海南杜鹃在河岸带弯道两侧的空间分布格局和年龄结构差异

河岸带是河流与陆地生态系统的交错带, 孕育了丰富的生物多样性。河流的冲刷与地势的作用使得河岸带往往形成弯道, 弯道内外两侧水流速度、泥沙性质与植物繁殖体聚集程度不同, 影响到植物生长与种群动态, 可能导致河岸带弯道凸岸、凹岸两侧植物空间分布格局和种群结构存在较大差异。该研究以中国特有、狭域分布的海南杜鹃(Rhododendron hainanense)为例, 揭示溪流弯道对其两侧海南杜鹃种群空间分布格局与种群动态的影响。在海南岛3个国家级自然保护区内各设置2个河岸弯道样带, 用方差/平均值法对弯道凸岸、凹岸两侧海南杜鹃的空间分布格局和种群动态进行对比研究。结果显示: (1)海南杜鹃弯道凸岸的种群结构呈增长型, 凹岸基本呈衰退型; (2)凸岸一侧上下游的海南杜鹃种群基本呈增长型, 凹岸一侧上下游的海南杜鹃则出现断龄现象; (3)距离弯顶位置或河岸越远, 各龄级海南杜鹃种群个体数量基本呈下降趋势; (4)在2 m × 2 m尺度下, 弯道凸岸及其下游的海南杜鹃主要呈聚集分布, 上游近似随机分布, 凹岸及其上下游则均呈随机分布。海南杜鹃分布的山区河岸带水流较快、坡度较大, 可能是导致弯道凹岸个体较少、种群结构呈衰退型的主要原因。因此, 河岸弯道内外两侧可能存在个体生长与群体差异, 在开展河岸带植物种群动态与物种保育研究中需给予重视。

Difference in spatial distribution patterns and population structures of Rhododendron hainanense between both sides of riparian bends

Aims Riparian zone is the intersection of stream and terrestrial ecosystem, which has rich biological diversity. Stream is usually curving because of water flush and topography effect. The flow velocity, sediment properties and plant propagule aggregation are different between inner and outer sides of the bend, which leads to differences in plant population structures and spatial distribution patterns between riparian bends’ both bank. Our aim was to reveal the differences in spatial distribution patterns and population dynamics of Rhododendron hainanense, a shrub endemic to Hainan and Guangxi of China, on each side of a stream.Methods Three nature reserves on Hainan Island were chosen as studied sites. In each site, two riparian bends were set as transects. The spatial distribution patterns and population dynamics of R. hainanense were compared for the convex versus concave banks by a method of “variance/mean method”.Important findings (1) Rhododendron hainanense populations were at increasing stage on convex banks, but at declining stage on concave banks. (2) Rhododendron hainanense populations were at growing stage on both upstream and downstream of convex banks, but the ages of the individuals on concave banks were not continuous. (3) The number of individuals of each age-class declined away from the top of bend or river bank. (4) At scale of 2 m × 2 m, the individuals showed an aggregation distribution on convex banks and their downstream, but a random distribution on the upstream of convex banks, and on concave banks and their upstream and downstream. The fast flow and steep slopes may be the main reason for the fewer individuals and declining population on concave banks than on convex banks.