Trees come at a cost

Professor Tony Rebelo, Bergvliet

The call to plant 10 million trees is a great initiative for urban streetscapes, but the Bulletin’s report is decidedly unbalanced (”Call to plant 10 million trees could give Cape Town back its shade,“ Bulletin January 20).

Certainly, we do need trees. But trees come at a cost. Planting trees has environmental consequences, and we ignore these at our peril.

The first of these is water: Cape Town is water limited, and Day Zero will always be around the corner (and our population continues to increase). Invasive alien trees in our catchments consume the equivalent of a large dam each year. Pines, hakeas and wattles are sucking our city dry. Cape Town must remove these trees, and drastically too or Day Zero will become an annual phenomenon.

But even in our suburbia, trees use more water than the local indigenous vegetation. The Cape Flats aquifer is an important water resource for both the city water supply and garden boreholes. Too many trees will impact on the aquifer. Put simply, we have too much water in winter and a major shortage in summer. Aquifer depletion will result in boreholes running dry at the end of summer. Most Capetonian gardens are not designed to survive a summer drought and most city street trees are not Mediterranean-climate adapted – they are profligate summer drinkers.

One of the arguments for planting trees is carbon sequestration. However, in these temperate latitudes, trees warm the soil and liberate more soil carbon than they store. So trees are bad news in Cape Town.

Fynbos, renosterveld and strandveld – which have no natural large trees – are far more efficient at storing carbon. Veld invaded by trees results in the liberation of carbon into the atmosphere increasing global warming, which is antithetical to what we are trying to achieve.

In addition, trees impact biodiversity. Natural vegetation in the Cape is largely tree-free, and trees compete with and destroy natural biodiversity. Cape Town is already the biodiversity extinction capital of the world, number two worldwide only to Hawaii. We need to protect our natural capital – both species and ecosystems, and trees have no part in this.

Some trees (like pines, gums and wattles) are also a major fire hazard. They increase the fuel loads, making fires more frequent, hotter, and much more difficult to control. In addition, palms and cedars on the urban edge are major sources of embers, causing nearby structures to ignite. Fires are natural: alien fires are not!

Trees also have other negative effects. They clog drains, raise pavements, crack walls, cast shade, block access, produce allergenic pollen, drop limbs, dump leaves, etc. These issues are best dealt with on a case-by-case basis, but clearly not all trees are equal. Tree species need to be carefully matched to the desired local outcome, by considering both the benefits and the costs.

But what the article also neglects is that in five to 10 years’ time Cape Town will have lost all its oaks, planes, maples and many other species. The polyphagous shothole borer beetle has escaped from quarantine at Somerset West and is spreading. It has already been recorded in Tokai, Sweetvalley and Constantia. Unless we start planting borer-resistant trees now, we face entire suburbs devoid of trees within five to 10 years. We will need thousands of trees just to replace these. We need to get planting. But it must be appropriate trees in appropriate situations.

The willy-nilly planting of trees would be a grave mistake. And planting invasive alien trees in inappropriate areas is a recipe for disaster.