Aloidendron is a genus of seven species of succulent tree aloes in the family Asphodelaceae (subfamily Asphodeloideae), order Asparagales. The genus was established in 2013 when phylogenetic studies demonstrated that a group of species long classified within the much larger genus Aloe were genetically distinct and constituted an entirely separate clade. The segregation was confirmed by Manning et al. in 2014, and the genus name reflects its character: the Latin–Greek compound aloe + dendron (tree) means simply "tree aloe."
Members of Aloidendron are large, long-lived succulents that form trees or substantial shrubs, typically with a tall trunk, forked (dichotomous) branches, and a rounded crown. The largest species can reach 20 metres in height. Bark is grey to yellowish, smooth or longitudinally fissured, and often carries a powdery bloom. Leaves are arranged in rosettes at the branch tips. Flowers are borne in branched inflorescences with cylindrical racemes; colour varies geographically — yellow in the western species, pink to orange-red in the eastern species. Fruits are dry capsules that split to release seed. All species flower in late autumn to winter except Aloidendron pillansii, which flowers in spring.
Southern Africa is the centre of diversity for the genus: five of the six or seven accepted species are endemic or near-endemic there. Aloidendron barberae and A. tongaensis grow in coastal forests of eastern and northeastern southern Africa; A. dichotomum ranges across the Northern Cape of South Africa and Namibia; A. pillansii and A. ramosissimum are restricted to the arid Richtersveld on the South Africa–Namibia border. The remaining species extend beyond the region: A. eminens is found only in Somalia, and A. sabaeum in Saudi Arabia and Yemen (its generic placement remains controversial).
The best-known member is A. dichotomum, the quiver tree, named for its hollowed-out branches, which the Khoi-San people traditionally used to make arrow quivers. It is also a flagship species for monitoring climate-change impacts on biodiversity. A. pillansii, the giant quiver tree, is listed as Endangered; all species appear on CITES Appendices regulating trade.
Etymology
The genus name Aloidendron is a compound of "aloe" and the Greek dendron (tree), literally meaning "tree aloe." It was formally established in 2013 when the tree-forming aloe species were recognised as genetically distinct from the genus Aloe and transferred to their own genus by Klopper & Gideon F.Sm.
Distribution
The genus ranges across sub-Saharan Africa and the Arabian Peninsula, with southern Africa as its centre of diversity. Aloidendron barberae and A. tongaensis occur in coastal and sand forests of eastern southern Africa into Mozambique; A. dichotomum is widespread in the arid Northern Cape and Namibia; A. pillansii and A. ramosissimum are restricted to the Richtersveld region. Outside southern Africa, A. eminens is confined to Somalia and A. sabaeum to Saudi Arabia and Yemen.
Ecology
Tree aloes are highly drought-tolerant succulents adapted to arid and semi-arid habitats. Seeds are dispersed in winter or spring, timed with rains in the summer-rainfall zone. Key pollinators are sunbirds, short-beaked weavers, starlings, and white-eyes. Individual plants are exceptionally long-lived; one A. dichotomum was estimated at 100–145 years old. The winter-rainfall species, particularly A. dichotomum, are regarded as flagship indicators of climate-change-driven habitat shifts.
Conservation
All Aloidendron species are listed on CITES Appendices; A. pillansii appears on the more restrictive Appendix I, permitting trade only in exceptional circumstances, while the remaining species are on Appendix II requiring export permits. South African plants are additionally protected under national and provincial environmental law. On the IUCN Red List, A. pillansii is Endangered, A. dichotomum and A. ramosissimum are Vulnerable, A. eminens is Endangered, and A. barberae and A. tongaensis are Least Concern. Principal threats are habitat degradation, livestock trampling of seedlings, illegal harvesting, and climate change.
Cultural Uses
Aloidendron dichotomum takes its Afrikaans common name kokerboom (quiver tree) from the practice of the Khoi-San people, who hollowed out its fibrous branches to make quivers for their arrows. The same species' light wood has been used to construct cool rooms and small buildings in Namaqualand. Tree aloes are also planted as living fire barriers around homesteads in fire-prone regions. In northern KwaZulu-Natal, local tradition holds that tree aloes protect against evil spirits and serve as lightning conductors, so they are customarily spared when surrounding forest is cleared.
Cultivation
Tree aloes are widely grown as bold focal plants in large gardens and tolerate drought well. All species prefer a freely draining position with minimal to no frost. The three western arid species (A. dichotomum, A. pillansii, A. ramosissimum) need full sun and are difficult to grow in high-rainfall areas; A. barberae and A. tongaensis accept shadier conditions. Most species propagate readily from stem cuttings taken in the warmer months, which normally root within one to three months. All species can be grown from seed sown spring through autumn, germinating within about three weeks; seedlings can be planted out within a year. A. pillansii, A. tongaensis, and A. dichotomum are notably slow-growing.
