Paper Presented at TAF Annual General Meeting (AGM) 23rd
2007 Dodoma Tanzania
THE EUCALYPTUS CONTROVERSY IN TANZANIA
Department of Forest Biology
Faculty of Forestry and Nature Conservation
Sokoine University of Agriculture
PO Box 3010 Morogoro Tanzania
The genus Eucalyptus has over 600 species, with origin in Australia where they are found in a wide variety of ecological zones and occupy some of the most important catchment areas in Australia. Eucalyptus plants vary in size from small shrubs to giant trees. Many species of Eucalyptus grow quickly and produce large quantities of wood when grown in well managed plantations both within and outside their natural range. Most of the introduced Eucalyptus species in Tanzania and elsewhere are fast growing and were introduced to provide various products such as fuel wood, pulp and paper, sawn timber, essential oils e.g. for medicine and perfumes, and services including reclamation of degraded lands, saline areas and drainage of water-logged areas. Eucalyptus planting in many parts of the world where they have been introduced has shown very promising results in producing wood for various purposes to suit local conditions. On the other hand there are recent controversies and critiques from adverse public reactions against planting of Eucalyptus though in some cases the arguments are not based on technical, ecological and socio economic arguments. The major argument has been that Eucalyptus removes too much water from underground reserves and streams, inhibit the growth of other vegetation and its leaf litter has adverse effects on soil humus among others. This paper highlights some of the controversies around Eucalyptus putting forward the positive and negative effects of Eucalyptus planting. It is argued that the controversies surrounding Eucalyptus effect on soil water budget and the benefits accruing from Eucalyptus are complex and no need to take sides here. The ecological effects of the species vary widely depending on sites (climate, soil, topography) and the benefits from Eucalyptus vary depending on the needs of respective communities. It should be noted that each site where Eucalyptus may be planted has its own peculiarities and buffering mechanisms and the response to Eucalyptus growing by each site should be judged separately. This means that decisions as to Eucalyptus planting must be specific to each case and should be based on adequate assessment of physical, biological and society factors. The controversies surrounding eucalyptus planting in Tanzania especially as relates to water use and effects on water resources still requires a more intensive investigation before one can make a conclusive advice given the variety of eucalyptus species and the different ecological regions of the country. Generally, it seems that there are no profound reasons to discontinue or discourage eucalyptus planting in Tanzania.
More than 600 species of Eucalyptus are known and new ones are still being identified. Some of the giant trees grow up to 90 m though in scrub areas with low annual rainfall dwarf forms exist. When planted outside their natural habitat many species of Eucalyptus have shown a high degree of tolerance to extremes of latitude and altitude in contrast to many plants which cannot flourish when introduced to different latitudes or altitudes (FAO, 1988). This fact has made it possible for some major Eucalyptus species to be introduced and widely grown in plantations and small scale plantings in many countries allover the world. According to FAO (1988), Eucalyptus globulus occurs naturally between latitudes 38.50 S and 43.50 S and not above 1000 m altitude. However it successfully grows in Addis Ababa Ethiopia (120 N and 2,500 m altitude) and at 3400 m in Peru. In Tanzania it grows successfully at altitudes above 1600m and latitudes less than 100 S and is among the major species adopted for planting by most small-holder forest practitioners in Tanzania (Munishi et al., 2004, Munishi et al., 2007). Today there is a very lucrative trade in Eucalyptus poles from Tanzania to Kenya and other Eastern African countries (Munishi – personal observations).
Eucalypts are increasing in importance in the middle and lower latitudes of the world because many species have the ability to improve conditions in treeless areas (FAO, 1988). In many places Eucalyptus have helped to raise people’s living standards by providing building materials, fuel wood, poles and farm timber. According to FAO (1988), it is common that people can grow and cut crops of eucalyptus using their own efforts to meet the needs of their families without having to import goods from elsewhere. In 1955 the total area under eucalyptus plantations was estimated at about 700 000 ha worldwide rising to 4,000,000 hectares within 25 years (FAO, 1979) and the trend towards planting eucalyptus in many countries has been on the increase.
Eucalyptus planting has been said to have both advantages and disadvantages. Some of the advantages of eucalypts include quick provision of benefits associated with fast growth, short rotation plantations for production of fuel wood and farm timber, produce good shelter-belts and rapid cover, under proper management they are good source of forage for bees (honey production), essential oils and tannins, can produce fiber for pulp and paper industries under relatively short rotation, can grow adequately in a wide range of ecological conditions and sites among others. On the other hand eucalypts have been said to have their disadvantages (FAO, 1988). These disadvantages include consumption of much ground water associated with its high growth rate, may affect soil fertility under certain circumstances, may not provide good soil protection against erosion and may not provide good habitats to support wildlife.
Planting of eucalypts has faced recent controversies and critiques based on technical, ecological and socio economic arguments. The major argument has been that eucalypts removes too much water from underground reserves and streams, inhibit the growth of other vegetation and its leaf litter has adverse effects on soil humus among others. In Tanzania such critiques have led into re-thinking about further planting of eucalypts by some people though the arguments are not based on rigorous data on the alleged impacts of eucalypts. Eucalyptus impacts and responses at different sites are complex and may require substantial research and adequate information before conclusive arguments can be made. Lack of adequate data in Tanzanian conditions to justify conclusive decisions on eucalypts will likely be a major constraint to solutions on these controversies.
2.0 Socio-economic Aspects of Planting Eucalypts
According to FAO (1988, 1989) eucalypt plantations have become a subject for argument in some countries because the needs of the local people and the forestry objectives likely to be achieved by planting eucalypts have not been well matched. In other places eucalyptus plantations are making a very good contribution to the needs of the community because the advantages obtained outweigh the disadvantages and it would be quite unwise to grow eucalypts if they were not beneficial to majority of the local people. In Tanzania eucalypts are among the major species grown under small holder forestry and contributes significant source of building material (poles) and fuel wood. Today eucalypts from small holder farms contribute a significant proportion in the timber market and large scale trade in transmission poles (Munishi et al., 2004; Munishi et al., 2007). Eucalypts can play an important role in developing the objectives of social forestry in Tanzania such as creating new sources of forest produce in rural areas (firewood, charcoal, building poles and small timber), help farm owners by growing useful trees including those that can be sold as cash crops, conserving soil and water and providing amenities such as recreational areas among others (FAO, 1988; Munishi et al., 2004; Munishi et al., 2007). It has been argued that in Ethiopia the introduction of eucalyptus species was a great success (Henry 1973) and is undeniable fact that eucalyptus plantations have played and will play a tremendous role in alleviating the fuel and construction material problems of the community in Ethiopia (Tesfaye 2007). In India the average annual productivity of eucalypts is 20 times that of natural forests putting it at a higher rank in terms of biomass production. Though still undesirable ecologically to replace natural forests with the plantations of eucalyptus, there is nothing wrong if its cultivation is done on marginal agricultural or degraded lands and farmlands as most eucalypts can adapt to different agro ecological conditions (Sapra, 2007).
In 1992 the world production of essential oils was estimated to be about 4000 tones of which 60-70% went into the medicinal market, with China dominating the market. Tasmanian Blue Gum (Eucalyptus globulus) is the major source of essential oils. The price of eucalyptus oil was $4/ kg by the mid 1990s. This species is also planted in Tanzania but not exploited for essential oils. Worldwide there are about 17.9 million hectares of eucalypt plantations mostly in Brazil, India, China, South Africa, Spain and Portugal.
Commercial production of eucalyptus began in Victoria, Australia in 1860. The nineteenth century eclectic doctors adopted eucalyptus as a treatment for fevers, laryngitis, asthma, chronic bronchitis, whooping cough, gonorrhea, ulcers, gangrenous tissue, edema, and gastrointestinal disturbances. European doctors used eucalyptus oil to sterilize their surgical and medical equipment. Eucalyptus leaves were often made into cigars or cigarettes and smoked to relieve asthma and bronchial congestion.
Modern medicines around the world have included eucalyptus in their practices. Indian ayurvedics use eucalyptus to treat headaches resulting from colds. Eucalyptus is listed in the Indian Pharmacopoeia as an expectorant and in the Chinese Pharmacopoeia as a skin irritant used in nerve pain. In France, eucalyptus leaves are applied topically to relieve congestion from colds and to treat acute bronchial disease. A standardized eucalyptus tea is licensed in Germany to treat bronchitis and throat inflammations. Eucalyptus is also an ingredient in German herbal cough preparations. The German Commission E has approved the internal use of eucalyptus to treat congestion of the respiratory tract, and the external use to treat rheumatic complaints. In the United States, eucalyptus is a component of many decongestant and expectorating cough and cold remedies, such as cough drops, cough syrups, and vapor baths. Eucalyptus is often used in veterinary medicine. It is used to treat horses with flu, dogs with distemper, and to treat parasitic skin conditions.
Out of the 600 species only a few are used in Tanzania including Eucalyptus saligna, E. grandis, E. camaldulensis, E. globules, E. viminalis , E. citriodora, E. regnas and E. microtheca. The main attribute, which has favored the choice of eucalyptus in Tanzania, and in many other tropical and subtropical countries, is the fast growth rate of the selected species. By selecting the best seed source within a species, one can produce poles in short rotation of 4 to 6 years and fuel wood in a shorter period. Eucalyptus trees were mainly introduced to provide fuel wood for rural and urban communities and transmission poles. At Sao Hill large areas were planted with Eucalypt to feed the pulp and paper mill. Eucalyptus was also used for marking of government forest reserves boundaries. Some farmers in the highlands of Kilimanjaro, Iringa, Mbeya and Arusha regions planted Eucalyptus trees for boundary marking or protection of fallow land. Some of these trees are very big, and given the current shortage of timber in these areas, and improvement in sawing and drying technologies, these trees are now sawn for timber and playing an important role in the local economy.
Eucalypts have been planted widely for commercial use in Brazil and other South American countries, Africa, the Indian subcontinent, and the Middle East. They are used extensively for fuel and construction and are an important component of the developing world economies. Foliage of some species yields essential oils for medicines and perfumes. Tannins are extracted from the bark of certain species (http://www.answers.com/topic/eucalyptus?cat=health)
3.0 Ecological Impacts of Eucalyptus Trees
3.1 Effect on Water Sources
A lot of concern has been expressed on effect of eucalyptus planting on the hydrological cycle with various claims that their presence on the landscape is causing the drying of water sources, rivers and springs. It has been advocated many times, and indeed in several regions large areas planted with Eucalypt have been clear-cut in the hope of increasing stream flow. For example in Mbeya, fuel woodlots (Eucalyptus maidenii and E. saligna planted to supply fuel wood to Mbeya town were cleared from both catchment and non-catchment areas, with no obvious plans for species replacement. The hope is that indigenous species will automatically take over even without assistance. This approach is ill advised since the sudden drastic vegetation removal over large areas on steep slopes invariably leads to increased soil erosion, landslides, and silting of waterways and water bodies, and increased floods on farmlands habitats.
The role of fast growing Eucalyptus on soil water budget is a controversial subject, and we do not wish to take sides here. We believe that each catchment has its own peculiarities and buffering mechanisms (nature and permeability of the geological substratum, soil depth, slope, and natural vegetative cover etc). This means that the response to Eucalyptus growth by each catchment should be judged separately.
It may be instructive to ask why were Eucalyptus planted there in the first place. Apart from boundary planting, farmers living in catchment areas planted Eucalyptus for various reasons as explained above. Eucalyptuses, because of their adaptability to marginal sites, have also been planted in highly eroded and degraded areas where other trees could not survive. Sites where it has been planted and succeeded to prevent further degradation included severely eroded slopes, and areas degraded by landslides.
The effects of eucalyptus on the water budgets will depend on the species in question, climate of the areas, surface soil conditions, nature of rock substratum, vegetative cover, slope gradient and length, tree growth stage and tree density, crown depth and leaf density, amount of rainfall and soil moisture conditions and rooting depth. As already mentioned fast growing species consume more water than relatively slower growing species. However, 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. In shallow soils the volume of water that can be retained is limited by texture and permeability. Sandy soils have lower water retention capacity, higher permeability and permit deeper percolation than clayey soils, which retain more water but have lower percolation rates. The amount of organic matter, including litter thickness on the soil surface will affect precipitation water retention and percolation; while a bare soil will encourage more surface runoff. The porosity of the geological substratum will determine the aquifer water storage capacity, and the slope gradient and length of the catchment area will determine whether vertical flow and hence deep percolation or surface runoff predominates.
According to Teshome, (2007), Sapra, (2007) there is a misconception by most people that eucalyptus consumes a lot of water more than any other tree species and agricultural crop. There are quite a number of research results which revealed that eucalyptus is efficient water user. For instance, Davidson (1989) reported that on a “leakproof hectare” at Nekemet (with annual rainfall of 2158mm), 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 eucalyptus produce higher amount of biomass which is economically profitable and acceptable.
Most eucalyptus species need on average 785 liters of water/kg of biomass produced as opposed to cotton/coffee/banana (3200), sunflower (2400), field pea (2000), cow pea (1667) soybeans (1430), potato (1000), sorghum (1000) and maize (1000) liters/kg biomass produced (Davidson 1989). This shows that eucalyptus species are efficient water users.
One of the primary concerns about eucalypt plantations is that they lead to a diminished rainfall in their area of influence. The most significant hydrologic effect of a eucalypt plantation, as well as any other tree plantations or forest cover, is its interception of rainfall. Studies in São Paulo indicated that a 6-year-old Eucalyptus saligna plantation lost 12.2% of rainfall water by canopy interception. Two 13-year-old pine plantations (Pinus caribaea and P. oocarpa) have shown losses of 12% (Lima 1976), and savanna-like vegetation showed a loss of 27% (Lima and Nicolielo, 1983). Secondary Atlantic forests varied from 12.4% (Castro et al., 1983) to 18.2% (Cicco et al., 1986), and the range in Amazonian rain forest was 8.9% (Lloyd et al. 1988) to 19.8% (Franken et al., 1982). Munishi and Shear (2005) observed that tropical sub montane forests in the Eastern Arc Mountains Tanzania intercept about 20% of the rainfall. A review of the work and data available in Brazil as well as other parts of the world suggest that, on the average, the water interception loss by eucalypt plantations is less than that of other tree plantations or native forests (Lima, 1993).
Yet, while some fast growing eucalypts consume a lot of water, there are other eucalyptus species which are adapted to grow in semi arid and arid areas, with acute soil water shortage where their fast growth relative to indigenous species cannot be attributed to high water consumption. The main explanation lies in the adaptation of these species to survive and grow under low water supply, including high water use efficiency to produce biomass.
With respect to water quality it has been demonstrated that watersheds covered by natural forests of eucalypts in Australia produce good quality water (Hatch 1976). Charley and Richards (1983) showed that the most positive effect of forest cover on watershed water quality occurs during rapid tree growth. This correlation may suggest that eucalypt plantations in Brazil may have a beneficial effect on the water quality by virtue of their very fast growth rate and short rotation.
Although some variation exists in the influence of various eucalypt species, native forests, and other tree species on water quality, studies have shown that watershed water quality depends more on the geology, soil, and precipitation regime of the region (Lima 1993). In the Valley of Paraíba River in São Paulo, a study conducted in two small watersheds covered by Eucalyptus saligna showed that the nutrient balance and quality of the water was not unlike that found in similar studies in other parts of the world (Ranzini 1990). The typical leaf area indexes of some eucalypt species are smaller than those of other forest species which suggests that the total interception by eucalypts may be comparatively low (Gash 1979)
A comparison of soil moisture in a 5-year-old E. grandis plantation, a 5-year-old P. caribaea plantation, and a savanna-like native forest showed similar pattern of annual variation in soil water for the three forest covers. On the other hand the eucalypts used water more efficiently than did the natural vegetation in terms of timber production, (Lima et al., 1990).
Vegetative cover protects the soil from surface runoff, and deep root channels permit more percolation. Stem flow directs water to recharge the soil near the tree, while the thickness of the tree canopy will determine the amount of water intercepted and hence lost through evaporation. Most Eucalyptus trees planted in catchment areas tend to have thin crown with open canopies that intercept less water than indigenous species. It is also believed that most of the water absorbing and nutrient feeding fine roots of Eucalyptus are concentrated on the topsoil and strongly protect the soil from erosion. Exposed, bare soils on steep slope void of vegetative cover like in degraded farmlands will obviously permit more runoff and less aquifer water recharge. In such cases planting Eucalyptus trees as a pioneer to mimic secondary succession makes sense. Mixed planting of Eucalyptus with indigenous shrubs or trees makes ecological sense. Otherwise, once the canopy of a Eucalypt stand has closed, timely opening of the canopy accompanied by enrichment planting with indigenous species should be encouraged to minimize the negative influences attributed to Eucalyptus trees.
The growth stage of trees affect the amount water consumed with higher uptake before canopy closure, which levels off thereafter. The age to canopy closure will be partly influenced by growth rate and canopy characteristics of the tree species in question, as well the density of the plants. Trees planted at high density tend to close canopy before widely spaced ones, with corresponding higher water consumption. We can safely deduce that a denser Eucalypt stand will consume more water than widely spaced trees. This implies that the planting pattern is of considerable effect as far as water consumption by trees is concerned. Factors such as planting space which determines the density of trees need be considered. Further the impacts on water resources by planted trees depend on the climate especially rainfall. Areas with higher rainfall than evapotranspiration hence moisture surplus can be more suitable for planting fast growing tree species and eucalyptus planting in these areas may not be an issue. In drier areas planting pattern may be manipulated such as allowing bigger spaces thus few trees per unit area to reduce the amount of evapotranspiration. Again, the possibility of mixing compatible pioneer indigenous species with Eucalyptus may reduce water consumption from the site.
Given the above facts eucalypts seem to behave as any other tree plantation or natural forest cover with respect to the soil water dynamics and with respect to the water balance of the watersheds. Considering the dynamics of water use by trees, eucalypt plantations can have both positive and negative effects on the water quality and quantity like any other managed vegetation and the net effects may be negative or positive depending largely on management practices.
4.2 The Case of Planted Eucalyptus Trees
As explained earlier one of the major exotic tree species planted in Tanzania are eucalypts and have widely been adopted at different scales in different parts of the country making not less than one-third of the species planted by small holders. The controversy as to whether to abandon planting of eucalypts or uprooting of the existing eucalypts may be baseless. Based on the dynamics of water use by trees the case of Eucalyptus trees in catchment areas is more interesting, since often the trees were planted with no harvesting objective in mind. It is however generally accepted that most of the stems of old Eucalyptus trees comprise of dead heartwood conducting sufficient water for maintenance requirement, but not growth, and hence water consumption is in equilibrium with site water supply. Clear-felling or uprooting of eucalyptus stands will likely cause more instability to the sites by exposing it to soil erosion and landslides especially on steep slopes. The best approach would be to clear the stand in narrow strips at a time, leaving buffer zones of standing trees in between. The opened canopy area can be replanted with light demanding indigenous pioneer species.
As already discussed above, it is unlikely that water in springs, streams and rivers in different part of Tanzania have dried up because of the presence of Eucalyptus trees. Most eucalyptus plantations are old with reduced water consumption and one would have expected drying of streams (in case eucalypts consume too much water) to happen in the young stages when the trees were fast growing and their water consumption was higher.
From the above discussion it is apparent that eucalyptus planting is not harmful be it in ecological and socio-economic terms. In Tanzania more than 90% of the rural communities depend on fuel wood for heating and lighting and there is no other energy alternatives that can outstand fuel wood. Further construction materials in the rural areas depend on wood from small holder forestry woodlots with no much alternative from natural forests/woodlands. Given the multitude of benefits from Eucalyptus it would be unacceptable to disfavor or abandon eucalyptus planting or critique eucalyptus on socio-economic or ecological grounds.
Eucalyptus species are tolerant to severe periodic moisture stress, low soil fertility and fire and insect attack. The comparative advantages of the species has made it part of the life of the rural people in Tanzania. Although some indigenous species may also have such advantages as they are adaptive to the local conditions and could be used for various purposes little is known about their growth, yield, soil nutrient/plant interaction, water consumption, silviculture and management. The challenge is to fill this knowledge gap and come up with more indigenous species pool for reforestation and afforestation purposes before critiquing the role of eucalyptus. Until this is achieved, eucalyptus will continue to be planted widely in Tanzania.
The general consequences of drying up of streams may not be attributed to eucalyptus unless other causes such as probable climate change with frequent prolonged drought of the past few years and the dynamics of land degradation resulting from unsustainable land use can objectively be ruled out. Clearing eucalyptus trees without more rainfall will not help rectify the problem of streams drying
The controversies surrounding eucalyptus planting in Tanzania especially as relates to water use and effects on water resources still requires a more intensive investigation in Tanzania before one can make a conclusive advice given the variety of eucalyptus species and the different ecological regions of the country. Generally, it seems that there are no profound reasons to discontinue or discourage eucalyptus planting in Tanzania.
The costs and benefits of planting fast growing trees including eucalyptus need careful assessment based on detailed site specific studies giving due consideration to both ecological and socio-economic needs.
The controversies surrounding Eucalyptus planting in Tanzania seem to be a matter of management (understanding relevant species or species choice, where and how to plant) and not a problem inherent in any particular species characteristics. Meanwhile given the dynamics of water use by different tree species and the water consumptive capacity of fast growing species like Eucalyptus it would be advised against planting of Eucalyptus in water sources/catchments, riparian areas or as gap planting species in catchment forests especially in arid lands. This also applies to other fast growing tree species unless its water consumption rate is ascertained.
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Paper Presented at TAF Annual General Meeting (AGM), 23rd – 24th 2007 Dodoma Tanzania