【目的】林下光环境异质性是造成林下幼树生长差异的重要原因,研究幼树在不同光环境下的生长差异,可为林下人工更新提供指导。【方法】以华北落叶松林下人工更新的10年生白扦幼树为研究对象,调查幼树的地径、树高,并利用半球面影像技术获取光照指标,在分析幼树生长势及相应林下光环境特征的基础上,通过双因素方差分析及线性回归研究不同光照指数类型对幼树生长的影响。【结果】白扦幼树的地径、树高均服从正态分布(P<0.05),中小径阶的幼树较多,占70%以上,主要受较差的林下光环境影响。不同类型的林下光照指数[散射光指数(ISF,I_ISF)、直射光指数(DSF,I_DSF)、总光照指数(GSF,I_GSF)]对幼树生长的影响差异不显著(P>0.05),但林下散射光对幼树生长的影响比直射光与总光照略强;林下光照指数大小对幼树的地径与树高生长均有显著影响(P<0.05),且对幼树地径的影响较强。当I_ISF<0.4、I_DSF<0.5、I_GSF<0.5时,各组幼树地径、树高差异不显著(P>0.05);当各光照指数高于相应的临界值时,各组幼树生长之间差异显著(P<0.05),且光照指数与幼树生长之间呈显著的正相关。【结论】林下光照指数类型对白扦幼树的生长影响不显著,光照指数大小对幼树生长的影响存在临界值,超过临界值时,幼树生长与光照指数大小呈显著线性关系。在华北落叶松林下进行白扦人工更新时,应选择或使其光照指数大于0.5的地点。

【Objective】Heterogeneity of light conditions is an important factor of growth differences of saplings under the crown. Growth differences of saplings under different light conditions were studied to provide a guidance for an artificial regeneration under pure plantations. 【Method】Artificially planted 10-year-old Picea meyeri in a Larix principis-rupprechtii forest was selected for the present study. The ground diameter and tree height of sapling were investigated, and the light index was obtained by a hemispherical photography. Based on the ground diameter and height of saplings and characteristics of light conditions in this study, the effects of different types and sizes of light indices on growth of saplings were determined by a two-factor analysis of variance and linear regression. 【Result】The ground diameter and height of saplings both conformed to normal distribution (P<0.05). There were more saplings in the middle and small diameter class; they accounted for more than 70% and were mainly affected by poor light conditions. There was no significant difference among the types of understory light indices (indirect site factor,ISF, I_ISF; direct site factor,DSF,I_DSF; and global site factor,GSF,I_GSF) on growth of saplings (P>0.05); moreover, ISF was slightly more significant than DSF and GSF. The light index had a significant effect on the ground diameter and height growth of sapling (P<0.05), and it had a greater effect on the ground diameter. When I_ISF<0.4, I_DSF<0.5 and I_GSF<0.5, there was no significant difference in the ground diameter or height growth of understory saplings among the different light index groups (P>0.05). However, there were significant differences among them (P <0.05) when the light index was higher than the threshold. The light index was significantly and positively related to sapling growth. 【Conclusion】The type of light index has no significant effect on the growth of Picea meyeri saplings. There is a threshold for the effect of light indexes on the sapling growth to be significant; beyond the threshold, the growth of understory sapling has a significant linear relationship with the light index. The planting point with the light index >0.5 should be selected or created when afforesting Picea meyeri in the understory of Larix principis-rupprechtii forest.