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3000万年前曾经遍布非洲的最丰富的热带雨林遗迹。特有树种占树木植物区系的24%，包括所有演替阶段的林下物种和各种传粉和传播系统。本研究探讨了一种特有的鸟类分散演替树——乌桑巴伦瘦衣兰（Sterculiaceae）在30公顷的森林碎片中繁殖不如在附近3500公顷的连续森林中繁殖的潜在原因。热带系统中的森林破碎化可能对树木产生生物或非生物后果。生物后果包括传粉者或传播者的限制，因为授粉花或传播种子的昆虫、鸟类或哺乳动物的损失或减少，在受干扰的栖息地中更普遍或更具破坏性的病原体或动物的种子或幼苗存活率降低，以及只有少数相关个体的小孤立种群的近亲繁殖减少。非生物效应可能很多，但通常反映了对改变光照条件的反应，随着森林斑块大小的减少，边缘面积比增加。拟议的研究将对森林破碎化后果的持续调查延长一年。我们的研究结果表明，在连续森林中传播种子的鸟类数量减少或在片段中没有，种子在远离亲本树的地方传播较少，幼苗在亲本附近比在更远的片段中比在连续森林中更多，超过1岁的幼树在片段中比在广阔的森林中更不容易获得。需要评估的剩余问题需要进行互惠的幼苗移植实验，以测试潜在的近交效应以及栖息地和边缘效应对幼苗再生的影响。本研究将20株连续林和20株破碎林的2000株立苗移植到边缘和内部，以测试：(1)在竞争条件下是否存在潜在的近交效应；(2)在粗放型和破碎型生境中，连续林树木的幼苗对边缘和内部生境的适宜性。如果显示出其中一种或两种影响，将要求进一步的支持，以剖析这种和其他特有树种和泛非树种的原因和后果。这项研究的更广泛影响在于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的物种，以及在任何给定的单位面积内丰度较低或罕见的许多物种。这一事实使热带树木特别容易受到森林破碎化的影响，使大多数树木密度低，分布在零星的小块土地上。这项研究首次清楚地证明了热带森林碎片中的树种是有限的，它可能是第一个通过种子源相互作用清楚地记录了一种环境，这将提醒生态学家注意在小型孑遗林分中普遍存在的近亲繁殖萧条的威胁。该研究支持一名非洲学生的研究生培训。

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.