Reference article: A Guide to On-Farm Eucalyptus Growing in Kenya
Eucalyptus and water use
There is a lot of misconceptions concerning the environmental impacts of eucalyptus spp (blue gum or the gum tree). It is often claimed that the presence of these species of trees on a landscape leads to the drying up of rivers, springs, and other water sources. The reality, however, is that numerous scientific research exists on eucalyptus’ effects on the environment with contradicting findings.
In some studies (such as Munishi 2007), it is reported that eucalyptus requires less amount of water, compared to most tree species, to yield one kilogram of biomass. For example, while a typical eucalyptus tree will consume about 785L of water to produce 1kg of biomass, coffee, cotton, and bananas each require 3,200L whereas sorghum, potatoes, and maize require 1,000L for the same. Table 2 on page 11 demonstrates that eucalypts do not consume more water than other native forest tree species.
The Kenya Forest Service (KFS) has indicated that the effects of eucalyptus spp on the water budget is largely dependent on several factors such as soil and climate conditions, the nature of rock substratum, tree density, vegetative cover, amount of rainfall, tree growth stage, slope, and the type of eucalyptus species in question. Nevertheless, it is generally agreed that the growing of eucalyptus in dry areas may lead to adverse environmental effects because of eucalyptus’ high competition for water.
Eucalyptus and soil fertility
It is also argued that eucalyptus spp adversely affects soil fertility, especially in monoculture. Consistent with this claim, research shows that eucalypts can deplete soil nutrients rapidly when grown as a short rotation crop for high biomass production and removal.
However, compared to areas under crops like tea, eucalyptus farmlands tend to exhibit a higher level of micronutrients. In fact, it is reported that the long-term growth of eucalyptus enhances soil fertility and even restores the fertility of degraded soils in arid and semi-arid areas. The litterfall of eucalyptus also has net positive contributions on the fertility of degraded wastelands and hillsides where the tree exhibits good potential for topsoil retention.
The allelopathic effects of Eucalyptus spp
One of the most important findings on eucalyptus spp is that it exudates allelopathic chemicals. These chemicals are known to hinder the regeneration of undergrowth. Because of this, eucalyptus can influence agricultural production. However, allelopathic effects of eucalyptus on other crops is reversed by high rainfall.
Eucalyptus and bio-diversity conservation
Another claim is that eucalyptus plantations have limited biodiversity. While native plant advocates, however, are yet to provide scientific evidence to support this view, eucalyptus cultivation saves biodiversity in many ways. For instance, eucalyptus replaces indigenous species for fuel woods, timber, and numerous other uses. Consequently, the growing of eucalyptus trees helps to prevent desertification and lower the destruction and degradation of natural forests. Furthermore, “Contrary to popular belief, many animals, both vertebrates and invertebrates, have adapted to life in the Eucalyptus groves,” Million Trees.