次生常绿阔叶林群落林冠结构对林下植被的影响

采用经典群落学调查方法和半球面影像技术，以珠江三角洲城郊次生常绿阔叶林群落为研究对象，运用聚类分析、非参数检验（Kruskal-Wallis test），指示种分析（ISA）和典范对应分析（CCA）等方法，定量研究次生常绿阔叶林群落林冠结构对林下植被的影响。主要结果如下:①次生常绿阔叶林林冠结构和林下光环境各参数沿林冠梯度变化表现不一。立木密度和平均胸径沿林冠梯度差异极显著（P < 0.001），林下直射光、林下散射光和林下总光照沿林冠梯度差异显著（P < 0.05）。②林下植被物种丰富，物种数、平均树高和总盖度沿林冠梯度差异显著（P < 0.05）。林冠梯度2的林下植被物种最多，但群落平均高度和总盖度皆以林冠梯度3最大。③林下植物对不同林冠结构和林下光照参数的响应不一。水锦树Wendlandia uvariifolia和绿冬青Ilex viridis同为林下直射光和林下总光照的指示种。④典范对应分析（CCA）揭示，林冠结构以及林下光环境共同影响林下植被的物种分布，扇叶铁线蕨Adiantum flabellulatum，乌毛蕨Blechnum orientale，芒萁Dicranopteris dichotoma和狗脊Woodwardia japonica等蕨类植物均分布在林下光环境较弱的区域，油茶Camellia oleifera和锡叶藤Tetracera sarmentosa等喜阳植物则分布在林下光环境较强的区域，而木荷Schima superba和腺点紫金牛Ardisia lindleyana等中性植物受林冠层立木的高度和胸径的影响更大。不同林冠梯度下的林下植被物种组成因林下光照强弱而不同，对次生常绿阔叶林木本植物影响最大的林下光环境指标是林下总光照和林下散射光。

Influence of canopy structure on understory vegetation of secondary evergreen broadleaf forest communities

Understory vegetation, representing the main component of biodiversity in a forest community and playing a key role in forest functioning and canopy structure, are important characteristics of a forest community. To evaluate the effects of different gradients of canopy structure on understory vegetation, secondary evergreen broadleaf forest communities in the suburbs of the Pearl River Delta were studied. The effect on understory plants was tested using classical community survey methods and hemispherical photography. Analysis included use of Cluster Analysis, Non-parametric Test (Kruskal-Wallis test), Indicator Species Analysis (ISA), and Canonical Correspondence Analysis (CCA). Results showed that (1) Canopy structure and understory light conditions varied along the canopy gradient. The standing density and average diameter at breast height (DBH) showed a highly significant difference (P < 0.001); whereas, the transmitted direct light, transmitted diffused light, and transmitted total light significantly differed along the canopy gradient (P < 0.05). (2) The understory vegetation was rich; and the abundance and average height as well as total coverage, significantly differed with canopy gradient (P < 0.05). Species were richest in canopy gradient 2, but the average height and total coverage were both highest in canopy gradient 3. (3) The responses of understory vegetation to canopy structure and transmitted light varied. Wendlandia uvariifolia and Ilex viridis were both indicator species of transmitted direct light and transmitted total light. (4) The CCA showed that canopy structure and understory light conditions affected the distribution of understory vegetation. Ferns such as Adiantum flabellulatum, Blechnum orientale, Dicranopteris dichotoma, and Woodwardia japonica resided in areas of weak sunlight, but heliophytes like Camellia oleifera and Tetracera sarmentosa inhabited places where they received better light. Moreover, distribution of neutral plants like Schima superba and Ardisia lindleyana were more affected by the height and DBH of standing trees. This study demonstrated that understory vegetation varied according to understory light conditions along the canopy gradient with transmitted total light and transmitted diffused light being the main factors influencing the understory light environment of the secondary evergreen broadleaf forest.