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Agent Orange - Agent Orange Defoliation Damage

soil forest mangrove herbicide

By 1971, the use of Agent Orange in Vietnam had ended. However, the damage caused to the vegetation of the region by the spraying of Agent Orange is still visible today. Agent Orange applications affected foliage of a diversity of tropical ecosystems of Vietnam, but the most severe damage occurred in the mangrove forest of coastal areas. About 306,280 acres (1,240 km2) of the coastal mangrove forest of South Vietnam was sprayed at least once. This area comprised about 40% of the South Vietnamese coastal mangrove ecosystem.

The spraying killed extensive stands of the dominant mangrove species, Rhizophora apiculata. Barren, badly eroded coastal habitat remained, which had devastating effects on the local economy. The harvesting of dead mangroves for fuel would sustain fewer people than the living forest once did, according to a 1970 report commissioned by the United States National Academy of Sciences, because the supply of mangrove wood was not being renewed. Unless a vigorous replanting program was undertaken, the report warned of a future economic loss when the dead mangroves were harvested.

The destruction of mature seed-bearing trees has made regeneration of mangroves slow and sporadic. Weed species have become dominant. Indeed, the Academy of Sciences has estimated that full recovery of the mangrove forest might take 100 years or more.

Agent Orange was sprayed over 14 million acres of inland tropical forest. A single spray treatment killed about 10% of the tall trees comprising the forest canopy. Defoliation of the trees was much more extensive, but many of the defoliated trees continued to live. Smaller shrubs, protected from the herbicide by the high canopy, comprised the majority of vegetation in sprayed areas. The total loss of commercially useful timber caused by the military application of herbicide in South Vietnam is estimated to be 26–61 million yd3 (20–47 million m3). In areas that were sprayed repeatedly, the valuable tree species were replaced by a few resistant, commercially unimportant trees (such as Irvingia malayana and Parinari annamense), along with tussock grass and bamboo. In the dry season, the stands of grass easily catch fire, and if burned repeatedly, the land is less likely to return quickly to forest. It will take many decades before the tropical forest recovers and attains its former productivity.

Because Agent Orange herbicide remains in the soil for some time, there is concern that these residues might inhibit the growth of crops and other plants. Soil bacteria break down the herbicides into smaller molecules, but complete decomposition takes years to occur. Studies performed 15 years after the spraying in South Vietnam still found degradation products of Agent Orange in the soil. These byproducts, which can be toxic, can be passed through the food web. How much Agent Orange actually reached the soil is subject to question. A large proportion of the herbicide falling onto the forest was trapped by the canopy. Few drops reached the soil directly, but much of the herbicide was eventually delivered to the forest floor by a rain of dead foliage and woody tissue. In open areas, much more of the application reached the soil directly. The contaminant TCDD is quite persistent in soil, with a half-life of three years. (In that period of time, one half of the dioxin originally applied would still be present in the soil.) In studies conducted in the United States, samples of inland soil and sediment from mangrove areas treated with herbicide still had substantial levels of TCDD after ten years. An indirect effect of the Agent Orange spraying is the poor fertility of soil in many areas, due to erosion following the destruction of soil-binding vegetation.

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