Forest Trees in Human Modified Landscapes: Ecological and Genetic Drivers of Recruitment Failure in Dysoxylum malabaricum (Meliaceae)

Tropical agro-forest landscapes are global priority areas for biodiversity conservation. Little is known about the ability of these landscapes to sustain large late successional forest trees upon which much forest biodiversity depends. These landscapes are subject to fragmentation and additional habitat degradation which may limit tree recruitment and thus compromise numerous ecosystem services including carbon storage and timber production. Dysoxylum malabaricum is a large canopy tree species in the Meliaceae, a family including many important tropical timber trees. This species is found in highly fragmented forest patches within a complex agro-forest landscape of the Western Ghats biodiversity hot spot, South India. In this paper we combined a molecular assessment of inbreeding with ecological and demographic data to explore the multiple threats to recruitment of this tree species. An evaluation of inbreeding, using eleven microsatellite loci in 297 nursery-reared seedlings collected form low and high density forest patches embedded in an agro-forest matrix, shows that mating between related individuals in low density patches leads to reduced seedling performance. By quantifying habitat degradation and tree recruitment within these forest patches we show that increasing canopy openness and the increased abundance of pioneer tree species lead to a general decline in the suitability of forest patches for the recruitment of D. malabaricum. We conclude that elevated inbreeding due to reduced adult tree density coupled with increased degradation of forest patches, limit the recruitment of this rare late successional tree species. Management strategies which maintain canopy cover and enhance local densities of adult trees in agro-forest mosaics will be required to ensure D. malabaricum persists in these landscapes. Our study highlights the need for a holistic understanding of the incipient processes that threaten populations of many important and rare tropical tree species in human dominated agro-forest landscapes.热带农林业景观是全球生物多样性保护的优先重点区域。目前人们对这类景观维系大型晚演替林木的能力尚知之甚少，而此类林木正是多数森林生物多样性赖以存续的基础。这类景观易遭受片段化及额外的生境退化，这可能限制林木更新，进而损害包括碳储存、木材生产在内的多项生态系统服务功能。马拉巴尔葱臭木（Dysoxylum malabaricum）是楝科（Meliaceae）的大型冠层树种，该科包含诸多具有重要经济价值的热带用材树种。该物种分布于印度南部西高止山脉（Western Ghats）生物多样性热点区域的复杂农林业景观中，栖息在高度片段化的林块内。本研究结合近交的分子评估结果与生态学、种群统计数据，探究了该树种更新所面临的多重威胁。针对从嵌入农林业基质的低密度、高密度林块中采集的297株苗圃培育实生苗，本研究利用11个微卫星位点（microsatellite loci）开展近交评估，结果显示：低密度林块内的近缘个体交配会降低实生苗的生长表现。通过量化这些林块内的生境退化程度与林木更新情况，本研究发现：冠层开度增大、先锋树种丰度提升，会整体降低林块对马拉巴尔葱臭木更新的适宜性。本研究得出结论：成树密度降低引发的近交加剧，加之林块退化程度加深，共同限制了这一珍稀晚演替树种的更新。为确保马拉巴尔葱臭木在这类景观中得以存续，需制定管护策略，维持冠层覆盖并提升农林业镶嵌景观中成树的本地种群密度。本研究强调，需全面理解人为主导的农林业景观中，诸多珍稀、重要热带树种种群所面临的初始威胁过程。