Savannas are not old fields: functional trajectories of forest expansion in a fire-suppressed Brazilian savanna are driven by habitat generalists

Under fire suppression, many tropical savannas transform into forests. Forest expansion entails changes in environmental variables and plant community structure. We hypothesized that forest expansion into savanna results in a shift in community-weighted mean functional traits from stress tolerance to competitiveness, with generalist species having trait values intermediate between those of specialists of savanna and forest habitats. We studied 30 plots distributed over three savanna-forest boundaries undergoing forest expansion in the Brazilian Cerrado, capturing a gradient from open savanna to recently formed forest. We measured functional traits of 116 woody species of savanna specialist, generalist, and forest specialist functional groups and quantified changes in species composition and mean traits across the basal area gradient. We identified two main axes of species traits. The first separated forest and generalist species from savanna specialists, with the latter possessing traits associated with resistance to disturbance and stress – such as thick leaves, thick bark, slower height growth and lower shade tolerance. Our second trait axis separated shrubs and understory trees from pioneer species. Generalist species' traits did not differ substantially from forest species, nor did they tend to have a typical pioneer strategy. Community-weighted trait means changed linearly with forest development. There was a steady increase in traits associated with competitive dominance rather than stress tolerance and fire resistance, indicating a wholesale shift in the selective environment. Several of these patterns – e.g., increasing height and decreasing light requirements – are common in old-field succession. In contrast to old-field succession, we found that specific leaf area increased, leaf thickness decreased, and wood density stayed constant. The assembly of forests appears to be shaped by environmental filters that contribute to a functional trajectory distinct from most other studied ecosystems. Our results highlight the importance of the functional composition of the early community and of the early colonizers of the open environment. Differences between savanna and forest specialists reflect the selective effects of the contrasting environments, while the traits of generalists – and their interaction with environmental filters – drive dynamics of forest expansion.

在火抑制管理下，多数热带稀树草原会逐渐演变为森林。森林扩张会引发环境变量与植物群落结构的改变。我们提出假说：森林向稀树草原的扩张会导致群落加权平均功能性状（community-weighted mean functional traits）从胁迫耐受型向竞争优势型转变，其中广适物种的功能性状值介于稀树草原特化种与森林特化种的性状值之间。我们在巴西塞拉多（Cerrado）区域内的3处正在经历森林扩张的稀树草原-森林边界布设了30个样地，覆盖了从开阔稀树草原到新近形成森林的完整环境梯度。我们测定了隶属于稀树草原特化种、广适种与森林特化种3个功能群的116种木本植物的功能性状，并量化了沿断面积（basal area）梯度的物种组成与平均性状变化。我们识别出物种性状的两个主要主轴。第一主轴将森林物种与广适种同稀树草原特化种区分开来：后者具备与干扰及胁迫抗性相关的性状，例如厚叶片、厚树皮、较慢的株高生长速率与较低的耐荫性。第二性状主轴则将灌木与林下乔木同先锋物种区分开来。广适物种的性状与森林物种并无显著差异，也未表现出典型的先锋物种策略。群落加权平均性状值随森林发育呈线性变化。与竞争优势相关的性状稳步提升，而与胁迫耐受及耐火性相关的性状则逐步降低，这表明选择环境发生了全面转变。其中部分模式——例如株高增加与光需求降低——在弃耕地演替中较为常见。但与弃耕地演替不同的是，我们发现比叶面积（specific leaf area）有所提升，叶片厚度降低，而木材密度保持恒定。森林群落的构建似乎受到环境过滤作用的塑造，这使得其功能演替轨迹与绝大多数已研究的生态系统截然不同。我们的研究结果强调了早期群落以及开阔环境中早期定居者的功能组成的重要性。稀树草原特化种与森林特化种之间的性状差异反映了两种对立环境的选择作用，而广适物种的性状——及其与环境过滤的相互作用——驱动着森林扩张的动态过程。