In the midst of the worst drought on record, many Capetonians are asking themselves whether planting trees could be a viable way to bring rain.
This is undoubtedly the result of widespread perceptions supported by reputable popular publications like ScienceMag claiming that “forests make rain” and “planting forests could help drought-stricken regions”.
But could this work for the Cape? An area with very little indigenous (Afromontane) forest confined to kloofs? Dominated by fynbos shrublands and renosterveld? Positioned at the southern-most tip of Africa?
The only viable option would be to plant more alien trees and encourage alien-tree invasion. But would this be beneficial even if it did work?
It is important to note that trees, like all plants, do not “magically” produce water. Trees can only put water into the atmosphere that already exists in the ground.
We call this water “green water”. It soaks into the soil following rainfall, where it replenishes groundwater or flows into streams and rivers. This water eventually becomes the water we see flowing in our rivers and being captured by our dams.
There are two ways that trees can transfer water: the first is by “transpiration” and the second is by “condensation”.
Transpiration is the process of water-use by trees. Picture a straw: plants suck water up from the soil and release water vapour through openings in leaves (stomata). It is a very similar process to human “perspiration” (or sweating) but for plants is the equivalent of breathing.
The second way, condensation, occurs when the air is saturated with water and suddenly cools against a surface and precipitates droplets.
This requires either very warm humid air, such as in tropical cloud forests, or less moisture-laden air that is cooled, such as the offshore mists on our West Coast that come off the cold Atlantic.
However, these conditions are scarce and only in specific climates can sufficient cloud water be captured to more than offset the water-use of the plants themselves.
We do not have these sorts of conditions in the Cape. The scientific evidence is unequivocal. Changing vegetation from shrubs or grasses to forest results in a reduction of river flows, or thus of water available to society. In a few rare cases, notably tropical cloud forests, there is some evidence that the trees can capture sufficient cloud water to more than offset their water-use.
But this is certainly not the norm. So yes, you’ve guessed it! Trees do not “produce” rain. They are taking water from our aquifers, rivers and dams, and breathing it out into the atmosphere. So where is this amazing gain in rain that these articles are promising?
The trees in the Amazon really do seem to be able to create their own rain, but this is mainly as a result of their special climate and atmospheric circulation.
In the tropical rain cloud forests of the Amazon, atmospheric circulation brings moisture-laden air to the coast from the ocean.
Forests on the coast transpire this water which is then transported further inland. Due to the vast areas of tropical cloud forest, high humidity and circulation patterns, this moisture waters the Amazon.
In the Cape and indeed most of South Africa our rainfall is of oceanic origin (100% in the Cape, 60%-80% in the southern Cape).
This means that evaporated water from the oceans moves over land and falls as rainfall. Indeed because of the moisture circulation patterns in the Cape, we produce hardly any rainfall for other regions (between 0%-20%) and the Cape mountains actually cut off any potential moisture from the Karoo.
People often suggest that Joburg has lovely rain and is one of the largest man-made forests in the world, and that this is therefore something we need to try in the Cape. These people have short-term memories. Joburg has low rainfall and the catchments that supply its water have had several droughts over the last few years, threatening its water security.
In addition, you cannot compare the Cape with Gauteng. Both have completely different climates. The Cape has a Mediterranean-type climate and receives winter rainfall.
Joburg has a temperate climate with summer rainfall.
To extrapolate findings that “trees bring rain” from tropical regions to our own Mediterranean-type climate in the Cape is wrong. In water-scarce regions, “indigenous is best”.
The vegetation that occurs naturally in a region is adapted to those specific conditions: in the Amazon it is the rainforest, in the Western Cape of South Africa it is predominantly fynbos.
Converting fynbos to alien trees would not produce the desired effect of “more rainfall”. It would have the adverse effect of depletion of our already scarce water resources, as well as bringing a host of other problems. Dr Alanna Rebelo and Dr David Le Maitre said it is critical to remember that trees transpire only what water is already available to them (water in rivers, wetlands or aquifers) which is the same water that we humans need to survive.
Therefore, using water that is a precious resource to “possibly” produce rain elsewhere in the world is not a viable solution for a water-stressed place like the Cape.
* Rebelo is a post-doctoral researcher in the Department of Conservation Ecology and Entomology at Stellenbosch University and Le Maitre is a principal researcher at the Council for Scientific and Industrial Research and a research associate at the Centre for Invasion Biology at Stellenbosch University.