Eucalypts have been blamed for many evils, including the drying-up of water courses, adverse effects on nutrient cycling and soil properties, the suppression of other vegetation and erosion. These are serious accusation made by authors from many countries and they have even overshadowed the benefits of the eucalypts (Okia, 2009).
No single fact should be taken as sufficient evidence to promote or to discourage the planting of the eucalypts, though the results from a large number of studies taken together may yield valid generalizations (Jagger and Pender, 2000; FAO, 1988; Davidson, 1985). There is no question that trees in general and the eucalypts in particular utilize large amounts of water and nutrients, but the returns that can be realized in terms of biomass production per unit of input must also be considered.
Similarly, Davidson (1989) argues that the criticisms would equally apply to other exotic trees planted in many countries, not just the eucalypts. Therefore, any balanced argument should compare the nutrient depletion to the outputs produced for each unit of water and nutrient consumed than the absolute amount consumed in isolation. In the controversy over eucalypts there has been a tendency for the negative and positive aspects of the genus to be highlighted. Some of these are briefly discussed in the following subsections.
4.1. Water use by Eucalypts
Eucalypt has taken the vast area of the world and raising fears over water resources and eco
hydrological effects (Shi et al., 2012). Bewket and Sterk (2005) and Zerga (2015) indicated that among other types of land use changes, eucalypts and land degradation in the highlands of Ethiopia lead to the decrease stream flow more clearly in the drier season. Similarly, Chanie (2013) reported farmers’ response as eucalypts dries up springs in the highlands. Eucalypt is known by its high transpiration rate ranged from 0.5 to 6.0 mm per day and also believed that eucalypt plantations may extract water from shallow ground water (Shi et al., 2012). Soils dry up in the eucalypts up to 3m than other vegetation cover and it has indicated that vegetation changes affect evapotranspiration, water yield flooded area, water balance and other hydrologic variable (Nosetto et al., 2012). On the contrary, eucalypt is water efficient tree species than other crops though eucalypt is perceived as high water consumer by some environmentalist (Davidson, 1989; Teshome, 2007; FAO, 1988). Most eucalypt species averagely use 785 liter of water to produce 1kg of biomass (Davidson, 1989).
To the extreme, it has been thought that eucalypt species consume a lot of water more than any
other tree species and agricultural crops. However, this is far from reality. They have greater water use efficiency (i.e. they consume less water per unit of biomass produced) than most agricultural crops, conifers, acacias and broad-leaved tree species (Table 3.21). To produce one unit of woody matter young trees require between 300 and 500 units of water, but as they get older their efficiency decreases and more water per unit of wood is needed (Teketay, 2000). In fact, use of water is proportional to the amount of biomass produced (wood, branches, twigs, leaves, flowers, fruits, etc.), and the relatively high water consumption is consistent with their high growth rate and biomass production.
In arid regions, water is limited and plants with deep spreading roots take most water while
plants with shallow roots may be stunted or unable to survive. In low rainfall areas, eucalypt species may suppress other plants by competing for water, but this is unlikely to occur in areas of high rainfall (Davidson, 1989). If the planting is not well planned, it may reduce the groundwater level and thereby affect water supplies of the local people.
Davidson (1989) also reported that at Nekemte (Western Ethiopia) with annual rainfall of 2158 mm, E. saligna and E. grandis could produce 46.6 m3/ha/yr without drawing on water reserves (rainfall only) compared to 16.4, 16, 12.4 m3/ha/yr biomass production for the coniferous, acacia, and broadleaf species, respectively. These figures reveal that for the same amount of water consumed, eucalypt produces higher amount of biomass, which is economically profitable and acceptable.
In swampy areas, the groundwater level is near or at the surface, and some species of eucalypts have been used to drain the water away by drawing it up through the roots. E. globulus is useful for this purpose. Mosquito breeding swampy areas can sometimes be controlled in this way. Drainage removes swamps which provide a habitat for mosquito larvae, thereby reducing the risk of malaria. This method has been used in various parts of Ethiopia (Teketay, 2000).
Many studies reported that water use in eucalypt is comparable to other tree species. There are some cases where afforestation with eucalypts (or other tree species) has lead to reduced water run-off and supply of streams or changes in water table levels, especially in regions with limited rainfall. However, in many well documented cases eucalypt plantations do not have any significant negative impacts on hydrology. A key finding of many experiments revealed that eucalypt is highly effective in regulating its water consumption relative to available supplies and regulates its growth accordingly. Based on numerous comparisons that have been made between the potential hydrological impacts of eucalypt and other tree species, it is not expected that the eucalypt trials planted under these permits would be any more impactful on local hydrology than planting other fast growing trees species.
Study conducted in the Ethiopian highlands, Pohjonen and Pukkala (1990) reported that E. globulus converted energy and available water into biomass more efficiently when compared with exotic coniferous tree species. Therefore, although some species of eucalypts may consume more water than the indigenous forest or other plantations, which may lead to reduced water yield and leaves less available water for other crops growing in association with the tree, it is more efficient in terms of converting water into biomass.
Table 3.21
Water use efficiency or consumption of water per unit of biomass produced.
Plant | Liters of water/kg of biomass produced |
Sorghum | 250 |
Maize | 250 |
Caw pea | 500 |
Soybean | 500 |
Eucalyptus (tree) | 510 |
Albizia lebbek (tree) | 580 |
Potato | 600 |
Sunflower | 600 |
Field pea | 600 |
Horse bean | 600 |
Pddy rice | 600 |
Syzygium cuminii (tree) | 610 |
Cotto/coffee/banana | 800 |
Acacia auriculiformis (tree) | 860 |
Dalbergia sisso (tree) | 890 |
Conifers | 1,000 |
Pongamia pinnata (tree) | 1,300 |
Source: (FAO, 1988; Davidson, 1989) |
4.2. Effects on soil fertility
The effects of the eucalypts on soils have been studied in several countries over many years (Kindu etal., 2006, Poore & Fries, 1985). Most of the concerns related to effects on soil quality deal with the depletion of nutrients. Moreover, a number of studies indicated that changes in some soil properties are influenced by tree species (Poore & Fries, 1985).
Studies by Dessie and Erkossa (2011), Kidanu (2004), Chanie (2013) argued that eucalypts decreases soil nutrients within 20m distance from the trees. The comparison study of eucalypts with mixed plantation has revealed that eucalypt has three times more fine root biomass in surface soil which indicated that planting crops in association and adjacent to eucalypts should be avoided (Gindaba, 2003). However, eucalypt species exceptionally can extend the nutrient cycling deep to ground soil where other trees and crop could not access that much depth (Hailu et al., 2003).
The wetland conversion study has indicated that there is significant difference between wetlands and converted land to dominantly eucalypts by reducing major nutrients from the converted land which is eucalypts (Mekonnen and Aticho, 2011). Similarly, Soil nutrient and carbon pool under eucalypts is lower than the mixed plantation (Gindaba, 2003). In addition to the above, as reported by Chanie (2013), the soil under eucalypts becomes water repellent and the perceptions of the local farmers agreed with the experimental findings by reducing the crop productivity of the land. On contrast to the above, Tadele and Teketay (2014) has found that the maize dry matter production and grain yield planted on cleared felled eucalypts stand are significantly higher than adjacent field. According to Hailu et al. (2003) eucalypt does not over exploit the soil than the traditional fuel usage such as litter and cow dung collection. Similarly, the study has indicated that due to non browsed characteristics of eucalypts than other fodder tree, it is well fitted for soil protection purposes if it is incorporated with avoidance of liter and bark collection in overgrazing practiced places. Generally, there is lack of clear scientific evidence that shows eucalypts’ impact on soil nutrient that lead to soil degradation.
Although it is blamed to extracts substantial amount of nutrients and compete with crops, at the same time eucalypts impoverish the soil. It could be legitimate to raise such concern under poor management where there is lack of species-site match (Davidson, 1989; FAO, 1988; Nigatu and Michelsen, 1993 and 1994).
Eucalypt copes with such variability through a root system that has intimate contact with the large volume of soil. With the extension of its root deep into the soil, given its high degree of adaptability, it extracts nutrients outside the realm of crops feeding zone. That is why its nutrient requirement is significantly lower than that of many agricultural crops (Moges, 1998, 2010). As a result, the species flourishes with sustainable high yield without fertilizer on red ash and degraded land. Further, eucalypt is not a natural forest that has little disturbance. If it were a closed system, nutrients would have been recycled from decomposing litter back to the tree and increase the nutrient bank (Teshome, 2009). But, eucalypt is an open system and nutrients are removed from the site when the stem, leaves and bark are harvested for various uses (Nigatu and Michelsen, 1993 and 1994). This means that the nutrient capital of the soil could be diminished. Therefore, the secret lies in nutrient mining. This is equally true for crops under poor management. Under viable environment, soil nutrient levels can be improved through sound management without the carrying capacity of land being overstretched.
Very few comparative studies have been made in Ethiopia on soil nutrients among plantations of different species including eucalypt species and the adjacent natural forests (Michelsen et al., 1993; Nigatu and Michelsen, 1994; Michelsen et al., 1996; Alemu, 1998). These studies have shown that plantation stands of fast growing exotics such as E. globulus, E. grandis, E. saligna, Cupressus lusitanica and P. patula had lower nutrient contents than soils of the adjacent natural forest. This seems logical as they are fast growing, thereby drain, and consume more nutrients from the soil. Eucalypt species have high demand for nutrients, but this incomparable with other tree species and much lower than agricultural crops. Teshome (2009) pointed out that the nutrient consumption of fast growing species like eucalypt species need to be well studied before wrong conclusion and recommendation is being made.
4.3. Issues on soil erosion
Soil erosion is among the most important surface processes that result in land degradation in the tropics. Trees can influence soil erosion mainly through intercepting rainfall which dissipates its kinetic energy. The rain drops that are intercepted eventually fall to the soil surface with reduced erosive energy, depending on the size and orientation of the leaves. Large leaves produce larger size droplets which have greater impact on the soil. Accordingly, erosive energy of rain under the crowns would be least for Casuarina spp. with Acacia spp. (e.g. A. auriculiformis) and narrow-leaved eucalypts (e.g. E. camaldulensis) occupying the mid-range and the broad-leaved eucalypts (e.g. E. globulus) at the top of the range for the eucalypts (Jagger and Pender, 2000).
Consequently Jagger and Pender (2000) reported that there is no evidence to single out the eucalypts for special criticism with regard to soil erosion. It has been hypothesized; however, that-long term exposure to allelo-chemicals may result in increased risk off soil erosion, which may have implications for sustainable land use over time (Jagger and Pender, 2003).
Eucalypt has been found to have impacts on topsoil retention and soil erosion (Dessie and Erkossa, 2011; Sunder, 1993; Poore and Fries, 1985). Some studies have concluded that eucalypt can worsen soil erosion as an indirect result of frequent disturbance from repeated harvesting (Poore and Fries, 1985). Others argue that eucalypt plantations can help control soil erosion on sloped or degraded sites, but their efficacy depends on environmental factors such as intensity of rainfall, soil condition, slope and the presence of ground vegetation and litter cover. Though few Ethiopia-specific case studies exist, the limited evidence available suggests that eucalypt may be an ineffective choice for erosion control (Sunder, 1993). Rather, eucalypt trees are generally expected to lead to an increase in soil loss due to the reduced understory cover in densely planted eucalypt areas (Poore and Fries, 1985).
The litter which accumulates under most eucalypt plantations can help to form a protective
barrier against erosion, but in many places the litter is collected as fuel or removed to reduce fire hazard. For instance, the depth of the accumulated litter under eucalypt stands in Shashemene Munessa Forest Project area was found to be on average 20–30 cm (Teshome, 2009). However, under eucalypt stands around Addis Ababa and very big towns, the accumulation of litter is very low as a result of human and livestock disturbances. People take away most of the litter and cattle and foot traffic compact the soil. If the litter is left on the site uncollected, it would have been incorporated into the soil system to slow down runoff and improve infiltration, and substantial amount of nutrients may pass to the soil system, thereby improving soil fertility (Teshome, 2009). However, as a result of litter collection, the ground under the trees is left bare and the soil is exposed to erosion. Therefore, litter should be allowed to accumulate on the sites, particularly on sites that are easily eroded.
A realistic assessment of each area to be planted is needed to decide whether erosion will be a
serious problem, and if so, whether it can be controlled. Some places may not be suitable for
plantation establishment. Eucalypt plantations on steep slopes can provide effective erosion control if careful techniques such as contour planting are used (Teketay, 2000). The root systems of selected species for catchment protection influence the soil binding capacity and as a result reduce erosion. E. globulus, for instance, has a strong tap root and good lateral root system that makes it very reputable species for catchment protection (Teketay, 2000; Teshome, 2009).
In regard to soil erosion by water under trees, there is no evidence to single out eucalypts for special criticism. Erosive resistance (physical characteristics) of soils is more important than crop management and crop management is more important than the type of tree crop. Since, in nearly every example where the litter is removed, erosion increases substantially, it is important to focus more on ground cover and ground level activities (cultivation, compaction by foot traffic, livestock grazing, trampling, and harvesting/logging damage rather than on the species of trees planted. On erosion-prone slopes it is better to use a periodic, partial harvesting system based on cutting of trees on lines around the contour or on removal of small patches in a mosaic pattern (Teketay, 2000; Jagger and Pender, 2003 ).
4.4. Allelopathic effect
Allelopathy is the release of chemicals from leaves or litter that inhibit the germination or growth of other plant species (FAO, 1985), and consequently reduce the output of crops. Allelopathic effect of eucalypt is among the issues dominating the agroforestry literature. Allelopathic exudates from eucalypt tree components have shown an inhibiting effect on undergrowth vegetation regeneration and growth (Poore & Fries, 1985).
Most of the studies put forward as "evidence" for eucalypts being strongly allelopathic involve laboratory studies of extracts on germination of seeds or early growth of potted plants which may not accurately represent field conditions. Soil bioassay studies have been carried out with three agricultural crops: chickpea (Cicer arietinum), tef (Eragrostis tef) and durum wheat (Triticumturgidum) under laboratory and field conditions in the Ethiopian highlands. According to the findings, bioactive compounds from the decomposing litter of E. globulus did not affect the test crop seed germination nor root growth. However, a litter extract with 5% dry matter concentration significantly hindered germination and root growth of the tested agricultural crops. On a farm field experiment, declining barley yield was observed near a E. globulus plantation (Jagger and Pender, 2005).
Results evidently vary across a wide spectrum of conditions from humid, fertile sites to dry, infertile ones. The magnitude of the negative effects may be influenced by rainfall. Although it is likely that allelo-chemicals do accumulate in the soil, they are highly soluble and rainfall is likely to leach them out and the effects of Allelopathy are thus likely negatively correlated with rainfall. It has been noted that allelopathic effects are more severe in low rainfall regions prone to soil erosion than in drier regions. However, the hampering effect on growth of understory or adjacent intercropped crops may more often be the result of strong competition for water and nutrients than Allelopathy. Farmers in the highlands of Ethiopia linked this effect to competition for water and nutrients (Jagger and Pender, 2005).
The potential allelopathic effect of E.camaldulensis, Cupressus lusitanica, E. globulus, and E. saligna on seed germination and seedling growth was investigated with four crops: chickpea, maize, pea and teff (Eragrostis teff Zucc.) (Limenih and Michelsen, 1993). The results revealed that aqueous leaf extracts of all the tree species significantly reduced both germination and radical growth of the majority of the crops. It has been shown that the shoot and root dry weight increase of the crops was significantly reduced after ten weeks treatment with leaf extracts.
Allelo-chemicals can effect germination and growth of plants through interference in cell division, energy metabolism, nutrient uptake etc. (Nigatu and Michelsen. 1993; Fetene and Habtemariam, 1995; Teshome, 2009; Yirdaw and Luukkanen, 2003). In this regard, eucalypt has toxic allelo-chemicals that consist of phenolic acids, tannins, and flavonoids (Yirdaw and Luukkanen, 2003). When realized into the soil, these inhibit other plants and play a role in shaping plant communities. For instance, leaf decomposition product from eucalypt is shown to suppressed germination and growth of chickpea, field pea, maize, and tef (Nigatu and Michelsen, 1993) while it exerted an antibiotic effect on soil microorganisms (Limenih and Bongers, 2010).
However, concentration matters. For instance, allelo-chemicals from decomposed eucalypts’ litter in high rainfall areas did not accumulate in sufficient concentration to affect seed germination and root growth of crops. Different strength of water extract from leaves of eucalypts did not delay the onset of germination and seedling growth of Olea (Yirdaw and Luukkanen, 2003; Limenih and Bongers, 2010; Limenih and Teketay, 2004). In fact, positive results have also been reported concerning the interaction of eucalypts with other plants (Kidanu et al., 2005). The lack of susceptibility of certain crops and the regeneration of other species suggest that eucalypts provide some benefit rather harm. Again, it is not only eucalypts but other exotic tree species such as Grevillea robusta which is a common feature in Ethiopia has allelopathic effects on most agricultural crops (Teshome, 2009).
However, in Ethiopia, little attention has been given to Allelopathy as determinant of crop production and productivity (Kidanu et al., 2004 and 2005; Chanie, 2009) and plant community structure (Senbeta, 1998; Moges, 1998; Yirdaw, 2002; Limenih, 2004). Therefore, empirical information is needed to resolve such negative effect. Until then, Allelopathy can be minimized with sound management through compatible crops based on proper eucalypt species’ site selection.
4.5. Competition with nearby vegetation
One of the criticisms associated with eucalyptus is that it prohibits the establishment of understory plant species. Eucalypt is usually taller than other plants of equal age. This has determined the amount of gap that would be available for sunlight to penetrate through its canopy. When planted at high density, the shade created has adverse influences on the understory environment (Zerga, 2015). The consequence could be vegetation free surface. The dense stands not only affect the growth of colonizing woody species, but also nearby crops given the added competition for water and nutrients. Then, yields from crops close to eucalypts may not be as good as those farther from the edge. Then, it is not eucalypts rather the lack of sound management that is to blame (Teketay, 2000). On the other hand, not all eucalypt species cast heavy shadow to discourage understory plants. Some even cast less shade than broad-leaved trees because they have often narrow, patchy crowns and leaves positioned downwards on the twigs (Yirdaw and Luukkanen. 2003).
As a case in point, several eucalypt plantations in different agro-ecological zones showed richness of herbaceous plant species than under adjacent natural forest. Further, the less dense plantations harbored more regenerated indigenous woody tree species than high dense eucalypts stands (Yirdaw, 2001; Senbeta and Teketay, 2001; Michelsen et al., 1996; Limenih and Teketay, 2004). This indicates that an inverse relationship exists between eucalypts density and diversity of the regenerated species. In relation to economic crop, wheat production was not affected by eucalypts on heavy clay soil (Kidanu, et al., 2005). When used as shade tree for coffee, its cup quality was acceptable as that within the indigenous forest (Shiferaw and Tadesse, 2010).
Because of shading and competition for water, the yields from agricultural crops close to eucalypt plantations are sometimes not as good as they are further away from the edge (Teketay, 2000). However, some eucalypt species cast less shade than other broadleaved trees because their leaves are held vertically downwards on the twigs and their crowns are often narrow. It is widely accepted that shelterbelts increase crop yields. On the other hand, the study made by Onyewotu et al. (1994) on the competitive effects between a E.camaldulensis shelterbelt and an adjacent millet (Pennisetum typhoides) crop indicated that the yield of the crop grown very close to the belt was reduced because of competition with the trees for light, soil moisture and nutrients, and shading. On the other hand, the results indicated that the yield of millet grown adjacent to the shelterbelt increased substantially. It is worthwhile to note that in eucalypt species the shade is characteristically patchy because the leaves usually hang downwards, indicating that shading is not a major problem.
However, not all eucalypts’ cast shade which is heavy enough to discourage ground vegetation or understory shrubs, and shading can be adjusted by varying the density of the trees. There are complex interactions between light and water requirements of different trees that make generalizations difficult. Certainly there are several species of trees with larger leaves than the eucalypts, thus they cast more shade.
4.6. Scarcity of wild animals
Ethiopia has diverse wildlife of world importance. Yet, there has been the erosion of these resources due to the destruction of their habitat from introduction of agriculture, recurrent drought, war and conflict. Among others, the unpalatability of eucalypt leaves is supposed to reduce the number of wildlife and livestock in an area. Due to its competitive nature, eucalypt is considered not to provide adequate fodder to wildlife (FAO, 1988; Negash, 1999). There is thus the debate should be on whether wildlife would remain in their newly established eucalypts habitat or not.
In the first place, eucalypt is never established in natural forests that harbor wildlife. If there were some in the landscape, the destruction of the habitat might have forced them to migrates; obviously prior to eucalypts. Then, it is difficult to imagine that degraded area, barren and treeless landscape could have provided the suitable habitat to wildlife.
Instead, they would migrate due to deforestation of the original forest. The subjective perception of the species is in the absence of objective studies which demonstrate that eucalypt plantations host lower wildlife population compared to a barren landscape that is now rehabilitated with indigenous species under a similar setting. The objective reality, however, is that with the establishment of eucalypts, the canopy has provided shade for the emergence of undergrowth vegetation and the regeneration of indigenous trees. Now that they have a suitable habitat, some of the wildlife has returned. As a case in point the scenic evergreen eucalypts plantation on Entoto Mountain that surrounds Addis Ababa hosts diverse wildlife even with large human population around and has become a prime destination to tourists (Senbeta and Teketay, 2001; Senbeta et al., 2002).
Further, the flower of eucalypts that produces abundant pollen and nectar has been essential in the life cycles of many insects and birds. These are important in the pollination of crops and bees provide additional benefit through production of honey. This has become a lucrative business to many rural communities. Under sound management similar plantations could exploit such potential without adverse effect on the ecology or crops. Palatable leguminous trees, shrubs, forages, pastures, and grasses can also be established under appropriate sound management. Then, such rehabilitated areas can be made favorable to wildlife instead of the categorical blame of eucalypts as restriction to their proliferation (Davidson, 1989 and 1995; FAO, 1988; Pohjonen and Pukkala, 1988 and 1990).
The unpalatability of most species to browsing and grazing animals (FAO, 1988; Davidson, 1989; Pohjonen, 1989; Turnbull, 1999) and incapability of providing adequate food and habitat for wildlife (FAO 1988; Negash, 1999; Teketay, 2000) can thus reduce wildlife in an area. However, this problem can be alleviated by establishing mosaics of plantations, natural forests, pastures, grasslands and croplands. It is important to note that the biodiversity of a natural forest and that of eucalypt plantations are not comparable. The natural ecosystems are very diverse, whilst the biodiversity of eucalypt plantations is limited. Mammals and birds that used to live in natural forests can be encouraged to return to a forest replanted with a mixture of exotic species by leaving open spaces occasionally, and allowing undergrowth to remain here and there.
4.7. Climate change effect
One of the criticisms against eucalypt plantations is that they may cause a change in the local
climate. This is because of their very high evapotranspiration rate, which may lead to a lower water table. This high rate of soil water loss is claimed adversely to affect local rainfall levels, resulting in possible desertification of a given area. But others argue that the contribution of the land mass to the hydrologic cycle is small compared with that of the oceans (Lee, 1980). On the other hand, the mere presence of a forest in a certain area does not necessarily affect the occurrence of rainfall in that area.
Some studies have shown that the microclimate within eucalypt plantations may be different from those of other species and from native forests, but the data are not conclusive. In terms of their effects on regional rainfall or on other regional climatic parameters, however, there is nothing to distinguish eucalypts from plantations of any other tree or from different types of native forests of similar structure and albedo (Poore and Fries, 1985).
Generally, the greater the leaf area and the more horizontal the leaves are, the greater the shading effect and the higher the evapotranspiration rate. Eucalypts cast less shade, on average, than other broadleaved trees, but there are big differences in the amount of shade cast by different species because they have different leaf sizes and orientations. The influences which are the result of shading can be manipulated based on need, by increasing or reducing the density of planting. Therefore, there is no reason to distinguish eucalypts from other genera with similar crown architecture regarding micro-climate at local level.
Eucalypt trees do an excellent job of sequestering CO2 because they efficiently store carbon in all their live biomass. Especially in the tropics, where sun and rain are abundant, extremely high conversion of CO2 occurs in the biomass. With such high efficiency, it is only natural that eucalypt trees grow quickly and with high productivity. Eucalypt is an efficient biomass producer; it can produce more biomass than many other tree species. It is known that carbon sequestration is proportional to biomass production. Therefore, Eucalypt plantations can play an important role in generating additional revenues from carbon trade.
4.8. Effects on plant species diversity
One of the major criticisms that have been debated over years among the scholars concerning eucalypt is its impact on flora of the area. On contrary to the above, eucalypt has potential in encouraging the recruitment, establishment and successions of native species, which promote biodiversity improvement (Senbeta et al, 2002). Regeneration of Junipersprocera on previously eucalypts plantation was also possible at Entoto, Ethiopia, which has shown wrong believes that planting other trees after harvesting eucalypts is not possible (Woldu, 1998; Senbeta and Teketay, 2001; Senbeta et al., 2002).
Mostly, eucalypt is feared for ecological hazardous and ecosystem destructor, but eucalypt is an important tree in fostering ecosystem serving as conservation tree at beginning of restoration process of degraded sites. Eucalypt plantations in southern Ethiopia were used as a buffer for degrading natural forest and have reduced impact on the forest (Senbeta et al., 2002, Zerga and Woldetsdik, 2016). Thus, eucalypt plantation has reduced natural forest deforestation in one way and reduced human impact on the other hand that provide time for natural forest regeneration and allow improvement of biodiversity richness.
Additionally, eucalypt plantations can be used to foster natural forest re-colonization and succession process (Senbeta et al., 2002; Lemenih et al,. 2004). Similarly, it can also facilitate the regeneration of native woody species in the plantation through reducing soil erosion and facilitating attractive conditions for seed germination (Lemenih et al., 2004). In addition, eucalypt plantation can also foster the regeneration of other native woody species in degraded area by providing protection (Teketay, 2000; Hailu et al., 2003; Senbeta and Teketay, 2001).
Plantation stands of eucalypts and other tree species have been shown to foster or catalyze
the regeneration of native woody species under their canopy provided that they are established
close to seed sources and protected from human and livestock disturbances, thereby enhancing
biodiversity (Senbeta, 1998; Moges, 1998; Teketay, 2000).
The studies made by Mihretu (1992) and Kidane (1998) indicated that there is a good natural regeneration of J. procera under E. globulus plantations at Entoto hills in Addis Ababa. The juniper has grown effectively on the eroded areas competing well with the eucalypt trees Similarly, the studies made by Senbeta (1998) and Moges (1998) at Shashemene Munessa Forest Project Area clearly showed that plantation stands of E. globulus, E.saligna, C. lusitanica and P. patula have been found to foster the natural regeneration of several native woody species like P. falcatus, Prunus africana, Syzygium guineense and Crotonmacrostachyus. The source of seeds for the naturally regenerated native woody species is the adjacent to natural forest.
Many of the allegations on species of eucalypts and other exotics, e.g. the allegation of hampering native biodiversity, are unscientific and disproved by many recent research findings (Moges, 1998; Senbeta and Teketay, 2001; Yirdaw 2002; Senbeta et al. 2002; Limenih, et al. 2004; Limenih and Teketay, 2005). Thus, there is now increasing knowledge of the ecological pros and cons of eucalypts better than before and most importantly the biodiversity impacts are clearly disproved. Rather eucalypts are considered as a foster species that nurse the rapid re-colonization of native species when planted on barren and degraded lands.
4.9. Policy Options
As Janz and Persoon (2002) expressed, there are serious shortcomings in the supply and use of information needed for policy-making in the forestry sectors, particularly those of developing countries. It should be underlined that, for a successful forest policy process, it is often necessary to know, among several other things, more about plantations and their role for rural communities. There is a general prejudice against forestry, particularly against fast-growing trees plantation, as compared to agriculture (Cossalter and Pye-Smith, 2003).
The current policy issue regarding eucalyptus planting in Ethiopia seems not in the favor of the species. There is no encouraging concern in the country to raise eucalyptus seedlings from government nurseries and distribute them for smallholder farmers. The views of policy makers are largely less favorable to eucalyptus. Owning to its importance, the policy practice of discouraging and in some cases banning planting of eucalyptus by farmers need rethinking from the side of the policy makers. Taking into account the dwindling natural forests in Ethiopia, it would be necessary to pass legislation to require that almost all charcoal, poles and firewood be derived from plantations of fast-growing species such as eucalyptus, to prevent further loss of natural forests (Turnbull, 1999).
There is a need for care when comparing policy and actual practice, because stated intentions in policy documents sometimes bear no relation with how policies are interpreted and applied (Sithole, 2002). In another concern, interventions to support market prices for the products of tree growing and to ensure producers’ access to markets may be as effective as or more effective than subsidies. Agricultural policies should be complementary to tree growing. Subsidies for credit, price supports and incentives, including measures affecting land and tree tenure, should be seen in parallel to both agricultural crops and tree growing like eucalypts by farmers in order to avoid policy measures that likely to distort decisions against one at the expense of the other (FAO, 1981).
Actually, the poor management should be blamed rather than eucalypts. All things are always changing; and our way of thinking should change with the rapidly changing conditions of our world. At the face of heightened climate risks, the harmonization of forestry and agriculture policy unquestionably helps combat food insecurity and poverty. In reference to eucalypts, as elaborated, the evidence so far reveals that the species will continue to figure prominently in the life of both rural and urban people (Moges, 2010). Hence, its justifiable place in the development policy of the country means that individuals and communities can be encourage to accelerate the establishment of commercial exotic plantation and indigenous forest that will not contravene the ecology.