Exotic plantation forest: A green desert?
Plantations are done for several reasons including timber and fire wood production, soil and water conservation, and carbon sequestration. The role of plantations for biodiversity acceleration varies considerably depending on whether the original land cover is grassland, shrubland, primary forest, secondary forest, or degraded or exotic pasture, and whether native or exotic tree species are planted. The plantations can support biodiversity when they are established on degraded lands rather than replacing natural ecosystems like forests, grasslands, and shrublands, and when indigenous tree species are used rather than exotic species. The dominance of exotic species is attributed to increased disturbance, changes in light and soil conditions, and, in some cases, changes in land management, including exclusion of grazing. The changing amounts of exotic versus native species reflect the community structure. Exotic plantations are less species rich than natural and semi-natural ecosystems (shrublands, grasslands, primary, and secondary forests). Exotic plantations support a less diverse flora and fauna compared to native plantations. In most cases the influences of exotic tree plantations on some environmental parameters such as soil biogeochemistry are still largely unknown. Some species degrade soil organic matter and adversely affect soil flora and fauna. Sometime soil nutrients decline, and whole tree harvesting causes accelerated soil nutrient loss. Many exotic species are also detrimental to critical environmental factors such as soil physical properties. Exotic monoculture tree plantations do not help to maintain landscape and biological diversity. Uniform rows of monocultural plantations are the opposite of diversity. Diversity is considered to be the primary indicator of ecosystem sustainability. Exotic trees have been found to be poor habitat for native birds. Nesters, insectivorous, and other birds that feed on fruit and nectar are particularly absent from exotic plantations. Some exotic plantations act to cut off islands of remnant indigenous forest from each other, reducing the chances of native species populations exchanging genes. Exotic monocultures increase fire risk and can act as a source of pests and pathogens that spread into adjacent indigenous forest. Around the globe monocultures have been found to be susceptible to pests and diseases. With the narrowing of tree genetic material, increased resistance to various pests and diseases, and increased pesticide use, there is the longer-term risk that nature will retaliate and produce a whole new mutated set of insects, fungi and bacteria. The living part of the soil is particularly sensitive to changes in pH. There is evidence of acidification under some exotic plantations. The nutrient removal associated with fast growing exotic plantations cause nutrient depletion of the soil and lower productivity. Some exotic species can be aggressive pioneer by invading adjacent ecosystems. Invasive species are characterised by having abundant and easily dispersed seed, experiencing little competition when invading new areas having an absence of natural predators and successful establishment of mutualistic relationships. Afforestation with exotic species alters habitat substantially for native flora and fauna with particularly strong negative effects on specialist grassland and shrubland species. Invasive exotic species compete with indigenous species and change ecosystem functioning. Environmental stress from nutrient decline and climate change will likely cause a decline in the health of monocultural tree plantations. On the other hand, exotic plantation forests can result in rapid development of a forest structure beneficial for some wildlife species though it has less developed understories due to the intensity of site preparation and frequent uniformity of plantation forest structure, and can change ecological processes through litterfall and decomposition. Exotic plantations provide wildlife corridor to connect with the natural ecosystems. It is obvious that exotic plantations are far better than agriculture or urban development. Sometimes, exotics can have understory resembling native forests. Though exotic plantations do very little to conserve biodiversity, but they act as carbon sinks and conserve soil from further degradation. It is certain that higher level of biodiversity than human-modified landscapes. Exotic plantations may also support some endangered faunal species providing an alternative habitat. It often supports fewer specialist species and can play an important role in biodiversity conservation at the landscape level. To potential economic revenue, exotic plantations can help restore the degraded areas where native regeneration or native plantations may otherwise be inhibited. It improves soil condition through litter production and adding humus. Through timber production, carbon sequestration and other services for ecosystem, exotic plantations of denuded lands with low biodiversity can continue to provide economic revenue. The role of exotic plantations in soil and water quality, yield, biodiversity and ecosystem health cannot be ignored. They are efficient at storing carbon dioxide in biomass and can be considered as the carbon sink. An exotic plantation may produce an increase in diversity if planted into introduced grass or crop lands, or on severely degraded areas, as trees tend to increase the vertical complexity of vegetation and the structural complexity of a landscape. The tree can reduce soil erosion and cause sedimentation in adjacent water courses. Plantations can help reduce storm and peak flood water levels. They reduce stream flows working against huge erosions and act as a shelterbelt in the coastal areas. Exotic plantations in the riparian strips are currently the major area offering biodiversity protection. It can play for both diversity of stream fauna and water quality, and for providing biodiversity reserves within catchments. Exotic plantations generally increase diversity compared to fallow, denuded, degraded, pasture and croplands. It is noteworthy to mention that exotic forests are also source to timber, industrial fibre, building materials, and provide recreation and wildlife habitat to some extents. Following the clearance of our maximum native forests, largely into agricultural lands, any type of tree plantation can be considered as the emerging major land use. Undoubtedly there are benefits from planting trees on areas of introduced pasture and cropland, such as improved protection for soils and reduced erosion, better water quality, and increased vegetation complexity. Exotic trees can also act as a “filter” of the atmosphere, exchanging various gases as well as air-borne particles such as dust and rain. Some exotic trees have biological relationships that fix nitrogen from the air, as do some free growing soil bacteria and fungi. Exotic trees can contribute variable quantities of litter (dead plant matter) to the soil organic matter. This litter varies in its chemical composition, the force with which it breaks down on the soil, and the diverse range of soil flora and fauna that inhabit it. The byproducts of the litter breakdown are retained in the soil through chemical and biological processes. Nutrients such as phosphorus, nitrogen, potassium, calcium, magnesium and trace elements may be retained in a form that is not available for tree and plant growth (fixed pool), or may be in the plant available “pool”. Forest canopies tend to moderate soil temperatures, with further insulation from forest floor litter layers. Soil aeration can be altered by the tree roots, through them “breathing” or decomposing, and by changing soil porosity and density, and possibly structure. Many exotic species do not negatively impact on soil but actually benefit it. It is well known to all that any tree has some benefits to the soil, such as inception of dust, protection from the kinetic energy of rain, preventing some types of erosion, and moderation of temperature. We need to restore the forests. There has been a call by the wood industry for massive global planting programmes to meet an expected demand for wood. But the industry should pause for a minute and put aside the profit margins, and should ask themselves whether the exotic plantation can fulfil the criteria of sustainability or not. Tree plantations may have many benefits over other industrial land uses, but there are ecological costs, whether present, or potential. It will be not wise to replace one unsustainable system with another. Putting all our eggs in the monoculture basket does not make sense. Alternative species and ecologically sustainable forestry systems must be pursued as a safeguard. Mixed plantations can play the role of “greenwash” in the ecosystems. A considerable number of combinations are possible; usually planting higher value timber trees such Eucalypts, Acacias, Mangium and Bokain in a long rotation with selective felling regimes. The mixed species will be more responsive to site conditions, allowing the maximum use of site diversity and individual species requirements. They will provide more and better quality habitat for wildlife and if planted through a landscape plan, be more visually attractive and interesting. Woodlands could be established as patches within a predominantly agricultural landscape to assist the transition to a protected ecosystem network involving native species. However, exotic plantations can support biodiversity to some extent, even when compared to primary forest, and should not necessarily be considered as 'green deserts'.
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