Mangrove reforestation in Panama: an evaluation of planting in areas deforested by a large oil spill
Duke, Norman (1996) Mangrove reforestation in Panama: an evaluation of planting in areas deforested by a large oil spill. In: Field, Colin D., (ed.) Restoration of mangrove ecosystems. International Society for Mangrove Ecosystems ISME & International Tropical Timber Organization ITTO, Okinawa, Japan, pp. 209-232.
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Abstract
[Extract] Although the monitoring study commenced in February 1989, it was apparent that natural regenerative processes of R. mangle forests were under-estimated in the earlier planting program. Following the large oil spill in April 1986, the Refineria Panama initiated its large scale planting program based on the rationale that, they were responsible for the large oil spill and resulting deforestation of mangrove, and they wanted to restore these areas to their pre-spill condition. The methodology of planting was developed from preliminary experiments and observations which determined that, natural recruitment was disrupted and inadequate, and growth and survival of seedlings was seriously suppressed by residual oil in sediments. However, as described in this chapter, natural recruitment following the spill was both plentiful and effective in most areas, and growth and survival of established plants were not seriously affected by residual oil in sediment. Furthermore, planted seedlings were expected to die in most oil gaps since natural recruits were beginning to shade them, particularly in sheltered sites. Such observations imply that many sites might have been no worse off, had there been no planting. In support of this proposal, the largely successful restoration of 49 ha of mangroves from an earlier large oil spill in Bahía Las Minas in 1968, was completely unassisted (Duke et al. in press c). On the other hand, it is suggested that assistance was required to protect damaged foreshore areas because despite differences in assistance between the 1968 and 1986 spills, equivalent areas of exposed habitat were lost by erosion after each. Following the 1986 spill, both planted and natural seedlings were unable to develop sufficient size in exposed sites before protective above-ground roots of oil-dead R. mangle trees had decomposed, 5-6 yr post spill (Fig. 15.3). Young plants were damaged and killed by drift material and erosion. These seedlings required at least another 2-3 yr development before they could become independent in such exposed sites. Planted recruits might have met the requirement to be independent sooner since their growth rates were apparently increased with upland soil and fertilizer, but the 1988 planting was too late in exposed 1986 oil-damaged sites. As a result, many exposed oil-damaged sites were lost or seriously eroded, 7-8 yr post spill, after showing vigorous recruitment earlier when dead above-ground roots were present. In retrospect, it might have been better to have conducted an interactive restoration program, balancing the benefits of selected planting, enhancing natural recruitment and growth, with the installation of devices to protect seedlings and surviving trees where they were vulnerable to damage once protective oil-dead stumps and roots began to deteriorate. It was also apparent from the monitoring study that the planting might have altered or damaged some sites. A comparison of plots in planted and not planted sites showed planted ones had lower densities, shorter trees and smaller vegetated biomass, in both sheltered and exposed locations. It is not known how these factors might effect long term recovery of oil gap sites, but the effect is considered detrimental. It is possible also that planting activities may not have caused the situation since the planting program might have planted seedlings in more damaged sites, implying unintentional bias in the sampling strategy of the monitoring study. But, this seems unlikely for several reasons. First, it was difficult to distinguish and rank oil-damaged sites based on anything but size, without conducting an expensive hydrocarbon sampling program. Second, sites in the monitoring study were chosen randomly. And third, there were independent observations of greater site alteration and activities than reported (Fig. 15.3). These observations combined with other possible reasons for negative effects on mangrove forest recovery caused by planting activities include: cutting and removal of dead timber including boat access, removing shelter for seedlings and sediments, especially in exposed locations; trampling of sediment resulting in soil compaction and release of residual oil; digging holes for planting, assisting erosion and release of residual oil; and interfering with natural recruitment by direct damage to existing seedlings (from the previous three activities), and disruption to seedling establishment in the future. In summary, there are several recommendations from the monitoring study in Bahía Las Minas which might be applied to oil spills affecting mangroves in the future and elsewhere. The recommendations include: • assessing methods of cleaning and promoting the survival of 'key' fringing mangrove trees oiled in exposed locations, at the time of the spill; • mapping the extent of oiling at the time of the spill, and storing samples of floating oil; • mapping areas of defoliation and subsequent deforestation shortly following the spill; • measuring the concentration of oil in sediments 1-2 mo post spill, and regularly afterwards; • assessing the condition of deforested sites for original structure and composition; • assessing the condition of deforested sites for presence/absence of seedlings (old seedling bank); • determining seasonal variation and availability of local propagule supply; • measuring topographic and sediment profiles across the mangrove zone and below mean sea level, and monitoring these for at least 7-8 yr post spill; • evaluating the benefits and methods of physically protecting seedlings in oil-damaged exposed areas; • assessing methods to accelerate seedling growth (like upland soil and fertilizer), particularly in oil-damaged exposed areas; and • evaluating the benefits of planting, including density and species selection. It is strongly suggested that sites of long term biological evaluation and the restoration program be established as soon as possible after a spill to have the most benefit. In Panama, and most other places, restoration of mangrove forests following large oil spills had chiefly focused on replanting in deforested oil gaps (Wardrop 1987). In these instances, recovery might take at least 25-30 yr for mangroves to approach their pre-spill condition, based chiefly on plant growth estimates of Rhizophora species (e.g., Cintrón et al. 1981, Cintrón and Schaeffer-Novelli 1984; Lamparelli et al. 1991). The kinds of assistance applied in the past included removal of oil and/or replanting. However, in providing assistance we presume, suspect, or know, that natural processes, like recruitment and plant growth, are unable to repair habitat damage both in the short and longer term, or either. Unfortunately, our collective experience with these matters remains inadequate so all measures applied must be treated as experimental. Therefore, restoration programs must be subject to close scrutiny, and they must be carefully monitored to assess their success or failure. If this is not done, we run the risk of re-applying damaging techniques which might worsen already fragile conditions in oil-damaged mangroves.
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