Seedling recruitment under isolated trees in a tea plantation provides a template for forest restoration in eastern Africa

Natural regeneration is less expensive than tree planting, but determining what species will arrive and establish to serve as templates for tropical forest restoration remains poorly investigated in eastern Africa. This study summarises seedling recruitment under 29 isolated legacy trees (14 trees comprised of three exotic species and 15 trees comprised of seven native species) in tea plantations in the East Usambara Mountains, Tanzania. Among the findings were that pioneer recruits were very abundant whereas non-pioneers were disproportionately fewer. Importantly, 98% of all recruits were animal-dispersed. The size of legacy trees, driven mostly by the exotic Grevillea robusta, and to some extent, the native Milicia excelsa, explained abundance of recruits. The distribution of bird-dispersed recruits suggested that some bird species use all types of legacy trees equally in this fragmented landscape. In contrast, the distribution of bat-dispersed recruits provided strong evidence that seedling composition differed under native versus exotic legacy trees likely due to fruit bats showing more preference for native legacy trees. Native, as compared to exotic legacy trees, had almost two times more non-pioneer recruits, with Ficus and Milicia excelsa driving this trend. Implications of our findings regarding restoration in the tropics are numerous for the movement of native animal-dispersed tree species in fragmented and disturbed tropical forests surrounded by farmland. Isolated native trees that bear fleshy fruits can attract more frugivores, resulting not only in high recruitment under them, but depending on the dispersal mode of the legacy trees, also different suites of recruited species. When selecting tree species for plantings, to maximize visitation by different dispersal agents and to enhance seedling recruit diversity, bat-dispersed Milicia excelsa and Ficus species are recommended. [Study published here: https://doi.org/10.1371/journal.pone.0250859]

自然更新相较于人工造林成本更低，但在东非地区，确定哪些物种能够成功定植并作为热带森林恢复的参照模板，相关研究仍较为匮乏。本研究针对坦桑尼亚东乌桑巴拉山区茶园内的29株孤立遗留林木（其中14株隶属于3个外来树种，15株隶属于7个乡土树种）展开调研，并总结了其林下的幼苗补充情况。研究结果显示，先锋树种幼苗补充个体数量极为丰富，而非先锋树种幼苗补充个体占比则显著偏低。值得注意的是，所有幼苗补充个体中有98%均为动物传播树种。遗留林木的体量主要由外来种银桦（Grevillea robusta）主导，在一定程度上也受乡土种非洲楝（Milicia excelsa）影响，该体量可解释幼苗补充量的差异。鸟类传播幼苗的分布情况表明，在这片破碎化生境中，部分鸟类会同等利用各类遗留林木。与之形成鲜明对比的是，蝙蝠传播幼苗的分布则有力证明：乡土遗留林木与外来遗留林木林下的幼苗组成存在显著差异，这大概率是因为果蝠更偏好乡土遗留林木。相较于外来遗留林木，乡土遗留林木林下的非先锋树种幼苗补充量几乎是其两倍，这一趋势主要由榕属（Ficus）植物与非洲楝（Milicia excelsa）驱动。本研究结果对于受农田环绕的破碎化且受干扰热带森林中乡土动物传播树种的扩散，具有多维度的热带森林恢复应用启示。结有肉质果实的孤立乡土林木能够吸引更多食果动物，这不仅会提升自身林下的幼苗补充量，还会依据遗留林木自身的传播模式，招募不同类群的定植幼苗。若要在造林选种时最大化不同传播媒介的访视频率、提升幼苗补充多样性，推荐选用蝙蝠传播的非洲楝（Milicia excelsa）及榕属（Ficus）树种。[研究原文发表于：https://doi.org/10.1371/journal.pone.0250859]