Aloe arborescens (Asphodelaceae): Arborescent forms on Mount Mulanje (Malawi)
2015
Thiede, Joachim | Campbell-Barker, Pastor Theo Peter | Clare, Russ | Downs, Philip E. | Hargreaves, Bruce J.
Aloe arborescens is a widespread and variable species, characterized as a much branched shrub 2–5m high by most authors. The remarkable single-stemmed, arborescent form endemic to Mount Mulanje (Malawi) has been largely ignored previously and is here described and illustrated in detail. In its vegetative and reproductive morphology, the arborescent form falls within the known range of variation, and is connected with the typical shrubby growth form by intermediates; it is considered a mere variant not worthy of formal taxonomic recognition but unique in its deviating growth form. The arborescent form is mostly unbranched or sparsely branched, but may also form specimens with thick stems and a huge crown. Within the revised classification of Aloe based on molecular data, the tree-like species with massive stems and crowns such as A. dichotoma were classified in the segregate genera Kumara and Aloidendron, so that the tree-like specimens of A. arborescens as well as some specimens of the Tanzanian A. volkensii ssp. volkensii represent the sole truly tree-like specimens with massive stems and crowns within the genus Aloe as now circumscribed (Aloe s. str.). The arborescent form occurs mostly between huge boulders which give some shade to the plants, which might discourage stem branching due to a lack of strong light for part of the daytime, and it might cause a micro-climate of cold air to accumulate between the rocks which might inhibit growth, in addition to protection from strong winds and frost.
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