African Wildlife & Environment Issue 74

Because Acacia sieberiana (the paper-bark thorn – now Vachellia sieberiana ) grow at higher altitudes, usually in grassland, it is not usually confused with the umbrella thorn ( Vachellia tortilis ). But they can co-occur in some places in the Lowveld and then their identities from afar can be confused by some

In this series I have written about a number of individual tree species and groups of trees in either a genus or a family, but I don’t think I’ve yet defined a tree? A question I am often asked is: 'What is a tree?' On scrutinising the literature few authors have an in-depth definition of 'a tree'. Why? My take on this is that defining a tree is not too easy. The concept of 'a tree' has no clear boundaries. Clearly when it comes to a baobab, this is easy because here we have a woody plant of significant stature with a strong, single trunk. But even here there could be some doubt, as when a baobab dies it simply collapses into a pile of fibrous debris. There is no useable timber in a baobab, yet one definition of a tree is that it has a woody stem, so then that begs the question, is a baobab a tree?

The doyen of Africa vegetation is undoubtedly Frank White who defined a tree as a woody plant with a single, unbranched trunk to 3-5metres . By that definition our South African tree list would be in the region of 500 species! Elsie Esterhuysen, of mountain fame, defined a tree as any woody plant that would provide sufficient shade for a sit-down-on-the-ground lunch spot! So clearly one’s definition is determined by where one lives and works. Frank worked a good deal in Zambia and Elsie in the fynbos heathland where there are few if any trees! Our literature does sort of define trees as woody plants at least twometres high and ideallywith a single stem, (with a diameter of 100mm or more). But many 'trees' have multiple stems (e.g. Dichrostachys and Grewia spp.) and others are substantial woody, or not so woody, climbers or lianas (e.g. Dalbergia armata and Adenia gummifera – the former is substantially

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