弄岗喀斯特季节性雨林不同群丛物种多样性随海拔的变化

为研究北热带喀斯特季节性雨林不同群丛物种多样性特征及其随相对海拔变化,在对广西弄岗15 hm2北热带喀斯特季节性雨林样地进行群丛划分的基础上,运用丰富度指数、物种多样性指数和均匀度指数对处于不同相对海拔高度群丛的物种多样性以及不同群丛乔木层、亚乔木层和灌木层的物种多样性进行比较。结果表明弄岗喀斯特季节性雨林各群丛的多样性指数存在极显著差异(P0.01),各群丛类型的物种多样性指数的顺序为:枝花流苏树-广西牡荆-蚬木群丛齿叶黄皮-黄梨木-蚬木群丛山石榴-广西牡荆-蚬木群丛南方紫金牛-苹婆-中国无忧花群丛垂茉莉-苹婆-广西棋子豆群丛割舌树-闭花木-蚬木群丛米仔兰-闭花木-蚬木群丛对叶榕-苹婆-董棕群丛;Patrick丰富度指数、Shannon多样性指数和Pielou均匀度指数在中坡以下(弄岗样地相对海拔76.3 m以下)随相对海拔变化规律符合"中间高度膨胀"理论,即处于该地段中部群丛的多样性高,相对海拔较低和较高的群丛的多样性低;而中坡以上(弄岗样地相对海拔76.3 m以上)多样性随相对海拔增高而增大;群丛各层次多样性与热带山地雨林一致,与亚热带喀斯特常绿阔叶林截然相反,呈现出乔木层亚乔木层灌木层。这些结果表明地形空间异质性导致的水热差异影响着北热带喀斯特季节性雨林物种多样性,相对海拔低、水热条件较好的中、下坡乔木多样性高,相对海拔较高、条件恶劣的中、上坡亚乔木、灌木多样性较高,所以在对喀斯特地区水热条件恶劣的中、上坡地带进行植被恢复时,应更多选择亚乔木、灌木。

Species diversity for various associations along an altitudinal gradient in the karst seasonal rainforest in Nonggang

Based on the association classification of northern tropical karst seasonal rainforest in Nonggang plot, species diversity for all associations, trees, sub-trees, and shrub layers along an altitudinal gradient were evaluated using the indices of species richness, diversity, and evenness. The species diversity indices were significantly different among associations (P < 0.01). The species diversity of various associations decreased in the following order:the Chionanthus ramiflorus, Vitex kwangsiensis, and Excentrodendron tonkinense association > the Clausena dunniana-Boniodendron minius-Excentrodendron tonkinense association > the Catunaregam spinosa-Vitex kwangsiensis-Excentrodendron tonkinense association > the Ardisia thyrsiflora-Sterculia monosperma-Saraca dives association > the Clerodendrum wallichii-Sterculia monosperma-Cychidendron guangxiensis association > the Walsura robusta-Cleistanthus sumatranus-Excentrodendron tonkinense association > the Aglaia odorata-Cleistanthus sumatranus-Excentrodendron tonkinense association > the Ficus hispida-Sterculia monosperm-Caryota obtusa association. The Patrick, Shannon, and Pielou indices indicated a "mid-altitude bulge" phenomenon below the middle slope (relative altitude < 76.3 m in the Nonggang plot); the species diversity of the associations located at middle altitudes was higher than those of associations at lower and higher altitudes. However, these indices increased as the relative altitude above the middle slope increased (i.e., when the relative altitude was greater than 76.3 m in the Nonggang plot). Species diversity indices for various layers decreased as follows:tree layer > sub-tree layer > shrub layer. This is similar to the pattern observed in tropical montane rain forests but contrary to that of subtropical karst evergreen broad-leaved forests. Differences in water and temperature reflect spatial habitat heterogeneity at various altitudes and influence the species diversity in northern tropical karst seasonal rainforests. The species diversity of trees was higher in relatively lower altitudes, which are characterized by beneficial water and temperature conditions. However, the observed species diversity of sub-trees and shrubs was higher for relatively higher altitudes characterized by harsh habitat conditions. Hence, sub-trees and shrubs are suitable for vegetation restoration in habitats for karst hills at high altitudes.