The Eucalyptus Tree and its Effect on Water and Soil Health

The Eucalyptus Tree and its Effect on Water and Soil Health

Over the past few years, there has been a wide debate on the effect eucalyptus has on soils in agricultural areas and its effect on wetlands and water sources. It is alleged that eucalyptus drains wetlands and poisons soils making them unfavorable for any other farming activities and should therefore be kept out of farmlands. To this effect, a lot of individuals have encouraged the uprooting of already planted trees and advocated for further legislation to prevent and discourage the planting of the tree species. This paper seeks to examine the truth of this and the effect the tree specie has on the environment, soil health and how it influences the levels of water in sites it thrives at. We shall also examine the difference in water consumption on output between different tree species when compared to eucalyptus.

Eucalyptus in Kenya
Eucalyptus is the most widely cultivated forest tree in the world. It is a large genus of shrubs and trees with over 800 species. It is native to Australia but is farmed across the whole world. In Kenya, the tree was introduced in 1902 from Ethiopia to provide fuelwood for the Kenya-Uganda railway. It has been popular since then due to its fast growth rate, high value and demand in the timber and fuel markets, and its ability to adapt to different environments. The Kenya Forest Service (KFS) estimates that various species of eucalyptus contribute about sh. 1.6bn to the Kenyan economy each year through its various products. There is about 100,000 hectares of the tree being farmed countrywide in gazetted forests, plantation farms and small scale farmers. It is not only an important commercial tree but also a windbreaker, an aesthetic tree, and useful in carbon sequestration and climate change mitigation.

The Politics of Eucalyptus in Kenya
However, there has been so much legislation on the cultivation of the tree specie despite its popularity with tree farmers. According to the Kenya Agriculture Act, “No landowner or occupier shall grow or maintain any eucalyptus species in wetlands and riparian areas” in the farm forestry rules. The Water Act discourages “the planting of exotic species that may have adverse effects on the water resource.” Numerous forest management and planning guides also discourage the growing of the tree in wetlands, around water sources and water bodies, and irrigated lands. In the 80s, the now defunct Permanent Presidential Commission on Soil Conservation and Afforestation (PPCSA) discouraged Eucalyptus farming, a move that resulted in reduced planting in state forests. In 2009, Hon. John Michuki, the then Minister of Environment, called for the uprooting of the tree in wetlands and called it a water guzzler. Recently, during the commemoration of the International Day of Forests at Nyanturago Wetland in Kisii, the County Forest Conservator, Mr. Wellington Ndaka revealed plans by the county government in partnership with the KFS to systematically uproot the eucalyptus trees within river lines and water shades in an effort to protect water sources from the high water consuming trees. Kisii County has not been the only one to encourage its farmers and foresters to uproot exotic trees. Numerous government officials and citizens have thrown their weight behind legislations that shall ban the planting of the tree in all riparian areas in favor of indigenous or fruit trees. There are reports that a single eucalyptus tree is capable of consuming up to 100 liters of water in a day. We sought to examine the truth of this and examined numerous research materials from different areas across the world. Some of the highlights can be followed below.

Eucalyptus and the Environment
There have not been wide studies on the effect eucalyptus has on the environment locally, but in other countries, plenty of research has been done.

Eucalyptus and its Effect on Water
According to a South African study on pine and eucalyptus, both species were found to dry a stream of river and after they were felled, the water flow returned to normal within five years. In an assessment over the effects of change in land use from grassland and indigenous forests to exotic trees of pine, eucalyptus and tea, there was no long-term reduction in water from the catchments, Silva et al. (2006).
Another study from India showed that when large scale eucalyptus plantations are introduced to a site, the water levels in wells declined until the trees reached their maximum rate of growth in six to eight years before the water levels reverted to their normal levels. Robertson (2005) observed that when the eucalyptus and other alien species were cleared from the river systems, the flow of the river was restored to normal within a decade, indicating that the ground water accumulates and springs up.
A similar observation was reported in the Mokobulaan region in South Africa when Scott and Welch (1996) were observing an afforestation scheme with Eucalyptus grandis and Pinus patula. After the trees were felled, the stream flow returned to normal within five years. We can easily observe from these that over a short term period, eucalyptus does consume a lot of water, but only for five to eight years or until they are mature. Afterwards, their consumption falls and water levels revert to their normal states.
When studying the water intake of eucalyptus in dry areas, scientists found out that the specie adopts different mechanisms to adjust to periods of drought; changes in leaf area were observed; the leaves were vertically arranged, there was high stomata sensitivity to air saturation deficit, deep rooting ability and osmotic manipulation to maintain turgidity in leaves (Kallarackal and Somen, 1997; Whitehead and Beadle, 2004). Eucalyptus is highly adaptable to different environments.

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