CRB: Consequences of Forest Fragmentation for an Endemic Montane Tree

坦桑尼亚东乌桑巴拉山脉的森林群岛拥有最丰富的3000万年前的雨林遗迹，这些雨林曾经遍布非洲。特有树种占乔木植物群的24%，包括所有演替阶段的林上和林下物种以及各种授粉和传播系统。本次调查测试潜在的原因，为什么Leptonychia usambarensis（梧桐科），一种特有的鸟类分散演替树，复制不太好的森林片段30公顷比附近的连续森林3500公顷。热带系统中的森林片断化可能对树木产生生物或非生物后果。生物后果包括授粉者或传播者受到限制，因为为花朵授粉或传播种子的昆虫、鸟类或哺乳动物减少或消失，种子或幼苗因在受干扰生境中更普遍或更具破坏性的病原体或动物而存活减少，以及只有少数相关个体的小规模孤立种群的近亲繁殖衰退。非生物的影响可以是众多的，但通常反映的反应改变了光的方案与增加的边缘面积比，发生减少森林斑块的大小。拟议的研究将把目前正在进行的关于森林区块化后果的调查延长一年。我们的研究结果表明，到目前为止，在连续森林中传播本物种的种子的鸟类数量减少或不存在的片段，种子分散远离母树，更多的幼苗发生在父母附近比更远的片段比在连续森林中，和少年植物一年以上的年龄招募不太好的片段比在广泛的森林。需要评估的剩余问题需要一个相互的幼苗移植实验，将测试潜在的近交效应和生境和边缘效应再生幼苗。将来自20个连续森林和20个片段树木的2000个建立幼苗移植到边缘和内部位置，以测试：（1）在竞争性田间条件下是否存在潜在的近亲繁殖效应，以及（2）来自连续森林树木的幼苗在广泛和片段栖息地中的边缘和内部栖息地适合性。如果显示出其中一种或两种影响，将要求提供进一步的支助，以剖析其原因和对这一树种和其他特有树种及泛非树种的后果。这项研究的更广泛影响涉及更清楚地了解热带森林多样性和稀有性的原因。这些森林通常包含一些常见物种，以及许多在任何特定单位面积中丰度低或稀有的物种。这一事实使热带树木特别容易受到森林破碎化的影响，使大多数树木在小的残块中密度很低。这项研究首次清楚地表明，在热带森林片段的树种是分散有限的，它可能是第一个清楚地记录环境的种子源相互作用，将提醒生态学家无处不在的威胁近亲繁殖衰退的小残株。这项研究为一名非洲学生的研究生培训提供了支助。

The forest archipelago of the East Usambara Moutains, Tanzania, holds the richest remnants of 30 million year old rainforests that once extended across Africa. Endemic tree species, comprising 24% of the tree flora, include overstory and understory species of all successional stages and a variety of pollination and dispersal systems. This investigation tests potential reasons why Leptonychia usambarensis (Sterculiaceae), an endemic bird-dispersed successional tree, reproduces less well in forest fragments 30 ha than in nearby continuous forest of 3500 ha. Forest fragmentation in tropical systems may have biotic or abiotic consequences for trees. Biotic consequences include pollinator or disperser limitation from loss or reduction of insects, birds or mammals that pollinate flowers or disperse seeds, reduction of seed or seedling survival from pathogens or animals that are more prevalent or more devastating in disturbed habitats, and inbreeding depression among small isolated populations of only a few related individuals. Abiotic effects can be numerous, but usually reflect responses to altered light regimens with the increase in edge to area ratio that occurs with reduction of forest patch size. The proposed study would extend for one year an ongoing investigation of the consequences of forest fragmentation. Our results to date show that birds that disperse seeds of this species in continuous forest are reduced in numbers or absent in fragments, that fewer seeds are dispersed away from parent trees, that more seedlings occur near parents than further away in fragments than in continuous forest, and that juvenile plants more than one year old recruit less well in fragments than in extensive forest. Remaining issues in need of evaluation require a reciprocal seedling transplant experiment that will test potential inbreeding effects and habitat and edge effects on regenerating seedlings. Two thousand establishing seedlings from 20 continuous forest and 20 fragment trees will be transplanted to edge and interior locations to test: (1) presence or absence of potential inbreeding effects under competitive field conditions, and (2) edge and interior habitat suitability of seedlings from continuous forest trees in both extensive and fragmented habitats. If either or both effects are shown, further support will be requested to dissect their causes and consequences for this and other endemic and pan-African trees species. The broader impacts of this study relate to a clearer understanding of the causes of diversity and rarity in tropical forests. These forests typically contain a few common species together with many species that are present in low abundance or are rare in any given unit area. This fact makes tropical trees especially vulnerable to forest fragmentation, leaving the majority in low densities in small relict patches. This study has been the first to clearly demonstrate that tree species in tropical forest fragments are dispersal limited, and it may be the first to clearly document an environment by seed source interaction that would alert ecologists to the ubiquitous threat of inbreeding depression in small relict stands. The study supports graduate training of an African student.