M. K. Hossain
Bangladesh has a very long history of plantation forestry starting from 1871 with teak (Tectona grandis), the seeds of which were brought from Myanmar. Teak was dominant in the plantation forestry until the mid 1960s, along with indigenous species like Dipterocarpus turbinatus, Shorea robusta, Artocarpus chaplasha and Syzygium grande. Records reveal that more than 130 exotic tree species have been tried so far in the plantation programs, but most of them did not show promising growth performances. The exotics that proved successful in trials and plantations are Tectona grandis, Paraserianthes falcataria, Hevea brasiliensis, Dalbergia sissoo, Swietenia macrophylla, Acacia nilotica, Eucalyptus camaldulensis, E. tereticornis, Pinus caribaea, Xylia kerrii, Leucaena leucocephala, Acacia mangium, A. auriculiformis and Gliricidia sepium. Conversion of natural forests, particularly the evergreen and semi-evergreen hill forests of high biological diversity relies on the rationale that plantations will produce better and higher yields. However, some of the successful species namely, Eucalyptus camaldulensis, Acacia mangium and A. auriculiformis have already been discarded from large-scale plantation programs due to controversy about their suitability or bad effects on the ecosystem. The paper discusses the comparative growth performances of the species and the possible pros and cons in the plantation programs of the country.
The total area of Bangladesh is 14.12 million hectares of which 1.30 million hectares comprising 9.2% of the total area is under the management of the Forest Department (ADB, 1992). These natural forests consist of three major vegetation types occurring on three different land types (Hassan, 1994). But, the natural forests are shrinking rapidly due to encroachments resulting from serious population pressure (Bhuiyan, 1979; ADB, 1992). Besides this, the management of these low yield heterogeneous forests was very complicated. So, at the beginning of the management of these forests, the main objective was to replace these heterogeneous forests by plantations of valuable species.
The plantations have some advantages over natural forest from management and economic point of views, such as concentrated production, ability to choose species with desirable characteristics. As because of the acute shortage of timber and fuel wood in Bangladesh, a priority program of introducing fast growing tree species was taken up. In plant introduction program of recent years, there has been increasing recognition of the importance of obtaining species adapted to a particular introduction area. After successive elimination, provenance, growth and yield trials, some promising species like Eucalyptus camaldulensis and Acacia auriculiformis have recommended for the large scale afforestation/reforestation programs (Zashimuddin et al., 1983; Davidson and Das, 1985; Latif et al., 1985; Amin et al., 1995; Hossain et al., 1989, 1994, 1996b). But, recently there is a decision against the cultivation of Eucalyptus and Acacia in plantation programs based on some media reports that these species have a damaging impact on the ecosystem. However, the decision is not backed by sufficient scientific information, research or field experiments. The paper briefly describes the growth performance of the species and possible risks and benefits in large scale plantation programs in the country.
The first attempt of raising forest plantation was made in 1871 with teak in the Chittagong Hill Tracts. Since then plantation forestry has become a part of the overall clearfelling management system. Some important hardwood species like Swietenia macrophylla, Lagerstroemia speciosa, Cedrella toona, Artocarpus chaplasha, Xylia dolabriformis, Syzygium grande were introduced in the plantation program along with Tectona grandis (Chowdhury, 1982; Das, 1982; Hossain, 1998). In the 1950s and 1960s, sal (Shorea robusta) plantations were established over substantial areas. In these way, the natural multistoried forests of Bangladesh were under the process of conversion from natural to artificial through the removal of existing growing stock and planting with more valuable species and also with softwood and firewood species (Hoque, 1977).
In 1974, the Forest Department began establishing plantations of fast growing species such as Gmelina arborea, Paraserianthes falcataria and Anthocephalus chinensis in Chittagong Hill Tracts and Sylhet Forest Divisions. In 1978 and onwards, extensive trials of Eucalyptus and Acacia species were started to find out the very fast growing exotics for some difficult sites. Plantation programs are getting priority in both public and private sectors. But, the choice of species is still under debate, particularly regarding the plantations of some exotics like Eucalyptus, Acacia and Pinus species.
Attempts have been made to replace the natural low yield forests by valuable indigenous as well exotic species since the beginning of the scientific management of these forests (ADB, 1992). From 1968, elimination, provenance, growth and pilot plantation trials were established with a number of indigenous and fast growing exotic tree species in different silvicultural research stations of Bangladesh Forest Research Institute (BFRI). Eucalyptus camaldulensis, E. tereticornis and E. brassiana emerged as the best species for planting in Bangladesh from the trials of 84 seedlots of 12 species of Eucalyptus imported from different parts of Australia (Davidson and Das, 1985). Another provenance trials of E. brassiana (12 provenances) and E. urophylla (12 provenances) were laid out during 1983 to select the superior provenances for Bangladesh (Hossain et al., 1996a, Islam et al., 1997). However, the seeds of a total of 216 provenances of 37 species of Eucalyptus were introduced in Bangladesh from which finally E. camaldulensis, petford-1 provenance was found suitable and promising (Davidson and Das, 1985; Hossain et al., 1989). A comparative study of the coppicing ability of different provenances of E. camaldulensis, E. tereticornis and E. brassiana showed that their coppicing ability was very high and E. camaldulensis showed the highest height and diameter (dbh) growth of their coppices (Hossain et al., 1994). Hossain et al. (1996b) also reports the superiority of E. camaldulensis in Chittagong University hilly areas over other eucalypt species and provenances.
Among the Acacia species introduced so far in Bangladesh, Acacia mangium and A. auriculiformis have shown promising growth in poor afforestation sites (Latif et al., 1985; Hossain et al., 1997). Provenance trials of Pinus caribaea var. hondurensis were established in 1983 and 1984 at Keochia, Chittagong and found that provenances grown well in the area (Latif and Zashimuddin, 1994) and Pinus caribaea var. hondurensis is doing better in comparison to Pinus oocarpa in the same location (Zashimuddin et al., 1991).
Another elimination trial of 23 dry zone hard wood species from Central America was laid out at Charkai (Dinajpur) in 1986 but the trial was failed (Hossain et al., 1989). Similarly, a provenance trial of Acacia mangium with 23 provenances (19 from Australia, two from Indonesia and two from Papua New Guinea) were established at Keochia, Chittagong (Zashimuddin et al., 1983). Records reveal that more than 130 exotic tree species so far tried in the plantation programs of Bangladesh (Hossain et al., 1989), but only a few of them proved successful in large scale plantation programs. Some of the successful exotic species and their status in the country are shown in Table 1.
Successful plantation technologies have been developed in many countries (Evans, 1992; Kanowski and Savill, 1992). Light demanding, colonizing exotic species have been the most successful in monocultures under plantation management (Hughes, 1994). The tropical and sub-tropical plantation forestry has focused on a small number of fast growing, colonizing species like Acacia, Eucalyptus, Gmelina, Pinus, Populus and Tectona (Evans, 1992). These species have the ability to capture the site rapidly and tolerate harsh soil and climatic conditions and abuse from animals, humans and fire etc. These characteristics of exotics are a pre-requisite for success on the often highly degraded sites. Higher yield advantages of exotics over indigenous species have been attributed to their greater tolerance of degraded sites and their escape from specialized pests and diseases. Thus, the diminishing natural forest resources are being compensated by rapid expansion of planted exotic trees worldwide (Davidson, 1995). Successful exotics are being planted in some other countries also, such as spruce in Europe, radiata pine in Australia and New Zealand, ipil-ipil in Philippines and Australia, eucalypts in India, China and Brazil (Evans, 1992).
There is an increasing concern among foresters, ecologists, botanists, conservationists and policy makers about the threat of uncontrolled introduction of aggressive tree species in the plantation programs. Invasion of exotics may cause major loss of biodiversity and species extinction either due to direct replacement by exotics or indirect effects on the ecosystem. Concern also exists on the degradation of the environment, e.g. the controversial effect of eucalypts on environment. There are also risks of the decline of growth and yield in second and successive rotations, or the infestation of pests and diseases, e.g. Psyllid in Leucaena leucocephalla, shoot borer in Swietenia and leaf defoliator in Tectona grandis.
In Bangladesh, there has been a net loss of forest cover over the years and the net yield per ha of original planted area has dropped significantly (ADB, 1992). The mean annual increment of present planting stock is extremely low by regional and international standards. There are also reports that 20-30% of all plantations established during the last 30 years have been destroyed, and of those surviving, the stocking is much less than the expected standards (ADB, 1992). The plantations appear likely to produce well below the capacity of the site due to lack of proper maintenance. All this has contributed to very low growth of 2.5 m3 ha-1year-1 in areas where 7.5 m3 ha-1year-1 was the original norm (ADB, 1992).
Teak, an economically important timber species initially was found promising in the hill forest areas, but later on critics arose that it depletes soil fertility, prevents undergrowth vegetation and productivity depends on only suitable specific site conditions. Though the expectation of productivity was more than 7-8 m3 ha-1yr-1 but, practically it is less than 2 m3 ha-1yr-1 in a wide range of plantations (ADB, 1992). However, Eucalyptus camaldulensis experimental plantations showed excellent yield to 69 m3 ha-1yr-1 in a closed spaced plantation (Davidson and Das, 1985), but in hilly areas the production reduced to as low as 14 m3 ha-1yr-1 (Table 2). The growth behaviors of some plantations in the hilly area was erratic and poor throughout the rotation cycle which may be due to the wrong selection of seeds and site (Amin, 1995). Similarly, Acacia mangium suffers from heart rot disease due to which the species is no more using in the new plantation programs.
The remaining most commonly used species is the Acacia auriculiformis, which is widely planted and have shown success in degraded sites (Ara et al., 1989; Hossain et al., 1997). Hossain et al. (1998a) reports the result of a provenance trial that leads to trees with single stems and good clear bole from a provenance trial in Chittagong University area. However, improvement for the clean cylindrical bole and other management practices will need immediately for going large scale plantation programs of the selected provenance. Some Acacia plantations showed better survival and growth in different areas of the country and the yield is 15-20 m3 ha-1yr-1 at 10-12 years rotation (Table 3). Some other successful species, viz, pine, Gliricidia, ipil-ipil are restricted to some localized plantations only.
In Bangladesh, large number of tree species have been introduced in the past; most do not naturalized, and most of those that naturalized do not become important invasive (Hossain, 1998). However, Eucalyptus camaldulensis and Acacia auriculiformis is widely used in forest plantations, agroforestry, community forestry and homesteads because of their fast growth, short rotation, non-palatability to grazing animals and specially their ability to thrive in poor soils. But, the news media and environmentalists publish articles on the possible bad effect of these species in Bangladesh and neighboring countries. There is an order from the government not to plant these species further more. In extreme cases, some established plantations were felled or cleared off also. Eucalyptus is blamed for use water differently (more) from other species, reduce soil fertility, leading to soil erosion, harmful for wildlife and kills native understorey vegetation. But, the criticisms and blames are not supported by sufficient scientific findings. Contrary, the leading forestry, agroforestry experts and scientists concluded that eucalypts may be a suitable species for afforestation and reforestation in denuded areas, marginal lands, roadside plantations and agroforestry programs (Hossain et al., 1997; Amin et al., 1995). Hossain et al. (1998b) also reported the luxuriant undergrowth vegetation and biomass production under eucalypt plantations in comparison to some other afforestation species. Acacia auriculiformis is blamed for pollen allergy, but there are no studies in favor of this criticism. Whereas, there are some weeds and grasses that are more responsible for pollen allergy than the Acacia species.
In Bangladesh, plantation program is increasing day by day and the exotic species are getting preferences over the indigenous ones (ADB, 1993). With deforestation, many degraded sites are available for plantation, though very little is known about how to manage such sites economically. Conversion of healthy prospective natural forests to plantations of alien species must be strongly discouraged. Productive plantations on previously deforested or non-forested sites may offer important options for conservation of natural forests and arresting the further deforestation activities. Another option of mixed species plantations of both the indigenous and exotics may also be able to contribute as an effective mechanism for sustainable resource management. Through the increase of productivity, pressures can be reduced on remaining natural forests, but emphasis must focus on existing plantations, and new plantations on degraded or denuded land, agroforestry and community forestry programs rather than on natural forest.
The wastelands and denuded hillocks adjacent to human habitation which are either covered with sun grass or overgrazed by leaving the sites with a thin vegetation cover and of very little water retention capacity may be reforested with exotics like Acacia and Eucalyptus species. When the techniques of successful teak, eucalypts and acacia plantations were being at standardized, the environmentalists became conscious of the danger of raising plantations of these species. Considering the demand of the peoples, artificial plantation programs may support but the major questions still concern is the correct choice of species for the site-whether mixed or in pure stands and the interactions of species and site, especially where exotic species like Eucalyptus and Acacia are used.
Plant introduction is a very time old practice and over a long period, plants of various types of economic importance were introduced in the country (Islam, 1991). Migration or introduction of plants from one place to another may be natural or planned one. Bangladesh, like many other countries have a long history of plant introduction from different countries or geographic areas of the world. In 19th century the British people were mostly contributed to the introduction of some economically important forest plants from almost all the continents. In the 20th century this trend continued to be the same, and some Australian tree species are getting preferences in plantation programs. However, we dont have any record of the introduction of weedy invasive species like Eupatorium odoratum, Mikenia cordata etc. in the country.
Tree plantation both at public and private sector got momentum in the recent year. Afforestation program also helps in improving the socio-economic condition of the rural people by generating employment in raising nursery and plantation activities. Indigenous species should given preferences in plantation programs first. If the native species are not successful or not able to provide the required productions, than the exotics may be considered for plantations only.
E. camaldulensis, A. auriculiformis and Acacia hybrid (A. mangium x A.auriculiformis) proved successful for afforestation, reforestation and agroforestry programs in Bangladesh. Absorption of much water by Eucalyptus or pollen allergy of A. auriculiformis is not proved as a serious problem in the context of Bangladesh situation. The embargo on cultivation of these species should be considered and farmers choice in the selection of species should be given preferences. Extensive studies should also be carried out by scientist and foresters on further effect of these species in the ecosystem.
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|  Institute of Forestry and
Environmental Sciences, Chittagong University, Chittagong 4331, Bangladesh.