【目的】研究2年生毛白杨(Populus tomentosa)细根对树种组成和林分密度的分布及适应策略,为杨树人工林初植以及混交配置提供参考。【方法】以3 m×3 m、3 m×6 m、6 m×6 m等3种林分株行距(密度)的2年生毛白杨‘北林1号’(P. tomentosa×P. bolleana)×(P. alba×P. glandulosa)纯林及其与刺槐(Robinia pseudoacacia)混交林(林分株行距3 m×6 m)为研究对象,采用根钻法取根样,垂直方向上取样深度为150 cm,每10 cm为1层;混交林与林分株行距为3 m×6 m和6 m×6 m纯林取样点分别在距树10、30、50、100、150、200、250和300 cm处,林分株行距为3 m×3 m纯林取样点在10、30、50、100、150 cm处,共取得1 305个根样。随后对根样进行扫描和烘干处理,并称干质量,得到不同树种组成和林分株行距下不同深度和水平距离的细根形态指标和生物量数据。【结果】①树种组成对细根生物量密度无显著影响,对根长密度和根表面积密度有显著影响。垂直方向上,混交林和纯林中的毛白杨细根变化趋势未发生变化,均随着土壤深度的增加而减少。水平方向上,纯林细根生物量密度主要聚集在水平距树[0,10) cm,混交林内毛白杨细根生物量密度则主要聚集在[0,50) cm。二维分布上,混交林内毛白杨的细根密集分布区更深,水平分布相对均匀,水平侧根更加舒展。②林分株行距对细根生物量密度无显著影响,高密度林分的根长密度、根表面积密度和平均直径显著大于其他两种密度林分的。垂直方向上,株行距3 m×3 m林分的细根生物量密度在[50,60) cm土层显著高于其他两种林分的(P<0.05),并随着林分密度的增大,林木逐渐向下层土壤分配更多的细根;株行距3 m×6 m和6 m×6 m林分的细根生物量密度随着土壤深度的增加而逐渐减小,而株行距3 m×3 m林分的细根生物量密度在[0,60) cm土层逐渐增多,呈单峰分布。水平方向上,在距树150 cm处,株行距3 m×3 m林分的根长密度和根表面积密度显著大于其他两种密度林分的。二维分布上,株行距3 m×3 m林分的细根密集分布区更深,株行距3 m×6 m林分的细根水平伸展更远。【结论】混交使毛白杨根长密度与根表面积密度显著降低。垂直方向上,随着林分密度的增大,林木细根逐渐向下伸长,向下层土壤分配比例增多;水平方向上,高密度林分细根生物量密度随着距树距离的增加而减小的趋势逐渐明显。树种组成和林分密度对2年生毛白杨幼龄林细根生物量密度影响均不显著,但株行距3 m×3 m纯林林分的根长密度和根表面积密度显著大于混交林和其他两种密度林分。3 m×3 m高林分密度毛白杨纯林的细根生长最旺盛,通过形态可塑性而非改变生物量充分利用土壤水养条件,在幼龄期相比混交林和其他密度林分内毛白杨更能有效地利用土壤资源。

【Objective】 The study aimed to investigate the response of the fine roots of the 2-year-old ‘Beilin 1’ cultivar of young Populus tomentosa to variations in the species composition of trees and stand density, and to study their distribution and adaptation strategies following variations in species composition and stand density at the young stand stage. The initial plantation and the mixed configuration were used as references.【Method】A mixed P. tomentosa forest (plant spacing 3 m × 6 m) and pure biennial P. tomentosa forests of three different densities (3 m × 3 m, 3 m × 6 m, and 6 m × 6 m) were selected as the research objects. The root drilling method was used for sampling, and the samples were collected from a depth of 150 cm in the vertical direction, with every 10 cm representing a different layer. The sampling points were located at distances of 10, 30, 50, 100, 150, 200, 250 and 300 cm from the trees in the horizontal direction, while the sampling points for the 3 m × 3 m stand were located at distances of 10, 30, 50, 100 and 150 cm, a total of 1 305 root samples were obtained. The obtained root samples were scanned, dried, and weighed for measuring the dry mass. The morphological characteristics of the fine roots and the biomass data at various depths and horizontal distances were assessed across different species compositions and stand densities. 【Result】The findings revealed that the species composition did not significantly affect the fine root biomass density, but significantly affected the root length density and root surface area density. The characteristics of the fine roots of P. tomentosa in the pure and mixed forests did not exhibit any alterations in the vertical direction, all decreased with an increase in soil depth. The fine root biomass density of P. tomentosa in the mixed and pure forests was primarily concentrated at distances of 0-50 and 0-10 cm, respectively, from the trees in the horizontal direction. The biomass density of fine roots of P. tomentosa in mixed forests was mainly concentrated at 0-50 cm. Analysis of the two-dimensional distribution revealed that the fine roots of P. tomentosa were more densely distributed at greater depths in the mixed forest, their horizontal distribution was relatively uniform, and the horizontal lateral roots were more elongated. The stand density did not significantly affect the fine root biomass density, and the root length density, root surface area density, and average diameter of the roots in the high-density stands were significantly greater than those of the other two stands with different densities. The fine root biomass density in the 40-50 cm soil layer of the 3 m × 3 m stand was significantly higher than that of the other two stands (P < 0.05). The finer roots were more densely distributed in the deeper soil layers. The fine root biomass density in the 3 m × 6 m and 6 m × 6 m stands decreased gradually at increasing soil depths, while that of the 3 m × 3 m stand gradually increased in the 0-60 cm soil layer, indicating a unimodal distribution pattern. The root length density and root surface area density of the 3 m × 3 m stand at 150 cm from the trees in the horizontal direction were significantly higher than those of the other two stands of different densities. Analysis of the two-dimensional distribution pattern revealed that the fine roots in the 3 m × 3 m stand were more densely distributed at greater depths, while those in the 3 m × 6 m stand were more horizontally elongated.【Conclusion】Mixing significantly reduced the root density and root surface area density of P. tomentosa. An increase in stand density caused the fine roots to be gradually elongated with depth, and increased their distribution in the deeper soil layers. The fine root biomass density gradually decreased in the horizontal direction at increasing distances from the trees in the high-density stand. The species composition and stand density did not significantly alter the fine root biomass density of young 2-year-old P. tomentosa trees, but the root length density and root surface area density in the pure forest and the 3 m × 3 m stand were significantly higher than those of the mixed forest and the other two stands with different densities. The growth of the fine roots of P. tomentosa was most vigorous in the pure forest and in the 3 m × 3 m high-density stand. The soil water and nutrients were fully utilized by morphological plasticity rather than by alterations in biomass, and the soil resources were more effectively utilized in the young forest than in the mixed forest and in the forest stands of varying densities.