Am J Bot. 1998 Jun;85(6):820.
The effects of isolation on primary succession are poorly documented. I monitored vegetation recovery on two Mount St. Helens lahars (mud flows) with different degrees of isolation using contiguous plots. Seventeen years after the eruption, species richness was stable, but cover continued to increase. That isolation affects community structure was confirmed in several ways. The dominance hierarchies of the lahars differed sharply. Detrended correspondence analysis on Lahar I showed a trend related to distance from an adjacent woodland, whereas vegetation on Lahar II was relatively homogeneous. Spectra of growth forms and dispersal types also differed. Lahar I was dominated by species with modest dispersal ability, while Lahar II was dominated by species with better dispersal. Variation between plots should decline through time, a prediction confirmed on Lahar II. Lahar I remained heterogeneous despite having developed significantly higher cover. Here, the increasing distance from the forest has prevented plots from becoming more homogeneous. At this stage of early primary succession, neither lahar is converging towards the species composition of adjacent vegetation. This study shows that isolation and differential dispersal ability combine to determine initial vegetation structure. Stochastic effects resulting from dispersal limitations may resist the more deterministic effects of competition that could lead to floristic convergence.
隔离对原生演替的影响记录不佳。我使用连续样方监测了圣海伦斯火山两次不同隔离程度的火山泥流上的植被恢复情况。喷发 17 年后，物种丰富度稳定，但盖度仍在继续增加。隔离会影响群落结构，这一点已通过多种方式得到证实。泥流的优势等级差异很大。对拉哈尔一号的去趋势对应分析显示出与附近林地距离有关的趋势，而拉哈尔二号的植被相对均匀。生长形式和扩散类型的光谱也不同。拉哈尔一号主要由扩散能力适中的物种组成，而拉哈尔二号则主要由扩散能力更好的物种组成。预测在拉哈尔二号得到证实，随着时间的推移，样方之间的差异应该会减少。尽管拉哈尔一号的盖度显著增加，但它仍然存在异质性。在这里，从森林不断增加的距离使样方无法变得更加均匀。在原生演替的早期阶段，两个泥流都没有朝着与相邻植被相似的物种组成发展。本研究表明，隔离和不同的扩散能力结合起来决定了初始植被结构。由扩散限制引起的随机效应可能会抵抗导致植物区系趋同的更具决定性的竞争效应。
