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by Abraham Parker, Danielle Ogurcak & Jennifer Rice
The mangrove habitat in Vietnam accounts for about 300,000 ha. Of this, 124,000 ha of true mangrove and 27,000 ha of rear mangrove were subject to military herbicide spraying during the Vietnam War, which began in 1957. The Herbicide Spraying Project was initiated by the U.S. Air Force in 1961 under the Code Name "Ranch Hand". In the early 1960's the objective of destroying forests was to keep a strip of land along 17th parallel (i.e. between North and South Vietnam, free of every kind of vegetation. The 1954 Geneva Agreement stipulated that this zone be kept neutral. Between 1961-1971, enough herbicides were sprayed to cover 30,305 mi2, or 20% of the total land surface of South Vietnam. A typical application was around 29 kg/ha. Almost 20 million gallons of herbicides were used in this time period. The U.S. administration had three aims for its large-scale campaign to destroy vegetation in South Vietnam: to guarantee "safety" in zones bordering roads and other communication routes, particularly by using flight missions to help check military movement by the Northern Liberation Force soldiers (Viet Cong) out of the central provinces and into the south of the country, to prevent Viet Cong from using dense jungle foliage as a means of camouflage, and to destroy rice fields and other food plants in order to expose the Viet Cong to hunger (Arison 1995). By the end of the War, after 72 million liters of herbicide, 13 million tons of explosives, and 400,000 napalm strikes (Jamieson et al, 1992) had been used, over 150,000 hectares of mangrove were affected, and toxins still, 25 years later, soak the soils.
Compounds and Defoliants
Used 
The following is a brief description of the compounds and defloiants that were sprayed during the Veitnam War:
2,4-D: 2,4 dichloro-phenoxy acetic acid. 2,4-D and its derivatives are easily absorbed by surface tissues of plants, especially leaves. Subsequent translocation is swift, so an herbicide does not have to cover all organs to achieve the desired effect. 2,4-D and its derivatives are broken down quickly in the soil. They will not remain effective over a period of months or even weeks in the conditions of high temperature and humidity in Vietnam (Grummer 1969).
2,4,5-T: 2,4,5-trichloro-phenoxy acetic acid. It is very valuable for control of woody species and scrub (Grummer 1969). The manufacture of 2,4,5-T produces Dioxin as a side product, which is an extremely toxic substance to organisms and has a very high persistence in soils (Baughman and Meselson, 1973).
Agent Orange: Defoliation occurred 4-6 weeks after application and effects last 9-12 months. Follow-ups were made after 4-6 months. It is quite effective in the thick jungle, because it is volatile and its high vapor pressure forces it into soil covering (Arison 1995). Agent Orange accounted for about 50% of total herbicide use during the war. (Arison 1995).
Agent White: Mostly used when on the spot placing is an important factor: It does not drift as easily as Agent Orange. Agent White accounted for 35% of total herbicide used during the war. (Arison 1995).
Agent Blue: Composed of cacodylic acid, which is 54% arsenic. Used to kill off rice paddies at any stage of plant development. Accounted for 15% of total herbicide use (Arison 1995, Grummer 1969).
Ecological Effects on the Mangrove Ecosystem
In mangroves, one application of herbicides most often destroys the entire plant community. Virtually nothing remains alive. A study of mangrove swamp conducted in the Ca Mau Peninsula (Hiep 1984) showed that herbicidal destruction reduced the area of the mangrove swamp, which had covered 82% of the peninsula before spraying, to less than 50% of its original state. The mangrove species, Rhizophora spp., is especially sensitive. Another species of mangrove, Avicennia spp., sometimes survives. Little, if any recolonization occurs in sprayed mangrove forests. It can take more than a century for the recovery of the ecosystem.
Lack of regeneration is due mostly to a lack of seed source. Extensive herbicide spraying resulted in a loss of seed/propagule-bearing trees in the sprayed areas (Westing 1984, Snedaker 1984). Furthermore, the soils of mangrove forests may prevent herbicides from decomposing (Orians and Pfeiffer 1970). If herbicides remain bound to soil particles, they might prevent seed germination in the future (Orians and Pfeiffer 1970). In an experiment performed by Walsh et al. (1973), it was found that mangrove seedlings are also very sensitive to residual herbicides used in Vietnam.
2,4,5-T has adverse effects on plant growth as the compound penetrates leaves, stems and root systems, then rapidly translocates inside the plant, inducing the breakdown of cell walls in both roots and leaves(Cung 1993). Lugo and Snedaker (1974) confirmed that the death of mangroves is due to the loss of meristematic tissue reserves, as they showed that mechanical (i.e. handpicked) defoliation does not prevent refoliation.
It is still unclear exactly why mangroves are so sensitive to herbicides, as opposed to inland forests, which are more tolerant of the attacks. Westing (1971) suggested that susceptibility relates to physiological attributes that permit growth in the tropical tidal environment. Rhizophora regulates ion uptake by a salt-exclusion mechanism in the roots, and Walsh et al (1973) demonstrated that while roots are destroyed by
herbicidal treatments, it is also possible that in addition to the direct effects of herbicides, death can be caused by disruption of this osmoregulatory ability. Furthermore, the physical conditions in the tidal environment could cause a greater herbicidal uptake and activity than in upland regions. The mangrove environment is very fertile, and it is known that high fertility coupled with abundant water increases the susceptibility of plants to herbicides (Walsh et al, 1973).
According to Westing (1984), a 1980 rough aerial
survey of Vietnam's 124,000 hectares of true man groves revealed:
(a) barren patches of land 5-50 hectares in size (5-10%), (b)
natural regeneration of Rhizophora(1%), (c) artificial
planting of Rhizophora (10%), (d) conversion to rice and
other crops (5-6%). In the rest of the area, there was natural
regeneration of undesirable species. Species such as atypical
grasses, the fern Acrostichum aureum, the palm Phoenix
paludosa, several species of vines, and lesser mangroves such
as Ceriops dominate regeneration in the defoliated
mangroves (Westing 1984, Snedaker 1984). Also, areas that were
colonized by new vegetation were being colonized on a large scale
by poor stands of bamboo (Bambusa sp.), which have a
lesser ability to store nutrients.
Artificial planting has been carried out in other areas as well, which will greatly benefit the recovery and assist in the regeneration of these forests. Between 1978 and 1981, the entire Duyen Hai District (a 22,000 hectare area), was successfully replanted with Rhizophora apiculata. To do this, seeds were collected from the Minh Hai mangroves and directly seeded into the mudflats at an intensity of 10,000 seeds per hectare. According to Williamson (1990), "form and growth rates appear reasonable and the overall result very impressive."
Effects on bird species
Many of the animal species inhabiting mangroves are restricted to that type of habitat (Orians and Pfeiffer 1970). These animals are therefore considered inhabitants of "islands" surrounded by unsuitable habitat and therefore, are expected to have higher rates of extinction due to disturbance and destruction of habitat than species of more continuous habitats (MacArthur and Wilson 1967). It is estimated that a 10% loss of mangrove habitat will eventually lead to 3 or 4% loss of indigenous plant and animal species (Westing 1984).
The massive defoliation had a profound effect on Vietnam's resident bird species in the mangrove areas. This is due, in large part, to a lack of habitat and a subsequent loss of native food sources. In defoliated areas, all species of insectivorous or frugivorous birds appeared to have been missing entirely, except for barn swallows (Hirundo rustica), which are migrants from the north (Orians and Pfeiffer 1970).
Piscivorous birds appeared to be less affected by defoliants and therefore more prevalent. (Orians and Pfeiffer 1970). Furthermore, two rare species that were once found in the Mekong delta are now thought to be extinct in Vietnam: the white-shouldered ibis (Pseudibis davisoni) and the giant ibis (Thaumatibis gigantea).
The mangrove forests of the Mekong Delta now support large numbers of herons, egrets, storks, and ibises. In fact, seven large breeding colonies have been located in recent years (Duc 1989). Furthermore, the wetlands of the Red River Delta are now an important migratory wintering site for thousands of ducks, geese, herons, egrets, shorebirds, gulls, and terns, as well as rare species such as the black-faced spoonbill (Patalea minor) and Saunders gull (Larus saundersi) (Duc 1989). The sarus crane (Grus antigone sharpii), once thought to be near extinction, was recently discovered wintering in large numbers in the northern delta, while several rare species such as the black-necked stork (Ephippiorhynchus asiaticus),the lesser adjutant (Leptoptilos javanicus), and the greater adjutant (Leptoptilos dubius) are now found to be residents of the Mekong and Red River Deltas (Duc 1989).
Effects on aquatic species
Indigenous fish populations can experience long-term effects as a result of herbicide usage in three ways. Fish can be indirectly influenced by disrupted vegetation on adjacent land areas. Secondly, herbicides can have toxic effects on food species such as aquatic plants, zooplankton, zoobenthos, and macro-invertebrates. Lastly, herbicides can have a toxic effect directly on the fish themselves. Long-term effects of herbicidal spraying in Vietnam on local fish fauna include a reduction in fish species diversity, an invasion of non-native fish species, and a reduction of fish biomass and productivity. These effects appear, primarily, to be the result of reductions in the fish species' natural food supplies. (Yen and Quynh 1984, Snedaker 1984).
The aquatic plants normally found in streambeds and ponds were absent altogether. Also, anomalous deformations were found among local freshwater algae (Yen et al. 1984). Therefore, macrophytophagous fish were absent in these areas. Other aquatic species were adversely affected as well. Aquatic invertebrates including mollusks, crustaceans, and rotifers were found in reduced numbers or missing altogether. Thus, indigenous species of shrimp, crabs, snails, mussels, and water fleas were also low in numbers. Various types of worms, larval two-winged flies, and mountain crabs appeared to be absent as well. (Yen and Quynh 1984)
On a more positive note, populations of various insects possessing aquatic larvae have managed to maintain their numbers, including mayflies, stoneflies, caddisflies, and dragonflies. These have accounted for an increase in the numbers of certain local fish populations. (Yen and Quynh 1984)
Destruction of coastal mangrove forests led to a decrease in estuarine and near-shore fishery yields. Although estuarine habitats are noted for their highly productive fisheries, between 1981 and 1982 only 24 species of fish were recorded in South Vietnam (Yen et al 1984). This is substantially less than would be present in a comparable unsprayed location. Due to the reduction and alteration of fish species populations, the catfish (Clarias fuscus) and loach (Misgurnus fossilis anguillicaudatus), both species from northern Vietnam, have successfully colonized within Mangrove areas (Snedaker 1984, Yen and Quynh 1984).
Effects on Soil Ecology
The defoliants applied to the mangrove forests have also had detrimental effects on the composition of the soils. The loss of tree cover due to defoliation exposes soil to more heat and rainfall. This causes the soil to lose many nutrients and chemical elements hindering its ability to grow anything on it. Also, topsoil are easily washed away by the heavy rainfall. This leads to the loss of most organic matter and nutrients, which ultimately leads to the deterioration of soil structure, the reduction in water capacity, and the decrease in soil fauna. Added up, this results in a long-term decrease in productivity (Zinke 1984).
Studies found a decreasing trend in organic matter and nitrogen due to erosion and an increase in organic matter and nitrogen in depressions where sediments were collecting. Along the coast, organic matter in the mangrove swamps was unaffected by spraying due to the level terrain (Huay 1984). A 1980 study (Snedaker 1984) found much site damage of barren mangrove swamps by erosion and wave action since the end of the war. Mangrove swamp functions to stabilize the shoreline. As the coastline accretes, mangroves invade the new land and hold the soil against wind, wave, current, and tide. The barren mud flats and channel banks have eroded at rapid rates. Estimates of soil fertility and physical characteristics of barren areas have shown that exposure has led to significant changes in structure and fertility, although fertility levels appear to have remained suitable for mangrove growth (Zinke 1984). The majority of areas that have remained barren are those that were marginal in vegetation growth before spraying.
Most notable evidence of the effect of erosion is the abnormally high turbidity of estuarine waters within the surrounding mangrove areas. It is not known whether the turbidity is due to local erosion in mangrove system or from the deposits of upstream erosion and downstream sedimentation from inland sprayed forests (Snedaker 1984). Deposition and shoaling of these sediments have an affect on local drainage and circulation patterns that can lead to a change in local salinity gradients (Snedaker 1984). Also, there is a deleterious affect on primary production and near shore filter feeders.
Deposited sediments may have retained some residual fraction of the more persistent water-insoluble herbicides, but there is no evidence to confirm their presence here or what, if any, their affect on colonizing organisms (Snedaker 1984). Continued erosion of the mangrove sediments does have the benefit of exposing reduced sediments and lowering land elevation to water level, thus possibly speeding re-vegetation, assisting in the removal of any persistent herbicides in the surface sediments, and promoting the beneficial influence of surface water circulation and flushing. (Snedaker 1984).
Herbicidal spraying has also had indirect effects on the soil ecology of mangroves. After defoliation, the contaminated biomass falls to the soil and decomposes. The tainted materials enter the soil creating more acidic (pH 4-5) levels. Some chemicals, like 2,4-D are rapidly decomposed in the soil by eubaceteria, but others, like 2,4,5-T, have a higher persistence. In some areas treated with Agent Blue, arsenic (a component of Agent Blue) was found in soil samples. The greatest concern, however, is Dioxin (a byproduct from the manufacture of 2,4,5-T) because of its high level of toxicity. Dioxin has a high persistence in soil samples, the greatest persistence of all the xenobiotics applied. Studies show that traces of Dioxin, which was thought to have a half-life of about 4 years, can be recovered from the top 15 cm of soil after 14 years of application. In one study, soil samples obtained from inland soils and sediments in mangrove areas sprayed about a decade earlier were found to contain up to about 30 ng/kg of Dioxin (Snedaker 1984). Qualitative analyses at low and high resolution found a high concentration of PCDDs and PCDFs (two forms of Dioxin) in herbicide-treated soil samples from Vietnam. Isomers of ICDD, including 2,3,7,8-TCDD, or Dioxin, were also identified. The half-life of Dioxin was also found to be between 5-10 years longer than expected (Thu et al, 1993). Picloram also remains in soil retaining its toxicity for decades. In one test, less than 4% had vanished from a test plot in 467 days. (Neilands 1969).
Effects on Hydrology
Mangroves serve as a transitional zone between land and sea and, thus, stabilize the shoreline (Westing 1977). Forest cover in the wetlands of the Mekong Delta is vital to ensure the proper flow of water through the delta's many channels, in order to protect its banks, fish, and prawn nursery areas, and provide refuges for waterbirds (Mackinnon et al. 1991).
The absence of a healthy and well-stocked vegetative cover produces more severe flooding, leading to severe erosion and nutrient loss. It has been determined that when the vegetable cover of the watershed is destroyed, peak and overall streamflow increases.
This is stimulated by the increase in surface runoff, the absence of the water storage capacity of the biomass that has been lost, and the reduction in water loss by evapotranspiration (Barnaby 1976).
There is a possibility that the altered inland watersheds continue to have an impact on mangrove ecosystems through altered hydroperiod, excessive erosion and deposition, and the introduction of deleterious materials that could effect flora and fauna (Yen et al 1984). Insufficient data exists to evaluate these impacts.
WORKS CITED
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Barnaby, F. (Editor). 1976. Ecological Consequences of the Second Indochina War. Stockholm International Peace Research Institute.
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Yen, M.d. and N.X. Quynh. 1984. Long-term changes in the freshwa ter fish fauna following herbicidal attack. Pages 91-93 IN: A.H. Westing (Ed.). Herbicides in War: The Long-term Ecological and Human Consequences. Taylor and Francis, Philadelphia, PA..
Zinke, P. 1984. Soil Ecology: an overview, p. 75- 81. In: Herbicides in War: the long-term ecological and human consequences by A. H. Westing. Taylor & Francis, Philadelphia, PA.
Jennifer Rice and Abraham Parker are seniors in the Department of Natural Resources. Danielle E. Ogurcak is a 1999 Cornell Graduate from the Department of Natural Resources.