| The Ziwu region of Loess Plateau is one of most fragile ecosystems in China. It plays a great role in keeping ecological balance for Loess Plateau, regulating regional climate, and keeping economical growing for eastern Gansu and northern Shaanxi. We choice the major advantage forest species: Quercus liaotungensis, Pinus tabulaeformis and Pinus tabulaeformisf.shekannesis forests to study the geographic distribution, basic biological characteristics, the population and community feature, in order to develop forest restoration and sustainable management in the Loess Plateau. Main results were as follows:1. Quercus liaotungensis forest mainly located in 1200-1750 m. It is single-priority community in general. The undergrowing shrubs have scattered distribution and grow lowly which cover 12% -25% and have 0.6 m on average. The undergrowing herbs cover 34.6% and have 12-24 cm on average. Pinus tabulaeformis forest mainly located in 1350-1650 m. It is single-priority community in general. The undergrowing shrubs cover 11% -29% and have 1.3 m on average. The undergrowing herbs cover 42.6% and have 9-26 cm on average. Pinus tabulaeformisf.shekannesis forest mainly located in 1200-1450 m. It is single-priority community in general. The undergrowing shrubs cover 36%-60% and have 0.5-1.6 m on average. The undergrowing herbs cover 10%-26% and have 9-26 cm on average.2. The distribution of Pinus tabulaeformis forests has the relatively same age; The distribution of Quercus liaotungensis and Pinus tabulaeformisf.shekannesis forest are uneven-aged, and with age increasing, this three communities's survival number is decreasing but the rate of death is different. The strength of death in Quercus liaotungensis is highest among three communities at early age. Pinus tabulaeformis and Pinus tabulaeformisf.shekannesis communities are belonged to increased population. Fitting equation of Quercus liaotungensis, Pinus tabulaeformis and Pinus tabulaeformisf.shekannesis forest between the diameter and the height were: y = -0.0081x2 + 0.6996x +0.023, y = -0.0177x2 + 0.8551x - 0.0874, y = -0.0056x2 + 0.8286x - 0.2747. The average diameter of Quercus liaotungensis is 21.06 cm, average high is 11.16 m, The average diameter of Pinus tabulaeformis is 11.49 cm, average high is 7.4 m, The average diameter of Pinus tabulaeformisf.shekannesis is 17.16 cm, average high is 12.30 m.3. The Quercus liaotungensis, Pinus tabulaeformis and Pinus tabulaeformisf.shekannesis spatial distribution pattern showed a certain changes in different sample sizes and age stages. With the increasing sampling area, the population aggregation showed small→large→small trend; with the increasing of forest age, population trends from the aggregate distribution to random distribution.4. The Pinus tabulaeformis species are abundantest and evenly distributed in three communities, Pinus tabulaeformisf.shekannesis forests have smallest species, and the characteristics of plant diversity from low to high are tree layer, shrub layer and herbage layer. The nature regeneration forest situations appeared significantly different interfering by stand structure, physical environment, man factors and so on. The number of young Quercus liaotungensis plants is 80-210/hm2. The number of young Pinus tabulaeformis plants is 27-60/hm2 in single-priority community and 55-190/hm2 in mixed community. The number of young Pinus tabulaeformisf.shekannesis plants is 150-220/hm2.5. The litter of Quercus liaotungensis, Pinus tabulaeformis and Pinus tabulaeformisf.shekannesis forest accounted to 32.65 t/hm2, 48.95 t/hm2, and 47.30 t/hm2 respectively. The nature water holding of the Quercus liaotungensis litter, Pinus tabulaeformis litter and Pinus tabulaeformisf.shekannesis litter is 105.5%, 125.9% and 121.5% respectively. The maximum water holding of the Quercus liaotungensis litter, Pinus tabulaeformis litter and Pinus tabulaeformisf.shekannesis litter is 50.93 t/hm2, 70.15 t/hm2 and 61.68 t/hm2 respectively. The effective water holding of the Quercus liaotungensis litter, Pinus tabulaeformis litter and Pinus tabulaeformisf.shekannesis litter is 38.13 t/hm2, 50.38t/hm2 and 43.81t/hm2 respectively.
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