Holocene natural rhythms of vegetation and present potential ecology in the Western Chinese Loess Plateau

Ecological restoration (reconstruction) must have a scientific basis on the biological zones of vegetation mainly controlled by the climatic factors (humidity and thermal conditions) and the sere of it. Holocene natural rhythms of vegetation in the Western Chinese Loess Plateau were reconstructed using pollen assemblages, a powerful proxy for paleo-environmental reconstruction and the potential natural vegetation (PNV) modeled using environmental variable-enveloping approaches with the Holdridge model. The succession of vegetation and geographical pattern of the potential vegetation are discussed. Both have very important implications for ecological restoration and management.During the Holocene (i.e., past 10,000 years), the virgin vegetation in the Western Chinese Loess Plateau has experienced a series of changes. A desert-steppe of late glacial was succeeded by a forest-steppe from 8850 to 7540 14C BP, and a Pinus-dominated forest occupied the landscape from 7540 to 6560 14C BP. A deciduous forest of high density and diversity existed from 6560 to 5790 14C BP. Afterwards the vegetation changed to a Pinus-dominated forest-steppe (5790–4950 14C BP) and then to an Ulmus-dominated forest-steppe (4950 to ∼4000 14C BP). The vegetation subsequently changed to a steppe from ∼4000 to 3120 14C BP and further to a desert-steppe from 3120 to 2900 14C BP. After a period of vegetation improvement (steppe) from 2900 to 2460 14C BP, a desert-steppe resumed (2460–2020 14C BP). Steppe vegetation was re-established around 2020 14C BP and seems to have then deteriorated again around 1000 14C BP.The potential vegetation pattern modeled using environmental variable-enveloping approaches with the Holdridge model indicates that, in the Western Chinese Loess Plateau except for the hill and mountain areas, there are three south–north potential ecological conditions zones corresponding to the S–N gradient of modern climate of this area. They are forests and steppe forest, steppe, and desert, respectively. In the hills and mountains, zonal vegetation is readily observable. Specifically, the lower elevations are dominated by steppes, the middle elevations by steppe forests, and higher elevations by forests. The lower limits of steppe-forest and forest zones are lower in east- and south-facing slopes due to the influence of the East Asian summer monsoon that brings precipitation from the southeast.Results indicate that the southeastern corner and the western fringe should be deciduous forests, the southeastern half steppe forests, and the northwestern half steppes, which will become the main regions of ecological restoration and agricultural development. Only the northwestern corner should be desert-steppe, and in such places human activities should be reduced. In hills and mountains, efforts will be made to prohibit hunting and grazing and plant trees on mountains, return the grain plots to forestry, and carry out water and soil conservation at present.