NO SCIENTIFIC EVIDENCE

FACT CHECK: Do Eucalyptus trees cause reduction of water level in rivers?

No empirical studies to support the theory that eucalyptus are high water consumers.

In Summary

• Expert Benjamin Wamugunda said just like any other plantation, eucalyptus only uses water during photosynthesis to produce wood.

• Dr. Sammy Carsan said diversion of water heads and clearing of vegetation along waterways is perhaps a major problem contributing to the loss of water volumes.

Eucalyptus trees along a river
Eucalyptus trees along a river
Image: COURTESY

Last month, Murang'a County Woman Representative, Sabina Chege,called for an end to the planting of eucalyptus trees on grounds that they cause reduction of water levels in rivers.

On her Facebook page, the MP claimed she had met KTDA officials at the Gituamba eucalyptus plantation and agreed not to replant the trees because water levels in a local river had reduced.

After public outrage on her comments, the MP pulled down the post but rephrased her statement, defending her sentiments on why she wanted the forest ‘done away with’.

“Today, I spent the better part of the day on a development tour within Kinyona Ward. I inspected the Gituamba Power line together with Rural Electrification Authority Officials and later met with KTDA officials in Gituamba Eucalyptus plantation, where they were planning to replant the same trees, days after harvesting those that were planted seven years ago against the wish of Kinyona residents. I informed them of the views and complaints of the residents despite several outcries therebefore,” she said.

A screenshot of the MP's Facebook post
A screenshot of the MP's Facebook post

“This community land must be used for the public good. The current use (planting eucalyptus woodfuel) is not for the local public good. The land is currently used by KTDA holdings and her managed factories (which are privately registered) as a source of their firewood project. Currently, Eucalyptus trees ( Minyua mai) are rejuvenating on the land for the wood fuel program by KTDA,” she added.

Chege said she was of the opinion that the land should be used in a more productive manner,  with less negative impact to the people.

She argued that the eucalyptus trees had impacted the area negatively, including reducing the water flow on the Kanyona River.

https://www.facebook.com/HonSabinaChege

But are these claims true?

The Star’s fact check desk contacted Benjamin Wamugunda, a senior forester and the National Chairman of the Forestry Society of Kenya.

The MP's original post
The MP's original post

In his response, Wamugunda said eucalyptus trees have been demonised for a long time due to wrong reasons.

He said just like any other plantation, eucalyptus only uses water during photosynthesis to produce wood.

“It is wrong to say that eucalyptus are to blame for the reduction of water in any place. That is a strange reason one would use to demonise a tree species,” he said.

Wamugunda’s sentiments are backed by a report by Kenya Forest Services on the eucalyptus and water use.

According to KFS, a lot of concern has been expressed on the effect of eucalyptus trees on the hydrological patterns, with various claims that their presence on the landscape is causing the drying up of water sources, rivers and springs.

KFS said these claims have not been conclusively supported by scientific evidence.

“However, studies have established that eucalypts exhibit high efficiency in water use for biomass accumulation,” read part of the report.

For example, it has been established that eucalyptus requires less water to produce one (1) kg of biomass than most crops.

Some comparative data to support this is as follows: 

  • Eucalyptus species require on average 785 litres
  • Cotton / coffee / bananas each require 3,200 litres
  • Sunflower requires 2,400 litres
  • Maize, potato and sorghum require 1,000 litres each

 According to KFS, the effects of eucalyptus on the water budget will depend on the species in question, climate, soil conditions, nature of rock substratum, vegetative cover, slope, tree growth stage, and tree density and amount of rainfall.

A senior scientist at the World Agroforestry said hydrological studies are needed in order to evaluate available water in the system, its use and losses. 

Dr. Sammy Carsan says, unfortunately, in most situations available long-term studies are lacking to evaluate the role of eucalyptus on water use and losses. 

He said while some studies exist and there is feedback to determine the impacts of eucalyptus on water use, one has to look at the total coverage of plantations in a given landscape rather than few isolated tree stands or woodlots on-farm plots.

“Decision to plant eucalyptus need to be informed by management knowledge on correct eucalyptus species for a given site but also more crucially an evaluation on whether the available land can support its production. It’s crucial to remember that eucalyptus is a plantation species with high efficiencies for wood biomass production- for this to happen, there is the demand for soil nutrients and water,” he said.

Carsan said currently, there are no empirical studies done in the country to support the theory that eucalyptus are high water consumers.

To him, some evidence indeed suggests major horticultural crops demand more water through irrigation than eucalyptus. 

“The difference is limited studies on eucalyptus water use attributes under varying site conditions and perhaps strong competition with agriculture crop production thereby making unsuitable under smallholder agroforestry production domains,”  he said.

With regards to arguments that a plant may take up less water but may reduce the volume of water because it is next to the river, Carsan said, “This is part of ‘myths’ associated with eucalyptus. Diversion of water heads and clearing of vegetation along waterways is perhaps a major problem contributing to the loss of water volumes than trees adjacent to waterways”.

Carsan said planting wrong crop choices in water catchment areas would, needless to say, lead to imbalances in water and soil nutrient use.

This story was produced by The Star in partnership with Code for Africa’s iLAB data journalism programme, with support from Deutsche Welle Akademie.