Shady proposition elicits illuminating responses
Reading Time: 6 minutes
Professors William Bond and Tony Rebelo respond to a misleading Constantiaberg Bulletin article, Call to plant 10 million trees could give Cape Town back its shade, promoting the Department of Forestry, Fisheries and Environment’s (DFFE) under-performing 10-million Tree Programme.
Erroneously conflating SANParks’ legislated removal of alien vegetation from our Protected Areas with a “loss” of “considerable tree cover” hurting “the city‘s cultural landscape” and depriving its citizens of “critical recreational value”, the article quotes spokespersons from several city-based community organisations voicing unquestioning support for a potentially harmful programme at a time of unparallelled global resource scarcity, habitat degradation, climate change and biodiversity loss.
Tree-planting drive a threat to ecology, livelihoods
Constantiaberg Bulletin 3 February 2022
I refer to the article (“Call to plant 10 million trees could give Cape Town back its shade,” Bulletin January 20). For over 25 years, the Western Cape has been running a world-class environmental programme removing trees. Surely that calls for some comment on why a government department, decades on, wants to establish millions of the same plantation trees that we have been removing.
The tree-clearing programme stems from a century-old debate on whether plantations of trees increased rainfall or dried up rivers. After many decades of research and monitoring we have a clear answer: trees dry up our rivers. To avoid future Day Zeroes, cutting down alien, invasive trees is a key strategy for sustaining our water sources.
The new call by the Department of Forestry, Fisheries and Environment (DFFE) to plant 10 million trees is part of a global and Africa-wide programme intended to soak up excess carbon dioxide in the atmosphere.
Anyone can see that trees lock up carbon in the tree trunk. What you can’t see is the carbon below ground, which can greatly exceed carbon above ground, in Arctic peatlands, for example.
With the support of oil and forestry companies, politicians (including former American president Donald Trump), and the World Bank, the gobal tree-planting programme has received massive support.
The science behind it is surprisingly thin and is being increasingly questioned by scientists. The trillion trees programme, for example, was promoted by a paper in Science (Bastin et al. 2019), but subsequent rebuttals showed that the carbon estimates were greatly exaggerated (they ignored soil carbon for example).
Plantations are an uncertain and very slow way of decarbonising the atmosphere. They are not only an inefficient mechanism compared, say, to reduction of fossil-fuel emissions, but tree planting also directly threatens the livelihoods of millions of people and thousands of sun-loving plant and animal species. The problem of plantations lowering water tables, very well understood in South Africa, has hardly entered public debate and tree-planting promotions.
It might help to visualise what 10 million trees means. Like all the other big numbers, it bamboozles people to talk about a million, a billion, a trillion. Even the largest trees start off as very small seedlings. So is the target 10 million seedlings? Or stately giants? How big will these 10 million trees be?
The answer really matters for how much carbon they sequester and the area needed to do so. If we choose tall trees and count how many trees you get in one hectare it’s not hard to work out. Two hundred trees per hectare would give you some stately looking tall trees.
Ten million of those would require 500km². That is an area exceeding the entire Cape Peninsula (470km²) including all its urban settlements. So when the enthusiasts for tree planting welcome the 10-million-trees programme do they envisage the whole of the Cape Peninsula covered under trees? Does “giving back Cape Town’s shade” mean swallowing up our famous sun-loving fynbos under pine trees?
This is not giving Cape Town back its shade – it’s creating shade that was never there before. Planting those 10 million trees would destroy all the biological uniqueness of the Cape Peninsula.
It makes sense to plant trees in cities and their gardens, for shade and recreation. But does it really make sense to cover remaining natural areas with even more plantations than those that already exist?
How should we deal with the flood of calls for tree planting to save the planet? What should we be doing to reduce climate change risks? Why plant a tree? Why not a solar panel or better roof insulation?
I like trees and have planted quite a few. If you really want to plant trees, it is helpful to ask: is this the right tree, in the right place and for the right reasons?
Avoid fast-growing alien invasives. Short-term gain will leave a centuries-long headache. Avoid planting in pristine, native vegetation. What are you destroying? Beware planting trees to decarbonise the world. You may lose more carbon from disturbing virgin soils, or if the stored carbon goes up in smoke in a plantation fire.
Beware the European forest fetish. Celebrate the magnificent views in South Africa, which plantations would obscure. Insist that Europe, and South Africa, fight climate change by the quickest and most effective means: reducing fossil-fuel emissions.
William Bond Emeritus Professor in Biological Sciences at the University of Cape Town
Trees come at a cost
Constantiaberg Bulletin 3 February 2022
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 (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 suburbs, 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 Cape Town 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 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 desired local outcomes by considering both their benefits and their 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 in Somerset West and is spreading. It has already been recorded at 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 a recipe for disaster.
Professor Tony Rebelo Bergvliet
This Post Has 2 Comments
Let’s get planting appropriate trees in appropriate locations and let our fynbos also flourish
Great Andy, and let’s also promote the removal of invasive alien vegetation including in our river corridors and bring back the Palmiet and other indigenous riverine plants which not only sequester carbon far more effectively but also combat channel erosion, retain flood waters releasing it slowly to feed the environment, and prevent it rushing off down to the sea unnecessarily fast.