Non-mangrove bio-shields along the coastal zone are popularly known as shelterbelts. Shelterbelts are strips of vegetation composed of trees and shrubs grown along the coast to protect coastal areas from high-velocity winds and from devastation caused by recent cyclones like HudHud or Katrina, and events like the 2004 Indian Ocean tsunami.Shelter-belts are also promoted as a means for reducing wind-speed and ameliorating the local microclimate. Bio-shields serve the purpose of sand binders and prevent sand erosion; these can also serve as a source of livelihood for the local communities, if designed as such. Some of the ecosystem services, social benefits and other benefits to animals from bio-shields are listed in the table below:
Benefits of bio-shields to agriculture
As a bio-shield, casuarina (Casuarina equisetifolia L.) of Australian origin, is the single most dominant exotic species planted in coastal areas in India and in many other south Asian countries (Odisha District Gazetteer, Puri, 1977; Odisha District Gazetteer, Cuttack, 1996). Since the 1920s, several state governments have established such plantations under coastal shelterbelt schemes mainly to prevent erosion, provide wind breaks, stop tidal water from damaging agriculture, and protect life and property from cyclones and tsunamis. However, planting casuarina on vulnerable coastlines, particularly in mangrove habitats, for protection from cyclonic wind or storm surges, has not helped protect coastal shores and life or property of the locals. But, casuarina mixed with native species of trees like cashew, does provide some level of protection during storms, and is socially acceptable as it helps farmers earn from such plantations. Properly established multi-species shelterbelts should be dense in their lower part, while being more open in the middle and upper parts. Cashew Nut Tree meets the following criteria for a mixed shelterbelt.
• Cashew has a deep and well-spread root system
• Cashew has a compatible crown, and a light branching habit
• Cashew is highly wind resistant
• Cashew is easy to propagate and maintain
• Cashew provides economic benefit to local communities
Cashew-Casuarina plantations along the Odisha coast
With these typical characteristics, cashewnut-casuarina plantings near the sea-shore in Odisha and other parts of coastal region,have been a successful soil-management module in place for decades. Odisha, on the eastern coast, has 480 km long, barren coastline of sand dunes. Presently, monoculture plantations of casuarina (Casuarina equisetifolia L.) in the outer belt and cashew (Anacardium Occidentale L.) in the inner belt have been created along the coastline to prevent wind blast and erosion of sand dunes. This has minimized the problem of coastal desertification and shore erosion. Being a key process in nutrient and carbon cycling in coastal ecosystems, decomposition of litter of cashew and casuarina in coastal sand dunes is an important factor for controlling both the vegetation structure and ecosystem function. In particular, litter decomposition plays an important role in nutrient cycling and organic matter turnover within ecosystems, and is an important determinant for maintenance of the biosphere. It also performs unique and indispensable activities on which larger organisms including humans depend. This has been documented in terms of very high Shannon- Wiener index in soil with cashew plantation than the barren sand dune that explains the long term effects of changes in soil condition of the plantation floors in coastal sand dunes of Odisha.
Many studies question the policy of planting solely casuarina on vulnerable coastlines, particularly in mangrove habitats, for protection from cyclonic wind or storm surges. But, casuarina mixed with cashew,has provided a good level of protection during storms, and better serves local needs of the people. This would certainly appear to be a better bio-shield approach than having casuarina monoculture plantations on cyclone-vulnerable coastlines. Therefore, effective ecosystem-based management of coastal areas requires a significant policy shift towards using casuarinas, along with cashew and other native species.
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