biomes. Examples include temperate deciduous trees that survived in cool mixed fore st in eastern Europe, and tropical evergreen trees that survived in tropica l seasonal forest in Africa. The sequence of biome shifts during a glacial- interglacial cycle may help account for some disjunct distributions of plan t taxa. For example, the now-arid Saharan mountains may have linked Mediter ranean and African tropical montane floras during enhanced monsoon regimes. Major changes in physical land-surface conditions, shown by the palaeobiome data, have implications for the global climate. The data can be used direc tly to evaluate the output of coupled atmosphere-biosphere models. The data could also be objectively generalized to yield realistic gridded land-surf ace maps, for use in sensitivity experiments with atmospheric models. Recen t analyses of vegetation-climate feedbacks have focused on the hypothesized positive feedback effects of climate-induced vegetation changes in the Sah ara/Sahel region and the Arctic during the mid-Holocene. However, a far wid er spectrum of interactions potentially exists and could be investigated, u sing these data, both for 6000 C-14 yr BP and for the LGM.