川中丘陵区4种乡土阔叶树根系拓扑结构特征

【目的】林木地下根系的养分竞争影响着林木生长、物种共存和森林群落动态,了解川中丘陵区乡土阔叶树根系构型的差异性和相似性,探讨它们在土壤环境中根系的适应策略,为该区域的防护林建设与植被恢复物种选择提供参考。【方法】以川中丘陵区“带状采伐改造”采伐带内补植的4种7年生乡土阔叶树桤木(Alnus cremastogyne)、香椿(Toona sinensis)、喜树(Camptotheca acuminata)、香樟(Cinnamomum camphora)作为研究对象,选择长势良好的桤木、喜树、香椿各7株,香樟6株,测定其地径、胸径、树高和冠幅。采用50 cm×50 cm的网格确定根系的分布位置,按1∶40的比例在50 cm×75 cm的坐标纸上准确绘制出根系的俯视图。将根系全部挖出后,测定各级根系长度及分支前后的直径、各级根数目,根系内、外部连接数目,以及连接长度等指标。【结果】①桤木根系以水平分布占绝对优势(水平分布半径是垂直分布深度的2.29倍),侧根十分发达,根系浅层化;喜树主根十分发达,垂直分布深度是水平分布半径的2.71倍,但其侧根不发达;香樟和香椿则主根和侧根均十分发达。喜树、香樟和香椿均属深根性树种。根系水平分布半径除香樟和香椿间差异不显著外,其余均差异显著(df=3,F=145.007, P<0.01);而垂直分布深度除香樟和喜树间差异不显著外,其余树种间均表现为差异显著(df=3,F=99.478, P<0.05)。②不同树种的根系拓扑指数之间存在显著差异,其中修正拓扑指数(q_a、q_b)、拓扑指数(T_I)存在极显著差异($df_{q_{a}}=3, F_{q_{a}}, P_{q_{a}}<0.01, df_{q_{b}}=3, F_{q_{b}}=4.066, P_{q_{b}}<0.05; df_{T_{I}}=3, F_{T_{I}}=69.561, P_{T_{I}}<0.01$)。桤木、香椿的根系为叉状分支结构,它们的q_a、q_b、T_I分别为0.160、0.097、0.673和0.122、0.047、0.635;喜树、香樟的根系则趋向于鱼尾形分支结构,q_a、q_b、T_I分别为0.582、0.547、0.885和0.364、0.266、0.799。③不同物种的根系平均连接长度差异显著(df=3,F=6.166, P<0.05)。4种阔叶树根系的平均连接长度按大小排序为:香樟>香椿>桤木>喜树。【结论】桤木和香椿主要通过增加次级根系来扩大根系的分布范围,从而增大对营养物质的吸收;而香樟和喜树则主要通过根系垂直向土层中快速延伸,以扩大根系的有效空间,从而更加有效地从土壤中获取水分和营养物质。研究表明,4种乡土阔叶树种均能在川中丘陵区生长,但根据它们根系构型的差异和植物的生态学特性,桤木适合在光照相对充足的地段栽种,喜树、香椿适合在土壤相对肥沃、水分条件较好、光照相对充足的地段栽种,香樟适宜在土壤相对肥沃、水分条件较好的地段栽种。

Topological characteristics of the root systems of four native broad-leaved trees in the central Sichuan hilly region

【Objective】Nutrient competition between the underground roots of trees has profound effects on tree growth,species coexistence,and forest community dynamics.An important trait in this regard is root system architecture,which can be described in terms of the geometric morphological characteristic parameters and topological structure of root systems.In this study,in order to provide theoretical and practical references for the development of protected forests and vegetation restoration in the central Sichuan hilly region,we examined the differences and similarities of the root systems of native broad-leaved trees in this region and their adaptive strategies in the soil environment.【Method】We focused on four native broad-leaved tree species(Alnus cremastogyne,Toona sinensis,Camptotheca acuminate and Cinnamomum camphora)in the central Sichuan hilly region,which had been replanted as part of a Cupressus funebris transformation experiment based on cutting strips in cypress stands 7 years age.For analyses of growth traits,we selected seven healthy individuals of A.cremastogyne,T.sinensis and C.acuminata,respectively,six individuals of C.camphora,then measured their basal diameter,breast-height diameter,height and canopy cover.All root systems(root diameter>3 mm)were extracted and their distributions were determines using 50 cm×50 cm grids.Furthermore,we drew 1∶40-scale overhead views of the root systems using 50 cm×75 cm coordinate paper.The length of roots in each root grade,the root diameter before and after branching,root number,the interior and exterior link number of root as well as the connecting length,were measured using electronic vernier calipers and a steel tape.【Result】①The root system of A.cremastogyne was found to have a predominantly horizontal distribution(the horizontal distribution radius was 2.29 times the vertical depth).Moreover,the lateral roots of this tree are well developed and the root system is shallow.Although the main roots of C.acuminata trees were developed,with vertical depth was 2.71 times the horizontal distribution radius,and the lateral roots were relatively underdeveloped.In contrast,both the main roots and lateral roots of C.camphora and T.sinensis were found to be well developed.With the exception of A.cremastogyne,the trees examined in this study are all deep-rooted species.Apart from C.camphora and T.sinensis,the horizontal distribution radius showed a significant difference(df=3,F=145.007,P<0.05)among the species.Moreover,in the case of the vertical depth of roots,there were significant differences(df=3,F=99.478,P<0.05)among species,with the exception of C.camphora and C.acuminata.②Topological indices were found to differ significantly among the species,of which qa,qb,and TI showed highly significant differences(dfqa=3,Fqa=38.007,Pqa<0.01;dfqb=3,Fqb=4.066,Pqb<0.05;and dfTI=3,FTI=69.561,PTI<0.01,respectively).The root branching structures of A.cremastogyne and T.sinensis was characteristically fork-shaped,and had qa,qb and TI values of 0.160,0.097 and 0.673 as well as 0.122,0.047 and 0.635,respectively.In contrast,the root branching structures of C.acuminata and C.camphora exhibited a fishtail topology,with qa,qb,TI values of 0.582,0.547 and 0.885 as well as 0.364,0.266 and 0.799,respectively.③We also found that the average connection length of root systems differed significantly(df=3,F=6.166,P<0.05)among the four species,and could be arranged in the following descending order:C.camphora>T.sinensis>A.cremastogyne>C.acuminata.【Conclusion】In order to adapt to the infertile soils and nutrient-deficient environment characterizing the central Sichuan hilly region,each tree species adopts strategies to enhance the effective utilization of nutrient space.A.cremastogyne and T.sinensis promote nutrient absorption via horizontal expansion of their secondary root systems,whereas C.camphora and C.acuminata obtain soil water and nutrients by means of the rapid development of vertical roots.Our results indicate that these four native species can normally grow in the central Sichuan hilly region.On the basis of their branching structure and ecological characteristics,we conclude that A.cremastogyne preferentially grows in locations receiving ample sunlight,whereas C.acuminata and T.sinensis favor fertile,moist,as well as sunny sites,and C.camphora shows a general preference for fertile and moist conditions.