目的 模拟系列密度西南桦Betula alnoides林分冠层光环境开展树冠遮阴试验，探究光环境对西南桦林木生长和枝条发育的影响，为合理配置造林密度进而实现西南桦优质大径材高效培育提供理论依据。方法 在系列造林密度试验基础上，以5年生、造林密度为4 m×4 m西南桦试验林的优势木和亚优势木为对象，设置3个遮阴梯度处理：遮阴率40%~45%、55%~60%和80%~90%，以无遮阴作对照(CK)，依据西南桦物候期，于10月中上旬开始对下部2/3树冠进行遮阴处理，每隔3个月(或半年)调查参试单株的生长和枝条发育情况。结果 西南桦各处理间的树高和胸径在各调查时间段和整个试验期内的增量均差异不显著 (P≥0.05)，胸径在1—4月份生长最快，各处理的胸径增量均超过0.73 cm。除2018年4—7月份期间枝条长度增量外，枝条发育在各时间段和整个试验期内均受遮阴显著影响(P<0.05)，表现出随遮阴率的增加枝条直径、长度增量呈显著降低趋势，在生长旺季(1—4月份)差异更加明显，CK组的枝条直径和长度增量分别为遮阴率80%~90%处理的1.74和1.78倍，达到1.197 mm和0.135 m。此外，西南桦幼林自然整枝率亦表现出随遮阴率增加而明显增大的趋势，其自然整枝集中发生在4—7月份，整枝率增量均超过47%。结论 适宜的高造林密度形成的弱光环境对西南桦幼林单株生长影响较小，但是可以显著控制枝条发育，促进自然整枝。生产上适当提高西南桦造林密度有利于其优质大径级无节材的高效培育。
Objective A canopy shading experiment was conducted by simulating the light environments of plantations with a series of planting densities, to explore the effects of light environments on tree growth and branch development, and provide a theoretical guidance for the efficient cultivation of large-sized and high quality timber of Betula alnoides through reasonable arrangement of planting density.Method On the basis of a series of afforestation density tests, three shading treatments (40%–45%, 55%–60% and 80%–90% shade rate) with 2/3 of the lower crown shaded and one control (CK) of no shade were set in plots with planting density of 4 m×4 m from a 5-year-old planting density trail of B. alnoides in early October following the local phenology of this species. The dominant tree and the subdominant tree were selected as experiment objects. The tree growth and branch development of all treated individuals were investigated every three or six months.Result The increments of tree height and diameter at breast height (DBH) in each interval and the whole experimental period did not differ significantly among shading treatments (P≥0.05). DBH increment was the highest from January to April, and exceeded 0.73 cm in each treatment. However, branch development in each interval and the whole experimental period was significantly affected by shading (P<0.05), excepting branch length growth from April to July in 2018. Both the increments of branch diameter and length decreased significantly with the increase of shading rate, and the increasing trend was more significant in fast-growing season (from January to April), in which the increments of branch diameter and length of CK was 1.74 and 1.78 times higher than those of 80%–90% shade rate treatment, and reached 1.197 mm and 0.135 m, respectively. Moreover, the natural pruning rate of B. alnoides young plantations also showed an obvious increasing trend with the increase of shading rate, and the natural pruning was focused from April to July with pruning rate increment over 47%.Conclusion The low light environment caused by appropriate high planting density almost has no significant effect on the individual growth of B. alnoides, while it can efficiently control branch development and promote natural pruning. The planting with an appropriate high density would be beneficial for large-sized and high quality timber cultivation of B. alnoides in practice.