Recovery of tropical mangrove forests following a major oil spill: a study of recruitment and growth, and the benefits of planting
Duke, Norman C., Pinzon, Zuleika S., and Prada, Martha C. (1999) Recovery of tropical mangrove forests following a major oil spill: a study of recruitment and growth, and the benefits of planting. In: Yáñez-Arancibia, A., and Lara-Domínguez, A.L., (eds.) Ecosistemas de Manglar en América Tropical / Mangrove Ecosystems in Tropical America. Instituto de Ecologia, Xalapa, México, pp. 231-254.
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Abstract
In April 1986, a large oil spill on the Caribbean coast of Panama killed approximately 75 hectares of tidal mangrove forests dominated by Rhizophora mangle. Since then, there has been extensive regeneration in sheltered sites, while those exposed to wind and waves were significantly disadvantaged by the scouring action of mobile wood fragments. The present study, conducted four and five years after the spill, compared annual growth of natural seedlings in both habitats with those in un-oiled natural light gaps nearby. Leaf node production rates, were greater in oiled sites, indicating that remaining oil in the substratum did not depress growth, and it was possibly greater because of higher light levels in more open, oil-deforested light gaps. In both places, advanced seedlings were found that appeared to be older than their respective light gaps. This is an important discovery for oiled sites, implying the need for greater care in post-spill clean-up operations, for example. But, the notion of seedlings previously growing under a shaded canopy has other important implications. Primarily, it means that such pre-established recruits have the better chance in filling gaps. For this reason, Rhizophora forests in Panama appear well-prepared for small-scale canopy damage, having this undercanopy community of established recruits in a 'seedling bank'. This bank turns over relatively fast since the plants are essentially shade-intolerant, dying after two or three years where the canopy remains closed. Nevertheless, their numbers are maintained by a seasonal supply of propagules in the second quarter of each year. For the advanced seedlings in light gaps, past growth was followed using the sequence of leaf scar nodes along the main stem, finding height production increased dramatically (4-6 times) after approximately one year following gap creation. This was the case in un-oiled and exposed oil-deforested sites, however, in more sheltered oiled sites, height production was apparently suppressed for two or three years afterwards, depending on the site. The growth of other seedlings in oil-deforested sites were also monitored, comparing natural recruits with those planted soon after the spill in an attempt at large-scale habitat restoration. Accordingly, it was found that planted recruits grew faster than natural ones (12-56%), aided by clean soil and fertilizer. However, this apparent benefit was out-weighed by both abundant natural recruitment and a significant negative effect on site recovery where planting took place. These findings bring into question the value of planting in this case. They also serve to emphasis a greater appreciation of natural recruitment and regeneration in future habitat restoration projects which may inadvertently contribute further to the destruction of already disrupted habitats.
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