Not all trees can make a forest: tree species composition and competition control forest encroachment in a tropical savanna

Forest encroachment into savannas is a widespread phenomenon, the rate of which may depend on soil conditions, species composition, or changes in stand structure. As savanna specialist trees are replaced by generalist species, rates of stand development may increase. Because generalists can persist in forests, they are likely to grow more quickly and survive longer in dense stands, compared to savanna specialists. Furthermore, the faster growth rates of generalists may allow them to overtop and outcompete savanna specialists, causing rapid species turnover.

We measured growth and survival of 6147 individuals of 112 species of savanna and generalist tree species over a period of 10 years in an ecological reserve in Assis, São Paulo State, Brazil. We modeled growth and mortality as a function of soil texture and nutrients, tree size, competitive neighborhood, and membership in savanna or generalist (species which can persist in forests and savannas) functional groups.

Tree growth an...

森林向稀树草原的侵占是一种广泛存在的生态现象，其发生速率可能受土壤条件、物种组成或林分结构变化的调控。当稀树草原特有树种被广适性物种取代时，林分发育速率往往会加快。由于广适性物种能够在森林生境中存活，相较于稀树草原特有树种，它们在高密度林分中通常生长更快、存活更久。此外，广适性物种更快的生长速率使其能够冠层超越并竞争排除稀树草原特有树种，进而引发快速的物种更替。

我们在巴西圣保罗州阿西斯市的一处生态保护区内，开展了为期10年的长期监测，对涵盖稀树草原树种与广适性物种（可在森林及稀树草原生境中存活的物种）在内的112个树种共计6147株个体的生长与存活情况进行了系统记录。我们以土壤质地、土壤养分含量、树木个体大小、邻体竞争强度以及所属的稀树草原或广适性功能群为自变量，构建了树木生长与死亡率的预测模型。

树木生长与