REHABILITATION OF OIL- DAMAGED MANGROVES

When mangroves have been killed by oil there is often a great interest in rehabilitation of the forests based on a desire both to re-establish the important mangrove ecosystems and to restore the appearance of the shoreline by replacing the unsightly dead blackened trees with live green ones. Positive steps can be taken to achieve this, though it is probably inevitable that rehabilitation schemes will have to focus on one or a few key species of tree—restoration of the full complexity of the mangrove ecosystem will depend upon subsequent natural processes.

It is obvious that regeneration either by natural recruitment or by artificial replanting immediately after an oil spill would be unlikely to restore the forest: whatever there was about the oil that killed the trees would almost certainly still be present to kill or inhibit the growth of natural recruitment or planted mangroves. An example of post-spill toxicity was demonstrated at the Refineria Panama spill of crude oil in 1986 where groups of red mangrove propagules were planted at sites at which the mangroves had been killed by oil. (The term ‘propagule’ in this case refers to seedlings growing out of fruits; they start doing this while the fruits are still attached to the trees, and are the normal, tidally-dispersed propagating organs of the red mangrove.) Those propagules planted three months and six months post-spill all died; however, some of the propagules planted nine months post-spill and larger fractions of those planted later survived.

The time that must elapse after an oil spill for the soil to become sufficiently nontoxic for natural propagules or replanted mangroves to survive depends on factors such as the kind of oil spilled, the type of soil, local tidal flushing and rainfall. That mangroves are able to grow does not require that all oil has been eliminated from the soil. Mangroves can become established and apparently grow normally at oil spill sites where there is residual weathered oil, as indicated by the disturbed soil giving off an odour of crude oil and an oil sheen appearing on the water surface. Such evidence of past oil spills may persist in mangrove soils for more than a decade.

Natural regeneration of oil-killed mangrove forest can be expected to occur, but the process may be slow as a consequence of residual oil toxicity or because propagules (or seeds) of mangroves, which are ordinarily dispersed by tidal waters, may be unable to reach the sites affected by oil because of barriers of fallen branches, aerial roots and trunks of the killed mangrove forest. In some cases regeneration may be slow because there are not enough live trees locally to provide an adequate seed supply.

One approach to restoring an oil-killed mangrove forest as soon as possible after the oil spill was used at the site of the Refineria Panama spill. Cylinders of oiled mangrove soil were removed by use of post hole diggers and nursery-grown seedlings planted in the holes or the holes filled with upland soil and propagules planted. In either case the developing roots of the seedling plants were protected from residual oil in the soil by a buffer of non-oiled soil in which the roots of a seedling could grow while the toxicity of oil in the soil continued to decrease through weathering and tidal and rainfall washing.

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