| Larix Chinensis Beissn,which is endemic species,only distributed in alpine and subalpine zone from 2800m to 3500m altitude in Taibai Mountain of Qinling Mountains,Shaanxi Province in China. It is the only tree which could consist of pure forest at the wood line in this region. It had been list in the China Red Data of Plants due to its special function in biodiversity conservation, watersouce conservation, national land preservation and ecological balance reservation and so on. Based on the materials of 44 phytoenosis investigative plots, the size-class and age-class structures of Larix Chinensis forests was analyzed, the distributing characteristics of Larix Chinensis forests and the characteristics of timberline at Taibai Mountain were quantitatively analyzed, the changing trend of timberline of Taibai Mountain was forecasted, and some corresponding protecting strategies were proposed. It was concluded as follows: 1. Size-class and age-class structures of Larix Chinensis forests: According to the characteristics of size-class and age-class structures of Larix Chinensis population 8 forest-types studying, the 8 forest-types were classified into 3 types: normal school type, double peaks curve type and reverse letter J distributing type. Utilizing the size-class and age-class structures chart of 8 forest-types, the structure, stability and regeneration regulation of Larix Chinensis forests were comparative analyzed, the results showed that they had very high consistency at describing community characteristics. Through regress analyzing, dependence is remarkable to the DBH and age of Larix Chinensis forests. The density distributing of the size-class and age-class of the 8 forest-types were analyzed, the results indicated that the density of low size-class and low age-class is high, density positively correlated with size-class and age-class. The restricting factor for the survival and development of the population was a short seedling. 2. Quantitative analysis of the distributing characteristics of Larix Chinensis forests and the characteristics of timberline of Taibai Mountain: The distribution of Larix Chinensis forests were analyzed, the results showed that altitude grads have remarkable influence on DBH, tree height, area at breast height, age and species abundance. Altitude grads is the main factors which influence the distribution of Larix Chinensis forests, exposure, slope gradient and soil thickness have some influence too, but not notable, wind direction almost have no influence. Ecological factors which have influence on the distribution of Larix Chinensis forests were arranged in an order. The order is altitude﹥slope gradient﹥soil thickness﹥exposure﹥wind direction. The main factors which decided the characteristics of Larix Chinensis forests were analyzed, the order is DBH﹥tree height﹥age﹥Area at breast height﹥species abundance. The characteristics of timberline of Taibai Mountain were quantitatively analyzed, 4 borderline were carved up according the alpine vegetation of Taibai Mountain. Altitude from3000 meter to 3100 meter was the original center of seed and species of Larix Chinensis forests. Altitude from 3400 meter to 3500 meter was the timberline of Taibai Mountain. Altitude from 3500 meter to 3600 meter was subalpine shrub meadow strip. Above altitude 3700 meter is alpine meadow strip. 3. Changing trend of timberline at Taibai Mountain: The temperature of Taibai Mountain had increased for about 0.6℃in recent 50 years. The timberline at Taibai Mountain is moving up. About the wood line, the Larix Chinensis forests is younger forests, it is also belong to evolve type succession community. The changing trend of timberline of Taibai Mountain was forecasted. In the later 50 years, the wood line of Taibai Mountain would increase for about 80 meters. They belong to different development stages. Forests managers should take different management measure to preserve Larix Chinensis population.
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