ANALYSIS: Thirsty species? The science behind the eucalyptus tree ban in Kenya’s wetlands
As the world marked Desertification and Drought Day on 17 June 2020, Africa Check revisited a persistent question about the impact of the eucalyptus tree on wetlands.
A reader contacted us through WhatsApp asking whether it was true that the tree “drinks water in wetlands and marshy areas like a drunkard drinks alcohol”.
The reader had posed the same question in 2019 through Facebook: “This thing about the eucalyptus tree being bad for wetlands and springs … is it fact or myth? What does the science say?” We promised to check it out.
Kenyan law prohibits eucalyptus in wetlands and near water sources
The law in Kenya prohibits the growing of eucalyptus species in wetlands and riparian areas.
“No agricultural landowner or occupier shall grow or maintain any Eucalyptus species in wetlands and riparian areas,” read the farm forestry rules in the Agriculture Act.
|What is a wetland or riparian area?
Kenyan law defines a wetland as “an area where plants and animals have become adapted to temporary or permanent flooding by saline, brackish or fresh water”.
Riparian land means any land which is between six and 30 metres from the edge of a river bank; a horizontal distance of 30 metres from the highest recorded water level in case of an ocean, lake, reservoir or stagnant water body; or a radius of between three and 15 metres from the source of a spring.
The Water Act also forbids “the planting of exotic species that may have adverse effects on the water resource”.
The Kenya Forests Service, the government agency in charge of forest management in Kenya, also has a guide to eucalyptus farming which prohibits the growing of the genus in wetlands, marshy areas, and on irrigated farmland, including near lakes, ponds, or any other body of standing water.
In February 2020, Kenya’s environment minister Keriako Tobiko called for eucalyptus trees in wetlands to be uprooted.
How accurate is the claim about the “water-guzzling” capabilities of eucalyptuses?
Forestry research institute reports conflicting study results
The Kenya Forestry Research Institute is the government agency tasked with forestry research. The agency’s booklet titled Facts on Growing and Use of Eucalyptus in Kenya said that “few studies have been carried out in Kenya on water use by eucalyptus”.
The institute cited other studies where “results showed that in high rainfall areas replacement of indigenous bamboo forest in water catchment with plantations of eucalypts, pine and tea did not result in any long-term reduction in water from the catchments”.
But it also pointed to results from South Africa where both eucalyptus and pine species “dried the stream after some years” and after they were felled, “the stream flow returned to normal within a period of five years”.
The institute also quoted a 1993 study from India which concluded that a “eucalyptus hybrid consumed more water on [a] per litre basis, but produced more biomass per unit volume of water”. Biomass in trees refers to the foliage, branches, stem and bark.
But another study from India cited in the booklet showed that when “large-scale” eucalyptus plantations were introduced “the level of water in wells declined until the plantations were six to eight years (when trees have their maximum rate of growth) and thereafter, reverted to the earlier level”.
So how bad is the eucalyptus for riparian land?
Eucalyptus highly efficient biomass accumulators – forestry research institute
Dr Jackson Mulatya is the deputy director of corporate affairs and quality assurance and the chief scientist at the forestry research institute.
“These claims have not been conclusively supported by scientific evidence,” Mulatya told Africa Check.
“However, studies have established that eucalyptus exhibit high efficiency in water use for biomass accumulation. For example, it has been established that eucalyptus requires less water to produce one kilogram of biomass than most crops.”
He pointed Africa Check to a 2007 paper which investigated eucalyptus in Tanzania, specifically mentioning “various claims that their presence on the landscape is causing the drying up of water sources, rivers and springs”.
“Eucalyptus species, like most other plants, adjust their water uptake to the available soil water, reducing their water uptake as the soil dries, and controlling water loss by regulating stomata opening. Thus, a high water table would allow a fast depletion, but as the water level falls water uptake decreases concurrently,” says the paper.
Stomata are tiny openings on leaves through which plants transpire.
Eucalyptus regulate own water consumption
The fact sheet linked to a doctoral dissertation titled Eucalyptus in Kenya: Impacts on Environment and Society. It further linked to guides by the two government agencies on planting eucalyptus.
The fast-growing eucalyptus species introduced in Kenya from Australia “requires a lot of water” to support quick growth, Garrett-Kluthe told Africa Check in an email.
“One unique thing about eucalyptus is their ability to regulate water consumption. If water is scarce, they are a lot more conservative with it but when water is abundant they will leave their stomata open, allowing for a lot of water loss through transpiration. Essentially the water flows through the tree and then evaporates into the air,” she said.
“For this reason, the recommendation is that eucalyptus woodlots should be planted at least 50 metres from water sources.”
Garrett-Kluthe, who continues to study the eucalyptus tree genus, noted that as an introduced species it was inevitable for the eucalyptus to pose some environmental challenges.
“Eucalyptus uses a lot of water and my research has shown that soil moisture levels are significantly lower in eucalyptus woodlots as compared with indigenous forests,” she said.
Eucalyptuses’ ‘thirst’ also used deliberately
This view is also echoed in a paper presented at the World Forestry Congress in 2003, hosted by the United Nations’ Food and Agriculture Organization. The paper notes that “eucalyptus has the inherent capacity for luxury consumption of water when moisture is abundantly available”.
“The unscientific myth about Eucalyptus species that it dries up the sub-soil moisture rapidly proves to be categorically wrong,” the paper concluded.
The Forestry Society of Kenya fact sheet also led us to a publication by Kilimo Trust, an East African agricultural non-profit headquartered in Uganda. It looked at Eucalyptus hybrid clones in East Africa and traced the “reputation for high water use” in eucalyptus to it often being “planted in wet and water-logged areas to dry them out”.
“An example is the planting of Eucalyptus robusta to help drain wetlands in Uganda,” the publication noted.
“But whatever the source of the criticism, the amount of water used to grow eucalyptus will depend on the variety, the climate (rainfall and solar radiation), soil type and depth, vegetative cover, tree growth stage, wood density, and tree rooting depth.”
Conclusion: Beware of eucalyptuses’ luxury water consumption
Therefore, the short answer is yes, but it is not that simple. Eucalyptus trees do use more water than many other species, but they are more efficient in how they use the water to convert into biomass.
Also, the more water available, the more these species “drink”, and the less water, the less they take in.
The science is not conclusive, but scientists warn that given the water consumption of eucalyptus, it is better to plant them away from wetlands and riparian lands in order to preserve those ecosystems. Kenyan laws and government policies enforce this view.
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