The Highveld grasslands cover a large area south of Johannesburg. If you have ever driven through them you may have been amazed at the vast openness – not a single tree in

sight – until you come across the ubiquitous gum tree at a lay-by. Trees manage to grow in towns and plantations in grassland areas, so they can obviously survive the cool climate. Why then do the grasslands have no trees? This was the topic of my recent Masters degree in Botany. In my travels I spoke to many people and everyone had their own theory, but not one that could really answer my question. Some were correct locally, for instance, many trees do not survive in soils that become seasonally waterlogged, and true, many of the flatter parts of the Highveld have soils like this, but certainly not the whole Grassland Biome.

A frosty answer‘Ah, it’s too cold and trees are killed by frost’, is probably the most common explanation. As I said before, trees do survive the winter, and grow in towns, plantations, along roadsides and in forest patches high up in Drakensberg valleys. And what about savanna trees? I wanted to explore how frost would affect the types of trees that normally grow within the grass layer in savannas. To do this I planted seedlings of acacia thorn trees in grassland and savanna areas and monitored their survival through the frosty winter of 2007. Near Piet Retief and Volksrust almost all the seedlings survived the winter frosts, whereas in a cooler area near Ermelo, only a few seedlings survived, but if those surviving seedlings could become adults, the area would turn into a wooded savanna. A few species, such as Ouhout Leucosidea sericea, do grow on hillsides in grassland areas – highlighting yet again that woody species can cope with the climate – but for the most part the Highveld grasslands are wide open grassy expanses.

Cut down in their primePerhaps people cut them down? This is a popular idea for explaining the tree-less nature of grasslands all over the world. But pollen cores and soil carbon analyses are revealing that the Highveld grasslands have been present for over 10 000 years (as have many other grasslands in the world), much longer than the influence of modern humans. There

are certainly tracts of land that have been cleared of trees within the last few hundred to 1000 years, but they are a small fraction of the entire Grassland Biome from Johannesburg to the Eastern Cape and Piet Retief to Bloemfontein!

A fiery debateFire, you think? I’m afraid not. Some grassland areas burn every year, yet others burn perhaps only every 10 to 15 years. This is also true for savannas, which are grasslands with an abundance of successful trees. But how do savanna trees cope with fire? Seedlings are easily burnt to the ground, but as the roots are not killed the plant can re-sprout. Seedlings grow into saplings, which, if they grow fast before the next fire, may grow tall enough to get their leaves above the height of the flames. If successful, they have ‘escaped’ into adulthood. If still too small, a sapling will burn to the ground and re-sprout from its roots again. A sapling can survive for decades by repeated re-sprouting, but if it is to reproduce successfully it needs to grow above flame height. Propagation is after all a goal of all living organisms. For the little tree, all depends on growing fast enough between fires. So what influences this growth rate?

Why do grasslands have no trees?by Julia Wakeling, with William Bond and Michael Cramer, Botany Department, University of Cape Town

TOP: An exquisite grassland with indigenous trees growing in the kloof and eucalypt plantations beyond. ABOVE: A Paperbark Thorn, Acacia sieberiana, re-sprouting after a severe frosting. Photos: Julia Wakeling.

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Is it in the soil?Soil nutrients seemed a likely candidate. The Highveld grasslands are generally on old soils that have been leached of their nutrients over time, while savanna soils on the same geological substrates are richer in nutrients. In an experiment, I measured growth rates of two acacia species to see if this relative lack of nutrients in grassland areas would cause trees to grow too slowly to ever escape the frequent fires. I collected soils from relatively ‘nutrient poor’ Highveld grasslands (near Ermelo, Volksrust and Piet Retief ), and the comparatively ‘nutrient rich’ savannas (near Vryheid, Pongola and Hluhluwe-iMfolozi Game Reserve), and grew seedlings in these soils. And indeed the Lowveld soils produced faster growing seedlings than the Highveld soils. But are the differences large enough to explain the absence of trees in grasslands and their presence in savannas? We know that a tree growing in savanna soils can grow to a height above the flames of a fire, in the time between two fires. In this experiment, seedlings grown in grassland soils grew only slightly slower than those in savanna soils, but they still grew fast enough to escape fires.

Ok, so what is it? We are running out of explanations. Why do savanna trees, which can survive fire, not grow in these vast grassland areas? I should point out that the Highveld grasslands are far from unique. There are similar grasslands throughout Africa: the Eastern Highlands in Zimbabwe, the Nyika Plateau in Zambia and Malawi, the Aberdares in Kenya; and the rest of the world: Madagascar, India, the tall-grass Prairies of North America, the Campos grasslands of South America and the Balds of South East Australia. In all these grasslands fire is important and fire-tolerant savanna trees occur nearby. Often there are forest patches within the grasslands, or trees growing in towns, highlighting that it is not too cold

for trees to exist. And yet there are no clear answers as to why trees don’t grow there.

Keeping cool A major aspect of the grasslands is that they are at greater altitudes or latitudes, and thus cooler, than adjacent savannas that do have trees. Most living organisms grow slower when it is cooler. I wanted to know how much slower savanna trees would grow in grassland climates and what this would mean for a savanna tree trying to outgrow fires. I planted seedlings of six acacia species at sites in cool grassland areas and warm savanna areas. When the experiment began, these seedlings were no larger than a coke bottle. After a summer of growth, seedlings at cooler sites were about the height of a spade, and those in warmer areas were double that, some of them large enough to provide a shady spot for lunch. In comparison to those in warm areas, the growth of seedlings in cool areas was so slow that they would not be able to grow above the height of flames between two fires. It is noteworthy that seedlings grew at all the sites. However, in the presence of fire, those that grew too slowly would be eliminated.

In summary thenFirstly, frost alone does not prevent trees from growing in the Highveld grasslands. Indeed, many of our grassland areas are frost-free. Secondly, low nutrient supply slows sapling growth as is true for the relatively nutrient-poor grassland soils. But by far the most important limitation on seedling growth in my study was cool temperatures associated with high altitudes. This slow growth in combination with frequent fire prevents trees from growing in the Highveld grasslands.

Implications for the future As temperatures are increasing with climate change, savanna trees will be able to grow fast enough to escape fire in grassland areas

where they could not before. A reduction in fire would also lead to tree invasions in areas where trees grow too slowly to escape under frequent fire regimes.

You may be thinking that I want savanna trees to grow in the Highveld grasslands. Quite the contrary, the grasslands are a diverse ecosystem filled with beautiful grasses and wildflowers. This research was aimed at better understanding savanna and grassland ecosystems so we know how to manage them. Many grassland areas are used for plantations, agriculture and cattle farming, and the small areas of pristine grassland that remain are threatened. What is left needs to be conserved.

When you next drive through these vast grasslands I encourage you to stop and appreciate them. Enjoy the patterns of the wind in the grass and examine the minute intricacy of grass flowers and the tiny wild flowers hiding between them. Ponder the travails of a tree seedling entering this grassy world and revel in the natural beauty of a wide open space without a single tree.

ACKNOWlEDGEMENTSThis work was made possible by the generous provision of land, labour, time, water, hospitality and enthusiasm by a number of land-owners and managers, including Robin and Sue Kirkwood, Emile Schmidt, Ian and Carol Goss, Henry and Ria Malan, Katy and Stephan Heitland, Clayton and Debbie Comins, Graham and Janine Ford, Nigel, Sue, Jen and Tracy Anderson, Mark and Ruth Armitage, Andrew and Amanda Shand, Mark, Evette, Michael and Andrew Hitchins, the Zululand Tree Project staff, and Mr Kosy Dongo, Lesego Bodibe, Samuel Zulu and Jan Theron of the Agricultural Research Council. The project was funded by the Andrew Mellon and National Research Foundations.

ABOVE: Five month old acacia seedlings planted in warm conditions near St Lucia, were much larger than the same type of seedlings growing in cooler conditions near Ermelo. BELOW: Five month old acacia seedlings planted in cool conditions near Ermelo were smaller than similar ones planted in warmer conditions in St Lucia. Photos: Julia Wakeling.

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