By taking the Broussonetia papyrifera seedings as the studied objects,the seedlings were divided into three classes by diameter-class of 0.5 cm ～ 1.0 cm,1.0 cm ～ 1.5 cm and ≥ 1.5 cm,and treated with three treating methods such as remaining seedling top and root,truncating top (maintaining aerial parts 30 cm) and cutting root + truncating top (maintaining taproot 5 cm and aerial parts 30 cm).One year after afforestation survival,the biomass,cumulative length and surface area of taproots,first level lateral toots,second lateral toots and fibrous roots of every treatments were surveyed.The results show that the effect of remaining seedling top of the seedlings of diameter-class 0.5 cm ～ 1.0 cm,was the best,the biomass,cumulative length and surface area of root system were 71.3g,758.1 cm,1371.2 cm2,the second effect was truncating top and that of cutting root + truncating top was the worst ; the seedlings of diameter-class 1.0 cm ～ 1.5 cm with the treatment of cutting root + truncating top had the best effect,the total root biomass,cumulative length and surface area were 111.1 g,988.4 cm and 1676.1 cm2,that of remaining seedling top and root was the next,that of truncating top is the worst; the total root biomass,cumulative length and surface area of seedlings of diameter-class ≥ 1.5 cm were 166.9g,2 173.8 em and 4 160.9 cm2.In sum,B.papyrifera nursery-grown seedlings being with diameter ≥ 1 cm need to truncate top or cut root; the seedlings of diameter ＜ 1 cm need not to trmeate roof or cut root.%以构树实生苗为研究对象,分别径级0.5～1.0、1.0～1.5和≥1.5 cm的苗木进行留顶、截顶(地上部分留30 cm)和切根+截顶(留主根5 cm,地上部分留30 cm)3种处理.造林成活一年后调查每个处理根系生长状况,通过测定主根、一级侧根、二级侧根和须根(d≤1 mm)的生物量、累计长度和表面积,研究其对根系生长的影响.结果表明:径级0.5～1.0 cm的苗木,留顶效果最好,根系总生物量、累计长度以及表面积分别为71.3 g、758.1 cm、1371.2 cm2,其次是截顶处理,切根+截顶处理效果最差；径级1.0～1.5 cm切根+截项处理最好,根系总生物量、累计长度以及表面积分别为111.1g、988.4 cm、1 676.1 cm2,留顶次之,截顶最差；径级≥1.5 cm的苗木,根系总生物量、累计长度以及表面积分别为166.9 g、2 173.8 cm、4 160.9 cm2.综上所述,构树造林苗木,在地径≥1 cm时需进行截顶(切根)处理；地径1 cm以下,则无需进行截顶或切根处理.
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