Center for Tropical Conservation, Nicholas School of the Environment, Duke University, P. O. Box 90381, Durham, North Carolina, 27708, USA.
Department of Biology and Florida Museum of Natural History, University of Florida, Gainesville, Florida, 32611, USA.
Ecology. 2017 Nov;98(11):2895-2903. doi: 10.1002/ecy.1991. Epub 2017 Sep 25.
Treefall gaps have long been a central feature of discussions about the maintenance of tree diversity in both temperate and tropical forests. Gaps expose parts of the forest floor to direct sunlight and create a distinctive microenvironment that can favor the recruitment into the community of so-called gap pioneers. This traditional view enjoys strong empirical support, yet has been cast into doubt by a much-cited article claiming that gaps are inherently "neutral" in their contribution to forest dynamics. We present concurrent data on seedfall and sapling recruitment into gaps vs. under a vertically structured canopy in an Amazonian floodplain forest in Peru. Our results strongly uphold the view of gaps as important generators of tree diversity. Our methods differed significantly from those employed by the neutralist group and can explain the contrasting outcomes. We found that seedfall into gaps differs both quantitatively and qualitatively from that falling under a multi-tiered canopy, being greatly enriched in wind-dispersed and autochorus species and sharply deficient in all types of zoochorous seeds. Despite a reduced input of zoochorous seeds, zoochorous species made up 79% of saplings recruiting into gaps, whereas wind-dispersed species made up only 1%. Cohorts of saplings recruiting into gaps are less diverse than those recruiting under a closed canopy (Fisher's alpha = 40 vs. 100) and compositionally distinct, containing many light-demanding species that rarely, if ever, recruit under shaded conditions. Saplings recruiting into gaps appear to represent a variable mix of shade-tolerant survivors of the initiating treefall and sun-demanding species that germinate subsequently.
树倒空隙一直是讨论温带和热带森林中树木多样性维持的核心特征。空隙使部分森林地面暴露在直射阳光下，并形成独特的微环境，有利于所谓的空隙先锋物种的繁殖。这种传统观点得到了强有力的经验支持，但一篇被广泛引用的文章质疑了这一观点，该文章声称空隙在其对森林动态的贡献上本质上是“中性的”。我们在秘鲁亚马逊河泛滥平原森林中同时记录了种子雨和幼苗在空隙与垂直结构树冠下的繁殖情况。我们的结果强烈支持空隙是树木多样性的重要产生者的观点。我们的方法与中性群体使用的方法有很大的不同，可以解释对比结果。我们发现，种子雨在空隙中的降落无论是在数量上还是在质量上都与多层面树冠下的降落不同，风传播和自播物种大大丰富，而所有类型的动物传播种子则明显缺乏。尽管动物传播种子的输入减少了，但动物传播物种仍占进入空隙的幼苗的 79%，而风传播物种仅占 1%。进入空隙的幼苗群体的多样性低于在封闭树冠下的幼苗群体（Fisher's alpha 为 40 比 100），而且组成明显不同，包含许多需要光照的物种，在遮荫条件下很少甚至从未招募过。进入空隙的幼苗似乎代表了一个混合的、有生存能力的荫蔽树种和需要阳光的物种，这些物种是由初始的树倒引起的。
