Environmental Engineering Reference
In-Depth Information
19.5
Discussion
Improper selection of mangrove rehabilitation sites took place frequently after the
seismic displacement of the intertidal zone. Even where experienced mangrove
rehabilitation practitioners were enlisted, the mechanisms of post-disaster project
planning and project management superseded careful assessment.
Although this paper is largely anecdotal, one single monitoring event's worth of
evidence of natural recovery versus planting success was undertaken, however it
was not performed as an academic study, but rather in the context of disaster relief
programming. What is notable, is that natural recovery, in some sites, had already
exceeded success criteria of the rehabilitation project. Stem densities per hectare
were higher than project targets in sites chosen for rehabilitation at both Simeulue
and Singkil. Species diversity was certainly higher due to natural recruitment than
planting alone, but should be described as a percentage of known local species
diversity present in nearby reference forests. Rate of recruitment was calculated
simply by dividing species density over time since disturbance, but should be
tracked through several monitoring events to develop a more realistic linear pro-
gression. A pair of assessment methods are given in the conclusions to assist project
managers in more deliberate planning.
In Teluk Dalam, Simeulue, natural recruitment was expected, as seismic uplift
was less than the tidal range, and signifi cant numbers of adult trees were noted not
only to have survived the disturbance event, but to have remained fecund. In Eastern
Simeulue, however, it was hypothesized that stem density would be signifi cantly
lower and natural recruitment rates signifi cantly slower than Teluk Dalam, as many
adult mangrove forests were uplifted entirely out of the zone of tidal infl uence.
Enough trees, however, remained both alive and fecund, and stem densities of
greater than 2,000 stems per hectare were encountered.
In Singkil Lagoon, which underwent seismic subsidence, adult trees died, but
enough remained alive and fecund in order to colonize newly attenuated intertidal
surfaces. Essentially, mangroves were noted to have shifted “upwards” along with
the tidal frame. Mangroves planted in the micro-deltas formed at the far end of
the lagoon grew initially but died off within a year due to rapid sedimentation,
evidenced by colonization of
Casuarina
. Caution needs to be taken at all times
when attempting to plant mangroves, so that their habitat requirements are met, but
an extra degree of caution is needed in the case where a coastline is resettling after
a period of disturbance. Such is the case in Aceh, where the subduction of tectonic
plates begins anew, and displaced substrates are in a state of fl ux, due to geomor-
phological deformation, changing patterns of sedimentation (Fig.
19.7
), compaction
and other factors.
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