Why the Highveld is treeless

I was surprised to see Julia Wakeling’s cautious endorsement of the frost and fire theory regarding the treelessness of the Highveld, in your March 2010 issue, being interpreted as a ‘debunking’ and in the words of the heading of Eugene Moll’s letter in March 2011, as an explosion of a myth. Perhaps a repeat of her conclusions would be in order, so that readers may judge for themselves as to whether she regards this theory as ‘rubbish’, or not. In her reply, she wisely remarks that there is no universal explanation for the lack of trees on the Highveld. ‘Exploding the myth’ and ‘debunking’ is hardly what she does in her original article, and I feel that your heading confuses a complicated issue.
In her reply to the letter, Julia invites comment, which is why I am about to offer her a view of the question from my spot, (very small, but intensely studied from a gardener’s, not a scientist’s, point of view), on the Gondwanaland steppe.
When I first came here twenty four years ago, I was puzzled and confused by the diurnal temperature extremes of a typical continental climate, until I saw a vegetation map of the world in a popular book on ecology, which marked the Highveld, and the high pampas, as ‘warm steppe’ with the Great Plains of North America, and of course, the Eurasian steppe as ‘cold steppe’. I have no idea whether the term steppe has any botanical or ecological validity, but for a gardener, it made everything fall into place. On my small patch of the vast patchwork of micro-climates that comprise the Highveld, I found the steppe theory fitted in very well. Here, the extremes of temperature are mirrored in the extremes of the weather, the endless strong winds, often turning to frightful gales, the bursts of excessively heavy rainfall, followed by long hot dry spells, all giving rise to what appear to be typical steppe soils. (And believe me, they do look as if they have been around since Gondwana). By identifying the woody aliens that are able to seed themselves in profusion in a disturbed and protected environment, (i.e. my garden) within the prevailing environment, I found that the part of the world that seems closest to my patch is Western China! I have not been able to pursue this theory very far in my garden, as you don’t find many plants from Western China around, but I have found that many garden plants native to the Great Plains thrive excessively. (Do not panic, in twenty years not one, not ONE has penetrated the surrounding veld. More about this later).
As Julia remarks in her article, altitude and what all botanists euphemistically refer to as a ‘cool’ climate, go together. In my experience, frost free grassveld is just that, grassveld, something between savanna and Highveld. But here I am out of my depth. Instead, for your amusement, let me describe a ‘frost free’ morning on a small-holding in the veld near the Free State village of Kestell.

Imagine a typical winter morning. The sun is shining, and there is no sign whatsoever of frost. (There hasn’t been for six weeks).Your toes are numb in your garden boots, but the botanists will tell you that it is ‘cool’, not cold. (To botanists, Omsk and Ottawa are cold in winter, and they are quite right). On this cool but sunny morning you decide to do a little digging. As you pick up your spade in the potting shed your fingers stick alarmingly to the freezing metal. Wrenching them painfully away, you go to find your gloves, reflecting that this is the fourth morning running (or even the tenth) that you have had to wash in water prudently tapped into jugs the night before, and that the toilet had to be flushed with a bucket of bath water left in the bath overnight for this very purpose. Not to worry, you will have running water by eleven, or half past one at the latest, when all the pipes, nine inches underground, (plus the ones in the roof, leading to the hot water geyser) have thawed out. A quick check on the min/max thermometer tells you that it is now minus eleven degrees Celsius, although it went down to minus thirteen overnight. (Or anything down to minus seventeen). You resume your digging and the spade immediately bounces back from something solid a few inches below the apparently dry surface. You know what it is; ice. Its depth will vary according to when you last watered the garden. In the veld, if it hasn’t rained for three or four months, you will have to dig for several feet before you find any signs of dampness. That is if you have the strength to get the spade into Gondwana. What tree seedling is going to survive these conditions, which endure (the right word) for three months and longer? Look at other steppes for your answer: None. Certainly, seedlings will germinate and survive in the nearby Berg, where they are fed and protected by snow and mist, which is why I do not consider the Berg to have anything to do with the Highveld Biome. Here, rare snow is welcomed as being a sign of a warm, gentle winter. Frost, too, is a luxury. Its presence means there is moisture in the soil and atmosphere, and as it melts, it may pass a drop or two of moisture to a seedling struggling through its first winter. Cold can be survivable. If water has evaporated or drained away, or is locked into ice, survival rates for woody plant seedlings in the veld drop to virtually zero. And as if these everyday trials are not enough, we also have extinction events to deal with.
Last year was a freak year. We had two faint dustings of frost in isolated patches early in May, and by mid June, my husband was moved to take a photo of the remarkable sight of our unfertilized, un-watered, Kikuyu back lawn a brilliant emerald green in midwinter. Next morning the frost reached the tops of the twenty meter cypresses (Cupressus arizonica) beside the lawn, (another photo) and the temperature was - 16 °C. This very sudden cold almost killed an infestation of wattle (Acacia mearnsii and A. dealbata) on a neighbouring hillside. With apparently all the top growth killed off, many of these trees are now stooling from ground level. Another similar freak event this year, and this re-growth could disappear. Of course, after the annual runaway fire (we are downwind from the village and township) that crosses the double lane N5 as if it wasn’t there, and races up the hill, this re-growth, with the dead top hamper to ensure a hot fire, could possibly take a fatal knock. The fire will of course stimulate the accumulated wattle seed to germinate, but these seedlings will be extremely vulnerable for several years.
This has relevance to your photo of the Acacia sieberiana sprouting from the base. I have found in my sheltered, watered garden that plants that are frost prone to the extent of the acacia do not have the reserves to keep repeating the performance. By the third year, the regrowth is weak and spindly, and the fourth winter finishes it off.
Flooding occurs irregularly, and was particularly severe this year, killing shrubs and saplings. With the freak late frost, this means that we have had two extinction events within eight months, with a near certainty of a third one, fire, within the next six months.
To return to the question of plants being able to penetrate the veld; you will know about C4 and C3 grasses, I don’t. But I wonder what it is that allows savannah trees to penetrate the grassveld, whereas here the veld seems impenetrable. Firstly, when I came here, I tried most of the familiar grasses around here in the garden with other ornamental plants, and found that they were strongly allelopathic. They created nice little bare patches around themselves, into the edges of which they duly seeded themselves in their millions. Hyparrhenia spp. were the worst, although I still have some, carefully corralled off from everything else. The other thing is that if you go into our local veld and dig up a sod, with difficulty, even if there are spaces between the grasses on the surface, underground there is an almost impenetrable mat of roots. The idea of anything woody coming up among these densely growing, highly allelopathic plants seems unlikely.
We have two species of indigenous trees that sow themselves on the property. One is Celtis africana, and the other Leucosidea sericea. The first only comes up in sheltered, middling to choice, well watered, garden soil, and the second only in a sheltered, narrow belt along our small river. In twenty years neither has penetrated the veld, nor has any other tree, except for a single Pinus radiata, which has reached sapling stage in a slightly sheltered land last ploughed twenty four years ago, and mowed for hay until about thirteen years ago, so it is not virgin veld. (With regard to the P. radiata which penetrates the veld (some of it apparently undisturbed) elsewhere in the district, it would be interesting to discover whether the soils of its native environment on the Pacific Coast of North America have any affinity with the toxicity of the Highveld soils proposed by Prof. Moll).
One last thought about the treelessness. I once had an apple establish itself to about knee height in some disturbed veld. (Apples like it here, they are thought to be native, originally, to the eastern end of the Russian steppe, up against the Tien Shan Mountains). Unfortunately for the apple, like most woody plants it came into leaf while the grass was still dead, and was eaten into the ground by a horse looking for something succulent. In the distant past, could this ability of woody plants to come into leaf while the grasses were still dormant and unpalatable be a partial reason for none of them getting the chance to develop the ability to adapt and establish themselves on the steppe, over millions of generations? It’s worth a thought: think of the fabled herds of antelope, the buffalo of the Great Plains, the horses of Mongolia, flattening everything in their path. Maybe not.
So, for my patch of the Highveld, prolonged low temperatures and prolonged drought, making a very short growing season for your hypothetical seedlings, (these two factors combined being the overriding factor), lack of shelter from the excessive winds (an underestimated factor), regular extinction events, unfriendly vegetation, and the possibility of “evolutionary grazing” having prevented the development of survival mechanisms, mean that there doesn’t seem to be the remotest possibility of trees establishing themselves on the local steppe, short of catastrophic global warming.
Di Moffett, Kestell


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Debunking the myth