科尔沁沙地黄榆种子散布的空间差异及规律

黄榆(Ulmus macrocarpa)是典型的风播植物, 在科尔沁沙地可以形成单一优势种林地。通过对其林下随机取样和林缘与孤立树不同方向从树基部向外的有序取样调查, 分析了黄榆林下和林缘外不同距离的种子沉降特征、林缘和孤立树种子散布方向的差异性与规律性。结果表明, 黄榆平均种子密度以林下最大, 林缘3 m处已显著减小, 林缘外随着距离增加黄榆平均种子密度呈指数减少态势, 且越远越不均匀。林缘和孤立树不同方向在26 m内, 单位面积连续分布的种子累积数量均以顺风向频率大的东北方向最多, 以逆风向频率大的西南或西方向最少。林缘和孤立树共10个方向的垂直断面从树基部向外的单位面积种子数量频度均符合Weibull分布和对数-正态分布密度函数(χ²_(α) < 0.900), 具有相同的种子散布格局。在科尔沁沙地自然条件下, 黄榆在各方向具有相同的“远距离”种子散布机制。

Spatial difference and regularity of seed dispersal of Ulmus macrocarpa in Horqin Sandy Land, China

Aims Ulmus macrocarpa is a typical anemochorous plant. It can form mono-dominant woodland in Keerqin Sandy Land, China. Our objectives were to investigate seed-fall characteristics of U. macrocarpa in the understory and away from both the woodland edges and isolated trees at different distances in different compass directions, and to illustrate the underlying mechanisms of seed dispersal in U. macrocarpa.
Methods In mid June 2011, we investigated seed dispersal after seeds fall in a nature reserve of U. macrocarpa woodland in Tongyu County in western Jilin Province. The woodland is about 50 km² in area which has nearly primitive landscape appearance. The seed density of U. macrocarpa was randomly sampled in the understory and orderly sampled at 1 m intervals from the tree bases away from the woodland edges and isolated trees along vertical sections of different directions. The quadrat was 50 cm × 50 cm in area. Differences in the seed density among the understory and parallel sections of different distance away from the woodland edges were tested by one-way ANOVA. Two types of density functions of theoretical distribution were used to reflect the patterns of seed dispersal away from seed sources along vertical sections of different directions.
Important findings The understory of U. macrocarpa had the highest mean seed density. The mean seed density was significantly reduced at 3 m away from the woodland edges, and it decreased exponentially with increasing in distance away from the woodland edges. We observed the highest cumulative seeds per unit area in the northeast direction; whereas the lowest numbers in the southwest and west directions, which may attribute to a high frequency of southerly winds. The pattern of seed dispersal matched the Weibull and Log-normal distributions (χ² (α) < 0.900). In conclusion, U. macrocarpa has the same long-distance seed dispersal mechanism in different directions under natural conditions in Horqin Sandy Land.