关键词: 川中丘陵区, 乡土阔叶树, 林木根系, 拓扑结构, 适应策略

Abstract:

【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 ofA. 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】① The 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. ② Topological indices were found to differ significantly among the species, of which q_a, q_b, and T_I showed highly significant differences ($df_{q_{a}}=3, F_{q_{a}}, P_{q_{a}}<0.01, df_{q_{b}}=3, F_{q_{b}}=4.066, P_{q_{b}}<0.05; df_{T_{I}}=3, F_{T_{I}}=69.561, P_{T_{I}}<0.01$, respectively). The root branching structures ofA. cremastogyne and T. sinensis was characteristically fork-shaped, and had q_a, q_b and T_I 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 q_a, q_b, T_I values of 0.582, 0.547 and 0.885 as well as 0.364, 0.266 and 0.799, respectively. ③ We 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 thatA. 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.

Key words: central Sichuan hilly region, native broad-leaved tree: tree roots, topological characteristics, adaptation strategy