塔河源荒漠河岸林群落物种组成、结构与植物区系特征

为了更好地了解荒漠河岸林植被动态与生物多样性维持机制,于塔里木河干流源头荒漠河岸林内建立6个1 hm 2样地,对群落物种组成、数量特征和空间格局等进行综合分析。结果表明,塔河源荒漠河岸林群落结构简单、物种多样性低,共有物种12科15属16种,杨柳科重要值最大(28. 97%),是群落优势建群科。取样面积在0. 04 hm 2时,包括了87. 5%的物种;样地内稀有种和偶见种均占所有物种的12. 5%。植物区系贫乏,单种科、属多,温带成分占绝对优势,古地中海成分也占有重要地位,具典型温带区系性质。样地内 DBH ≥ 1 cm 的活木本个体有2497株,灰胡杨多度占92. 95%,胸高断面积和重要值最大,在群落中占绝对优势地位。群落内科、属、种的数量随植株高度级的增加而减少,垂直结构层次分明,由乔木层、灌木层和草本层构成。样地内所有树种平均胸径15. 57 cm,径级与高度级结构均呈偏正态分布,缺乏高度≤ 2 m的个体,群落更新不良。优势种灰胡杨、胡杨径级结构均呈单峰型,其中胡杨幼树出现断层,灰胡杨幼苗向幼树发展过程中死亡率极高(92. 56%),均为衰退型种群。两个优势种空间格局呈聚集分布,不同生境下随径级增大空间分布格局变化不同;且两个优势种在空间上相互排斥而占据不同生境,暗示着种间竞争与生境异质性是影响荒漠河岸林物种空间分布格局和物种共存的重要因素。

Species composition,community structure,and floristic characteristics of desert riparian forest community along the mainstream of Tarim River

Desert riparian forest is a typical temperate deciduous broad-leaved secondary forest and is mainly distributed in the Tarim Basin. To better understand the mechanisms of community dynamics and species coexistence of this forest type, six 1-hm 2 plots were established and divided into 150 subplots. We analyzed species composition ( e .g ., importance value, abundance), community structure ( e .g ., vertical structure, size-class structure), spatial distribution of woody plants, and floristic composition. Results indicated that community structure of the desert riparian forest was simple and species diversity was low, with 16 species, belonging to 15 genera and 12 families identified. Salicaceae had the highest importance value (28. 97%) and was the dominant family in the community. When the sampling area was larger than 0. 04 hm 2, 87. 5% of species were counted. Rare and occasional species accounted for 12. 5% of all species, respectively. Floristic composition was poor, with single species, genus, and temperate components being dominant, although ancient Mediterranean components also occupied an important position, with typical temperate flora. We counted 2497 living free-standing trees in the plots. Species abundance, basal area, and importance values showed that Populus pruinosa was the obviously dominant species. The number of families, genera, and species decreased with increasing height class, and the vertical structure was composed of canopy, shrub, and herb layers, indicating apparent vertical stratification in this forest. The average DBH of all trees was 15. 57 cm, and DBH size and height classes showed skewed normal distributions, with few individuals with a height lower than 2 m, indicating weak regeneration across the community. The DBH structures of P. pruinosa and P. euphratica generally were unimodal;in particular, P. euphratica lacked small DBH trees and P. pruinosa had extremely high mortality during development from seedling to young tree, thus suggesting declining populations for these two species. The spatial distribution patterns of the two dominant species were clustered and varied with diameter in different habitats. In addition, the two dominant species were spatially mutually exclusive and occupied different habitats, suggesting that interspecific competition and habitat heterogeneity may be responsible for the species coexistence and spatial distribution of dominant species.