African Wildlife and Environment Issue 68

ENVIRONMENTAL EDUCATION FAUNA, FLORA & WILDLIFE

ENVIRONMENTAL EDUCATION FAUNA, FLORA & WILDLIFE

This time I decided I would take another different approach, and instead of talking about one tree species, or several related trees, I would discuss something about tree growth forms. Why? I think it is important that when one is travelling through any ‘treed’ countryside there should be a variety of questions that arise in your mind. For example, a few years ago I had the delightful privilege of joining Ernst van Jaarsveld and others on a field trip from Cape Town to northern Namibia. On the road there and back, we (all of us were botanists) had animated discussions about many topics to while away the kilometres travelled. I don’t recall how the topic of succulents came up, but as we travelled north through the Succulent Karoo, the Knersvlakte, and past the Richtersveld, we started to talk about why in the south most succulents are short and low to the ground, yet in the north they are so much taller? As you may know, we in southern Africa have well over one

Just as an aside, you should be aware that succulents store water that they then use slowly, thus whether they have more than enough water, or are thirsty, their metabolic activity hardly alters (whereas most plants grow quickly under optimum conditions and slow down when times are tough for them). This means that, if they do not get moisture (and they only need small quantities and many succulents can absorb moisture through their leaves and stems) regularly they can and will die, so prolonged droughts kill succulents. This is possibly why Australia has no succulents, other than in salt marshes. The succulent species in the Succulent Karoo are essentially low shrubs, and even so-called ‘stone-plants’ also occur (e.g. Lithops and Fenestraria spp.). Now, as one travels from Cape Town further north past the Knersvlakte to Springbok, there is a gradual appearance of taller species of succulents such as Aloidendron dichotoma (the Kokerboom) and in the Richtersveld, which is a little further north, there are several additional species such as Aloidendron pillansii , Stoeberia arborea , Pachypodium namaquanum , and Tylecodon paniculatus . By the time one reaches northern Namibia there are a much greater number of tall succulents. The biggest of all in Africa, namely the Baobab Adansonia digitata is found in northern Namibia. Meanwhile, along the coast, where the conditions are cool because of the influence of the Agulhas current, low-growing succulents can be found almost to the Cunene. Moving inland where the cooling influence of the ocean is lost, the succulents are generally tall. Somuch for the west coast, what about the east coast succulents? Starting along the south coast with the Klein Karoo, there are many succulents there, both dwarf shrubs and some taller species like Aloe ferox and Portulacaria afra . In fact, if you look at the Biome Map of South Africa in that comprehensive tome ‘The Vegetation of South Africa, Lesotho and Swaziland’ in Strelitzia 19 edited by Mucina and Rutherford (2006) you will see that they map the Succulent Karoo in a big arc from south of the Orange River or as it is now called the Gariep River. However, the local Nama people of Bushmanland still call it ‘Orange’ down through the inland bi-modal rainfall area into the Klein Karoo as far east as Port Elizabeth. East and north of Port Elizabeth there is the Albany Thicket and further north, Savanna. In these last two biomes the succulents, where present, are tall i.e. aloes and Spekboom in the thicket, and many aloes and euphorbias in the Savanna (starting with species like E. tetragona in the south, then E. triangularis in KZN and finally E. ingens ) the biggest euphorbia of them all)

should be dwarfed and thus close to the ground in the south and tall in the north? This question teased our minds for some time! Of course, Ernst is a global succulent expert, and he has grown a great number of species of many shapes and sizes over his long career as a horticulturalist, first in Kirstenbosch and now at Babylonstoren. After a lot of chit-chat Ernst intervened in our discussion, to give his reasons for this discrepancy in the size of succulents in relation to latitude. He told us that from his field experience, and observations, when he grew succulents, that they were heat sensitive. Thus, in the south, where winters are much colder, they need to be low-to-the-ground to keep warm. Whereas in the north, where it is much hotter (and more humid), they are tall so they can be air cooled. I don’t know of any experimental evidence for these observations, but clearly the field data certainly supports Ernst’s view, and his own

A lone Aloe dichotoma

A really tiny vygie wedged between sheets of metamorphosed shale

from KZN northwards). If you look at the group of aloes known as sensu lato , the biggest and tallest by far are Aloidendron kosiensis and A . barberae (previously these two species were lumped as Aloe bainesii ). To conclude, the reason for the growth form and height of many succulent species is for form to fit function. In the hotter and more tropical parts of our land, succulents need to be tall to be air cooled. In the colder climes, they tend to be dwarf shrubs where they can benefit from the warmth from the substrate. Eugene Moll Department of Biodiversity and Conservation Biology University of the Western Cape emoll@telkomsa.net

third of all global succulent species, with the ‘vygies’ species being by far the most numerous. We are also unique in that our succulents occur in many different plant families, such as Aizoaceae, Asphodelaceae, Asteraceae, Crassulaceae, Euphorbiaceae, Geraniaceae, Zygophyllaceae and a few more! An additional factor is that we have a wide range of succulents from annuals to perennials, leaf and/or stem forms, even plants with succulent stems, and succulents that also have deciduous leaves; etc. So, on this journey questions about succulents and the succulent growth form were many and very varied. For me one of the most interesting was the question of why succulents

horticultural experience is also indicative. Possibly some science can follow the observed natural history? So here we go with examples: The Succulent Karoo we know is a globally unique vegetation type, with many succulents and this makes it one of the global plant hotspots, and it receives bi-modal moisture (i.e. it receives RELIABLE precipitation in spring and autumn, although the AMOUNT can vary hugely). The literature tells me that this is the reason why it occurs where it does. (i.e. in the arid to semi-arid zone in southern Africa along the west coast and north as far as about Luderitz).

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