Remnant trees affect species composition but not structure of tropical second-growth forest

Remnant trees, spared from cutting when tropical forests are cleared for agriculture or grazing, act as nuclei of forest regeneration following field abandonment. Previous studies on remnant trees were primarily conducted in active pasture or old fields abandoned in the previous 2–3 years, and focused on structure and species richness of regenerating forest, but not species composition. Our study is among the first to investigate the effects of remnant trees on neighborhood forest structure, biodiversity, and species composition 20 years post-abandonment. We compared the woody vegetation around individual remnant trees to nearby plots without remnant trees in the same second-growth forests (“control plots”). Forest structure beneath remnant trees did not differ significantly from control plots. Species richness and species diversity were significantly higher around remnant trees. The species composition around remnant trees differed significantly from control plots and more closely resembled the species composition of nearby old-growth forest. The proportion of old-growth specialists and generalists around remnant trees was significantly greater than in control plots. Although previous studies show that remnant trees may initially accelerate secondary forest growth, we found no evidence that they locally affect stem density, basal area, and seedling density at later stages of regrowth. Remnant trees do, however, have a clear effect on the species diversity, composition, and ecological groups of the surrounding woody vegetation, even after 20 years of forest regeneration. To accelerate the return of diversity and old-growth forest species into regrowing forest on abandoned land, landowners should be encouraged to retain remnant trees in agricultural or pastoral fields.

遗留林木（remnant trees）指热带森林被清理用于农业或放牧时未遭砍伐而留存的个体，在农田弃耕后可成为森林更新的核心位点。既往关于遗留林木的研究多聚焦于当前牧场或弃耕2~3年的旧地，且仅关注更新林的结构与物种丰富度，未涉及物种组成。本研究是首批探究弃耕20年后遗留林木对周边森林结构、生物多样性及物种组成影响的研究之一。我们将单株遗留林木周边的木本植被，与同一片次生林（second-growth forests）内无遗留林木的邻近样地（"control plots"）进行了对比。结果显示，遗留林木下方的森林结构与对照样地并无显著差异。但遗留林木周边的物种丰富度与物种多样性均显著高于对照样地。遗留林木周边的物种组成与对照样地存在显著差异，且更接近邻近原始林（old-growth forest）的物种组成。此外，遗留林木周边的原始林专性物种与广适性物种占比均显著高于对照样地。尽管既往研究表明遗留林木可在初期加速次生林的生长，但我们未发现其在更新后期对局部茎干密度、胸高断面积及幼苗密度产生影响的证据。即便在森林更新20年后，遗留林木仍对周边木本植被的物种多样性、组成及生态类群具有明确影响。为加快弃耕地上更新林的物种多样性与原始林物种的恢复进程，应鼓励土地所有者在农业或牧场区域留存遗留林木。