Abstract: [Objective] The responses of the Thuja sutchuenensis seedling survival and growth to altitude gradient were studied to provide scientific basis for regional selection of reintroduction and expanding cultivation of T. sutchuenensis in the future.[Method]The experimental plots were located at altitudes of 178 (Baize), 1 360, (Yangban) and 2 250 m (Chethana) in the Xuebaoshan Nature Reserve of Chongqing Municipality and the surrounding area. The seedlings used in the experiment were 1.5 years old container-raised seedlings. After seedlings were reintroduced, air temperature and soil temperature in each plot were monitored using an automatic monitoring instrument; survival rate, height and ground diameter of the seedlings were monthly measured. The seedlings were randomly sampled from different experimental plots after growing season to measure their phenotypic traits, fresh mass and dry mass. At the same time, the soil samples were collected to measure their physical and chemical properties. On the basis of these, gray relation analysis (GRA) was applied to quantify the correlation between survival rate, growth and environmental factors.[Result] In the study area, when altitude increases 100 m, the mean annual air and soil temperature respectively decreased by 0.6 and 0.5℃, and ≥ 10℃ air and soil accumulated temperature respectively decreased by 205.3 and 171.7℃.Soil density, pH value and available phosphorus gradually decreased with increasing altitude, while other soil indicators (capillary porosity, total porosity, capillary water capacity, non-capillary water capacity, maximum water capacity, organic matter, total nitrogen, available nitrogen, available potassium) increased. The survival rate of seedlings of middle altitude (Yingpan plot) was the highest (100%), followed by the low altitude (Baihe plot, 94%), the lowest survival rate was found at the high altitude (Chechangba plot, 36%). Seedlings began to die during March to April. The peak of seedling mortality in high altitude appeared from May to August while that in low altitude appeared from July to September. Except for length of the longest first lateral branch and number of first lateral roots (middle altitude > low altitude > high altitude), other phenotypic traits, fresh mass and dry mass of the seedlings increased with increasing altitude (low altitude > middle altitude > high altitude) and showed significant differences among experimental plots (P < 0.05). The survival rate and growth were influenced by various environmental factors. Among them, air temperature and soil temperature were the main factors, followed by soil pH and soil density, finally other soil physical and chemical properties.[Conclusion]The heterogeneity of temperature, moisture and soil fertility caused by the altitudinal gradients had great influence on the survival rate and the growth of the seedlings, which were restricted due to lower air and soil temperature, and soil water for a long-term accumulation in high altitude area. However, the environmental conditions of low and middle altitudes can meet the basic requirement of seedling survival rate and growth. Therefore, the low and middle altitude areas, not the high altitude area (≥ 2 200 m above sea level) were preferred areas for the reintroduction of T. sutchuenensis.

Key words: altitude gradient, Thuja sutchuenensis seedling, survival rate, growth characteristics, gray relation analysis (GRA), response, reintroduction