Cordeiro Norbert J, Ndangalasi Henry J, McEntee Jay P, Howe Henry F
Department of Botany, Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605, USA.
Ecology. 2009 Apr;90(4):1030-41. doi: 10.1890/07-1208.1.
Forest fragmentation may have positive or negative effects on tropical tree populations. Our earlier study of an endemic African tree, Leptonychia usambarensis (Sterculiaceae), in the East Usambara Mountains of Tanzania, found poorer recruitment of seedlings and juveniles in small fragments compared to continuous forest, and concomitant reduction of seed-dispersal agents and seed dispersal. However, the possibility that other biotic or abiotic consequences of the fragmentation process contribute to diminished recruitment in fragments was left open. Here we test whether excessive seed predation, diminished fecundity, low seed quality, or adverse abiotic effects acted independently or in concert with reduced seed dispersal to limit seedling and juvenile recruitment in fragments. Extended observations of disperser activity, a seed placement experiment, seed predator censuses, and reciprocal seedling transplants from forest and fragment sources failed to support the alternative hypotheses for poorer seedling and juvenile recruitment in fragments, leaving reduced seed dispersal as the most plausible mechanism. Poorer recruitment of this species in forest fragments, where high edge-to-area ratios admit more light than in continuous forest, is particularly striking because the tree is an early successional species that might be expected to thrive in disturbed microhabitats.
森林破碎化可能对热带树木种群产生积极或消极影响。我们早期对坦桑尼亚东乌桑巴拉山脉的一种非洲特有树种——乌桑巴拉细爪木（梧桐科）进行的研究发现，与连续森林相比，小碎片区域内的幼苗和幼树补充情况较差，同时种子传播媒介和种子传播也有所减少。然而，破碎化过程的其他生物或非生物后果是否导致碎片区域内补充减少的可能性仍未明确。在这里，我们测试过度的种子捕食、繁殖力下降、种子质量低或不利的非生物影响是否单独或与种子传播减少共同作用，以限制碎片区域内的幼苗和幼树补充。对传播者活动的长期观察、种子放置实验、种子捕食者普查以及来自森林和碎片区域的幼苗相互移植实验，均未能支持碎片区域内幼苗和幼树补充较差的其他假设，这使得种子传播减少成为最合理的机制。在森林碎片中，该物种的补充情况较差，因为高边缘面积比使得碎片区域比连续森林接收更多光照，这一点尤为引人注目，因为该树种是早期演替物种，原本可能会在受干扰的微生境中茁壮成长。
