Development of a population of Boswellia elongata Balf. F. in Homhil nature sanctuary, Socotra island (Yemen)

Abstract

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.

A review on factors influencing fog formation, classification, forecasting, detection and impacts

“…Fog is seen as a source of water in a local ecosystem, where the precipitation levels are incapable of fulfilling water requirements for both flora (Emery 2016 ; Jacobson et al 2015 ) and fauna (Nørgaard and Dacke 2010 ; Parker and Lawrence 2001 ). The desert ecosystem is one such example where the vegetation is majorly dependent on fog for its moisture (Kalivodová et al 2020 ; Lvončík et al 2020 ; Riccardi et al 2020 ; Lehnert et al 2018 ) and nutrient (González et al 2011 ) requirements. Hamilton and Seely ( 1976 ) performed the most remarkable research that paved the way for studying the dependency of desert species on fog water; Seely and Hamilton ( 1976 ), reporting the fog basking physiology of tenebrionid beetle ( Onymacris unguicularis , Lepidochora discoidalis , L. porti , and L. kahani ).…”

Twenty years of biodiversity research and nature conservation in the Socotra Archipelago (Yemen)

“…A close study of individual trees from aerial photographs now provides the first direct visual evidence of this species' decline and indicates the imminent extinction of its largest subpopulation. Researchers witnessed a clear reduction in the largest subpopulation of Boswellia elongata on Socotra Island between 1956 and 2017, a period of 61 years, by about 78% (Lvončík et al 2020). Through experimental designs and assessment of the potential causes, the decline has been attributed to both natural and human-mediated factors.…”

“…The Socotra Dragon's Blood Tree, for example, does not only harbour a wide range of endemic species, it also plays a role in the island's hydrological cycle. Photo by Kay Van Damme, Socotra Island, 2014 of the rangeland), have made natural regeneration of most ground-dwelling palatable plants in accessible areas nearly impossible, in particular in times of drought when grazing pressures intensify (Miller and Morris 2004;Lvončík et al 2020). The process is catalysed by non-selective overarching events such as climate change, which could potentially give the final nudge for Socotran species that are on the edge at the downward slope towards extinction (Van Damme and Banfield 2011).…”

“…Weakened by the combination of overgrazing and damage by wind, the population was attacked by a local species of bark beetles on top of that. If the trend continues, the iconic subpopulation of frankincense trees in a well-known protected area on the island (Homhil Nature Sanctuary) will be gone in only a few decades from now (Lvončík et al 2020). The events are not localised; we may expect that the majority of exposed areas where this and other endemic ground-dwelling tree species occur on the island are facing a similar situation.…”

Conservation Assessment and Chemistry of Boswellia ogadensis, a Critically Endangered Frankincense Tree

“…Many Boswellia species are facing significant sustainability challenges and probable or confirmed population declines due to a variety of factors [ 7 , 8 , 9 , 10 , 11 , 12 ]. Key threats to many species include grazing by ungulates (goats, cattle, camels), fire, land conversion for agriculture, improper or excessive harvesting of resin, and attacks by insects [ 7 , 8 ].…”