We assessed seven decades of change in the largest known population of the endangered endemic Boswellia elongata Balf. F. (Burseraceae) on Socotra Island (Yemen). To quantify the population change we evaluated tree number and locations on digitized images from various sources in the period 1956–2017 and combined this with direct field measurements of the population between 2011 and 2017. Our study reveals that the Homhil Nature Sanctuary B. elongata population shows a continuous decline since 1956. The steady but slow natural decline was strongly accelerated by two catastrophic cyclones in November 2015, when 38% of the trees were directly destroyed by strong winds. During the following 2 years 29% of the remaining trees died additionally. The remaining population has a bell-shaped size distribution; most trees are around 40 cm in diameter (range 18 to 70 cm). Tree ring analysis of 11 dead trees with a diameter of 29 to 44 cm without bark, resulted in estimated tree ages between 80 and 101 years. We estimate that similar-sized trees showing strong signs of senescence have a maximum age of a little over 100 years. The age structure of the Homhil population is, therefore, unbalanced with large sized trees prevailing. Natural regeneration is absent for decades. Viable seeds are available and have been shown to germinate, but the development of seedlings into saplings is a bottleneck. If the decline continues at the current rate, only 30 trees will remain there in 2036. Protection, planting and awareness activities are needed to keep this unique frankincense tree in Homhil Nature Sanctuary.

中文翻译：

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乳香（Boswellia elongata Balf）种群的发展。F.在索科特拉岛（也门）霍姆希尔自然保护区

我们评估了濒临灭绝的地方性乳香elongata Balf的最大已知种群的变化七十年。F.（Burseraceae）在索科特拉岛（也门）。为了量化人口变化，我们在1956-2017年期间评估了来自各种来源的数字化图像上的树数和位置，并将其与2011年至2017年间的人口直接实地测量相结合。我们的研究表明，Homhil自然保护区B. elongata人口自1956年以来持续下降。2015年11月，两次灾难性飓风强烈地推动了自然平稳但缓慢的下降，当时38％的树木被强风直接破坏。在接下来的2年中，其余29％的树木另外死亡。其余的人口呈钟形分布。大多数树木的直径约为40厘米（范围为18至70厘米）。对11棵直径为29至44厘米且无树皮的枯树的年轮分析表明，估计树龄为80至101年。我们估计显示出强烈衰老迹象的类似大小的树木的最大年龄超过100年。因此，Homhil人口的年龄结构与流行的大树不平衡。几十年来缺乏自然再生。有存活的种子，并且已经证明可以发芽，但是将幼苗发展成树苗是一个瓶颈。如果以目前的速度继续下降，到2036年将只剩下30棵树。需要保护，植树和提高认识活动才能在Homhil Nature Sanctuary保留这种独特的乳香树。