Local scientists bust rain myth

Joint doctorate for Alanna Rebelo.

Bergvliet ecologist Dr Alanna Rebelo and Stellenbosch researcher Dr David le Maitre says it’s a myth that planting forests can help drought regions.

Dr Rebelo says the worldwide perception that forests can help drought-stricken regions is wrong. She says the only naturally occurring patches of indigenous, afromontane forest in Cape Town are confined to kloofs in the Table Mountain National Park.

“The rest of our natural area is fynbos shrub land and renosterveld. The only way to create forest would be to plant alien trees.”

She adds that trees, like all plants, do not magically produce water and can only put water into the atmosphere that already exists in the ground.

Dr Le Maitre says this water is called 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.”

Dr Rebelo describes it as being like 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 but for plants it’s the equivalent of breathing,” says Dr Rebelo.

Dr Le Maitre adds that plants significantly affect the water cycle in one other 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 offshore mists on our West Coast that come off the cold Atlantic,” he says.

“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,” says Dr Rebelo.

“We do not have these sorts of conditions in the Cape and scientific evidence is unequivocal.

That changing vegetation from shrubs or grasses to forest results in a reduction of river flows, or of water available to society.

In a few rare cases, such as tropical cloud forests, there is some evidence that the trees can capture sufficient cloud water to more than offset their water use.

“So trees do not produce rain; in fact they are taking water from our aquifers, rivers and dams, and breathing it out into the atmosphere,” says Dr Rebelo.

She adds that in the Cape all our rainfall is of oceanic origin (between 60% to 80% in the Southern Cape).

“This means that none of our rainfall is influenced by vegetation. Additionally, we reproduce hardly any rainfall for other regions.

“The Cape mountains actually cut off any potential moisture for the Karoo,” says Dr Rebelo.

Dr Le Maitre says some people suggest that Johannesburg has good rain because its suburbs make it one of the largest man-made forests in the world.

“However, Johannesburg has also seen its fair share of drought recently. Also, Johannesburg cannot be compared to Cape Town as they have completely different climates – the Cape is Mediterranean, Johannesburg is temperate,” he says.

They stress that in water-scarce regions, like the Cape, indigenous is best. The vegetation that occurs naturally in a region is adapted to those specific conditions.

Converting fynbos to alien trees would not produce the desired effect of more rainfall. It would have
the adverse effect of depletion of already scarce water resources as
well as bringing a host of other problems.

They say it is critical to remember that trees transpire only what is already available to them.

“This is the same water that we humans need to survive.

“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,” says Dr Rebelo.

* Dr Rebelo, 30, has also become the first student from the faculty of agricultural sciences at Stellenbosch University to be awarded a joint doctorate by Stellenbosch University and the University of Antwerp – a PhD in conservation ecology and entomology.

Dr Rebelo hails from a family of scientists.

Herdadecologist,DrTony Rebelo, works with the South African National Biodiversity Institute and her
mum, Patricia Holmes is a biophysical specialist with the City of Cape Town.

It was while Dr Rebelo was studying zoology and ecology at
UCT that she became passionate about rivers and wetlands.

She recently completed her Master’s degree research on wetlands in the Kromme River, where she investigated the hydrological benefits of restoration.

She recently completed her PhD on the ecosystem services of the Palmiet wetlands in the Cape Floristic Region.