| The loess plateau is a semi-humid and semi-arid area of China. Because of long time drought in recent years, soil desiccation caused by severe soil water deficit is an increasing serious limiting factor of artificial forest growth on the loess plateau. The desiccated soil layer is a hidden trouble of artificial forestland construction and lead to artificial tree growth become slowly and got deteriorated gradually, which further limit the step of vegetation construction of the loess plateau. Based on soil moisture observation in deep soil layers of the artificial forestland, the WinEPIC model, which was developed by USA and is a soil and water resources management and plant productivity simulation model, was used to simulate water productivity and soil desiccation effects in 0-10m soil profile on Black locust tree and Chinese pine tree forestland on semi-humid, highland and gully region and semi-arid, hilly-gully region of the loess plateau during 1957 to 2001. The trends water productivity change and soil desiccation of artificial forestland were analyzed based on simulated data, and stability of artificial forestland growth were determined, which provide the basic scientific foundations for withdrawing cropland to forestland and vegetation reconstruction on the loess plateau. The progresses of this study are as following:(1) Severe soil desiccation already occurred on artificial Black locust tree and Chinese pine tree forestland on the both semi-humid and semi-arid regions. Soil water overuse amount in 0-10m soil profile of Black locust tree and Chinese pine tree forestland on semi-humid regions was 363.9mm and 533.7mm separately, thickness of desiccated soil layers was over 7.3m and 9.5m respectively. On semi-arid area, soil water overuse amount and thickness of desiccated soil layers in 0-10m soil profile of Black locust tree was 758.5mm and over 10m.(2) The databases of daily weather variable sequences, physical and chemical properties of soil profile, crop growth parameters of WinEPIC model at Changwu, Luochuan and Yan'an were built for the simulation. The model verification results show that the WinEPIC model could simulate water productivity and soil water dynamics of artificial Black locust tree and Chinese pine tree forestland on the loess plateau precision.(3) During 1957-2001 simulation period, the average of simulated annual biomass of Black locust tree forestland at Changwu and Yan'an were 5.52t/hm2 and 5.37t/hm2 respectively, and simulated annual biomass decreased obviously and gradually with fluctuation after 5-6 years old; the average of simulated annual biomass of Chinese pine tree forestland at Luochuan and Yan'an were 4.29t/hm2 and 3.62t/hm2 respectively, and simulated annual biomass decreased obviously with fluctuation after 17 and 13 years old separately.(4) During 1957-2001 simulation period, drought stress occurred on 6-7 years old Black locust tree forestland at Changwu and Yan'an, drought tress days increased gradually with fluctuation after that year, and its at Yan'an was evidently higher than at Changwu; Drought stress occurred on 18 years old Chinese pine tree forestland at Luochuan and on 16 years old Chinese pine tree forestland Yan'an, and drought stress days increased obviously with fluctuation after that year.(5) During 1957-2001 simulation period, from 1 to 7 years old was strong soil desiccation period on Black locust tree forestland at Changwu and Yan'an, monthly available soil water amount in 0-10m soil layers on the forestland at both locations changed sustainable and fluctuated with rainfall at low level of 0-250mm for a long period after 8 years old. The 14 to 20 years old at Luochuan and the 9 to 20 years old at Yan'an were strong soil desiccation period on Chinese pine tree forestland, and monthly available soil water amount in 0-10m soil layers on the forestland at both locations changed sustainable and fluctuated with rainfall at low level of 0-150mm.(6) During 1957-2001 simulation period, soil moisture in 0-10m soil profile on Black locust tree forestland at Changwu and Yan'an deceased gradually with growth age increasing. Desiccated soil layers occurred and its thickness increased annually, it was over 10m when growth age was 7 years old. Thickness of desiccated soil layers on 19 years old Chinese pine tree forestland at Luochuan and Yan'an were over 10m, soil moisture in 2-10m soil layers kept stable desiccated conditions after 20 years growth at Luochuan and 16 years growth at Yan'an.(7) Concluded from simulated results, stable growth age of Black locust tree forestland was over 45 years at Changwu and under 40 years at Yan'an. Maximum soil water use age of Chinese pine tree forestland at Luochuan and Yan'an were 20 years. The water productive stability of Black locust tree and Chinese pine tree forestland were low and foreground of artificial vegetation reconstruction on the loess plateau may be pessimistic.
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