Abstract:

The community structure, species composition, and diversity are important indicators of forest quality. Investigating the relationship between community structure and species diversity can provide a theoretical basis for optimizing and regulating forest structure and improving forest functionality. The Chinese fir (Cunninghamia lanceolata) is an important fast-growing timber species in southern China. The sustainable management of Chinese fir plantation forests has become a core issue in forestry research because of overexploitation, which leads to soil degradation and a decline in productivity and ecological functioning. Previous studies have mainly focused on the effects of different densities, stand ages, and management practices on the species diversity of Chinese fir forests. However, few studies have highlighted the stand structure and species diversity of Chinese fir plantations following long-term abandonment and natural succession. The aims of this study were to 1) elucidate the characteristics of community structure, species composition, and diversity of Chinese fir plantations after long-term abandonment, 2) determine the relationships between quantitative population characteristics of Chinese fir and diversity indices of undergrowth, and 3) provide a scientific basis for the sustainable management of Chinese fir plantations and forest management decisions based on biodiversity conservation. A 50-year-old Chinese fir plantation abandoned for over 20 years was selected from the Yangmingshan National Forest Park, Jiangxi Province, China (25.6508° N, 114.3237° E). The plantation forest was established in 1973 with the same initial planting density and early forest management measures and has been abandoned for over 20 years. Six 20 m×20 m plots were established in the stands with an interval of 80-100 m. Each plot was divided into four 10 m×10 m subplots. All trees with a diameter at breast height (DBH)≥ 5.0 cm in each plot were measured for DBH and height. Three subplots were randomly selected from each plot to survey the shrub layers. The height, base diameter, and crown width of the shrub species were measured to calculate the coverage. Herbaceous plants were surveyed in five 1 m × 1 m quadrats installed at the four corners and center of the plot. The coverage and height of all herb species were measured. The number of individuals was counted. The tree and shrub layers were divided based on tree height, which is individuals with height≥5.0 m in the tree layer. Data processing involved species diversity analysis and the calculation of species importance values to reflect the status and dominance of species within the community. The importance value (V_I) for each species was calculated as the mean relative density, height, frequency, coverage, and dominance. Diversity indices, including species richness (S), Shannon-Wiener index (H), Simpson index (D), and Pielou’s evenness index (J), were calculated across different community layers. An improved M. Godron stability measurement method was employed to analyze community stability. Statistical analyses were performed using SPSS version 26.0. Pearson correlation coefficients were calculated to examine the relationships between the importance value of Chinese fir and the diversity indices of different community layers. Statistical significance was set at p<0.05. The frequency distribution of the DBH class of the tree layer appeared bimodal, with a high proportion of smaller individuals with DBH≤10 cm, which were mainly invasive broad-leaved trees. This indicates that the community belongs to a stable growth-type stand. The average V_I of the Chinese fir population was 77, and its basal area was 41 m²·hm⁻², accounting for 93.5% of the total basal area of the community. The frequency distribution of the DBH class of the Chinese fir population showed a normal distribution pattern, indicating that Chinese fir could maintain its dominant position in the community for a long time. However, reserves of Chinese fir seedlings and saplings are severely insufficient, indicating a declining population. In contrast, broad-leaved tree species (e.g. Schima superba, Castanopsis lamontii, and Diospyros morrisiana) in the subcanopy layer showed good regeneration and may become the primary community components in the future. Community stability analysis revealed that the tree and herb layers were relatively stable, whereas the shrub layer underwent dynamic adjustments with lower stability. This is closely related to changes in the canopy structure of the community and interspecific competition, particularly for light. A total of 72 plant species were identified, belonging to 55 genera and 35 families, including 51 woody species, six herbaceous species, and 15 fern species. Tropical elements dominated the flora, with 28 genera and 37 species of seed plants, accounting for 64% and 65% of the total, respectively. Species richness was highest in the shrub layer, followed by the herb layer, and was lowest in the tree layer. Typical subtropical evergreen broad-leaved tree species such as Schima superba and Castanopsis lamontii, appeared in the community and became dominant in the subcanopy layer. Additionally, evergreen broad-leaved species such as Cinnamomum camphora and Castanopsis carlesii occupy essential positions in the shrub layer, and may gradually enter the tree layer as succession progresses. This forest community had four species, Phoebe bournei, Machilus nanmu, Cibotium barometz, and Angiopteris evecta, which are endangered, endemic, and national key protected species in China. There were significant differences in species diversity among the different layers. The shrub layer had the highest richness and Shannon-Wiener index, followed by the herb layer, and the lowest in the tree layer. The Simpson index was highest in the herb layer and lowest in the tree layer. Correlation analysis showed that the importance value of Chinese fir was not significantly correlated with the Shannon-Wiener index, Simpson index, and Pielou’s evenness index of the shrub layer but was significantly negatively correlated with the diversity indices of the tree and herb layers. This indicates that the dominance of the Chinese fir population constrains species richness and diversity of the community. Chinese fir remains dominant in long-term abandoned C. lanceolata plantations. Due to the severe lack of seedling and sapling reserves, the population may gradually decline. The evergreen broad-leaved species, such as Schima superba and Castanopsis lamontii, with sufficient regeneration reserves may gradually occupy essential positions in community succession. Increased light transmittance in the canopy promotes the growth of understory shrubs and herbaceous plants, which enhances the species diversity of the community. However, the invasion of broad-leaved species intensifies interspecific competition, further restricting the regeneration of C. lanceolata so that its dominant position in the community may gradually be replaced by broad-leaved species

Key words: Cunninghamia lanceolata community, stand structure, species diversity, floristic characteristics, Yangmingshan Forest Park

摘要： 群落结构及其物种组成和多样性是评价森林质量的重要指标，探讨群落结构和物种多样性关系，可为林分结构优化及功能提升提供理论依据。选择江西阳明山国家森林公园弃管20年的50年生杉木（Cunninghamia lanceolata）人工林，采用样地调查法，分析其群落结构、物种组成和多样性的变化特征。结果表明，群落乔木层林木径级分布呈双峰曲线，径级≤10 cm的较小个体占比高，以侵入的阔叶树为主，属于稳定增长型林分。杉木种群的平均重要值为77，胸高断面积41 m²·hm⁻²，占群落总胸高断面积的94%；其径级分布呈正态分布型，可维持其在群落中的长期优势地位；群落稳定性分析表明其属于稳定种群，但其幼苗、幼树储备严重不足，属于衰退型种群。该群落共出现植物72种55属35科，其中木本植物51种38属22科、草本植物6种6属4科、蕨类植物15种11属9科。热带成分在区系中占优势，共有种子植物28属37种，分别占总数的64%和65%。群落不同层次的物种多样性差异较大，相关性分析表明杉木重要值与灌木层所有多样性指数之间均无显著相关性，而与乔木层和草本层的3个多样性指数（包括Shannon-Wienner指数、Simpson指数和Pielou均匀度指数）均呈显著负相关。这表明群落物种丰富度和多样性受制于杉木种群的优势度。

关键词: 杉木群落, 林分结构, 物种多样性, 区系特征, 阳明山森林公园