The Tree Myth

As Alanis Obomsawin’s observation highlights, we must understand that we are shaped by our past but not chained to it and, therefore, act for the best future of the planet. Conservation activities are necessary to protect our natural resources for current and future generations, and have nothing to do with the short-term exploitation of resources for the entertainment of current generations.

We need to move away from the colonial notion of plantations, specifically pine in Table Mountain National Park and other wild areas. Retaining alien tree plantations and plantings creates a colonial landscape, not the natural heritage Cape landscape.

The popular notion of planting a million or a billion trees originated in Europe and North America before recent research findings. It may be appropriate in some cases, but it is not the solution for most of Africa. Biomes such as savannah, wetlands, and fynbos are most suitable where they originally occurred.

The City of Cape Town has outlined its Urban Forest plan. This plan is based on the belief that trees will create a cooling effect, among other benefits. However, such beliefs need careful consideration, and actions need to be evaluated carefully.

For example, some thoughts, as outlined by Dr Guy Preston, are as follows:

1. Planting “the right trees in the right place” is crucial, and the policy needs to be placed in the broader context of conservation and safety needs in Cape Town.
2. The policy does not address the requirements of the National Environmental Management: Biodiversity Act (NEM:BA Act 10 of 2004).
3. The policy does not adequately handle the eradication of invasive species under the AIS Regulations of NEMBA.
4. There has been an unfortunate misinterpretation of the alien invasive species regulations.
5. The City of Cape Town is situated within the Cape Floristic Kingdom. There is no mention of fynbos, or the importance of protecting habitat for fynbos types and species.
6. The policy makes no mention of fire.
7. The goal of 10% tree cover is incongruous for greater Cape Town.
8. What the City needs is a Vegetation Management Policy, and not an Urban Forest Policy.

It is essential that we are clear about the tree species that we may wish to plant or introduce. For example, it has been shown that clearing alien trees generates more water for the City (Holden et al).

Holden et al also showed that climate change likely worsened the impacts caused by the alien trees. This is particularly important given our recent near day-zero experience for water in Cape Town, as well as last year’s crisis in Gqeberha. It is also of immense importance to those citizens who now depend on underground water for domestic or commercial use.

Comprehending the effects of forest planting is not a simple matter. It also depends on the intended benefits. Native forests are better for mitigating climate change, and not all forest-restoration initiatives provide equal biodiversity benefits. Researchers compared monoculture tree plantations with diverse native forests in 53 countries and found that the latter provides better carbon storage and water delivery, and are better at preventing soil erosion. By contrast, monoculture tree plantations yield more wood (Hua Fangyuan).

Moreover, it has been shown that forests as natural carbon sinks with cooling effects are only effective in tropical regions. In other areas, they either have no impact or contribute to global warming (Swaminathan). This is partly because plantations release aerosols, more so than natural forests, which further contribute to global warming (Salinger).

The calculus of forests’ climate effects is not straightforward, and new research indicates that a more forested world may not necessarily be cooler. In some regions, new forests could enhance warming by reducing the amount of sunlight reflected into space, offsetting any gains in carbon absorption (F Pearce).

How then do we explain why it feels cooler under a forest canopy? First, one is out of direct sunlight and, secondly, trees transport large amounts of moisture from the soil into the air through transpiration (a typical tree can transpire up to 100 litres of water per day). This transpiration in the form of water vapour cools the air around the tree.

However, this localised cooling does not imply cooling overall. Trees, especially in forests, can cause overall warming by altering the absorbance of sunlight, or albedo, of land surfaces. A surface such as fresh snow has an albedo of 0.8 to 0.9 (on a scale from zero to one), meaning they reflect most solar energy back into space. In contrast, a continuous canopy of broadleaf trees can have an albedo of just 0.15, implying that the trees absorb solar energy and radiate it in the form of heat. A canopy of conifers can have an even lower albedo, such as 0.08.

In Israel, around 4 million Aleppo pines were planted in the Negev Desert. Measurements of albedo and biomass conducted by Yakir show that the warming effect from the dark canopy of the pines now exceeds the cooling effect of its carbon capture.

Misguided and uninformed projects can cause more harm than good.