Mangrove biodiversity survey south of the Onilahy River - Frontier ...

Discussion
As the Lovokampy mangal has not been studied previously there is no map to compare it to but
residents of the local village stated that approximately five years ago the mouth to the creek became
blocked thus allowing no water flow through the mangal. This considerably changed the shape, the
mangal decreasing a lot in surface area as it became much drier (Frontier-Madagascar, unpublished
data). The creek opened up again and was open during the visits to the mangal in May and June
2002. However a recent visit to the mangal in November 2002 revealed that the creek had again
become blocked, and little seawater was entering the mangal.
Both A. marina and C. tagal had a higher number of seedlings than adults. B. gymnorrhiza, R.
mucronata, L. racemosa and X. granatum all had more adults than saplings and seedlings. This is
shown in Figure 23 to Figure 25 and Table 8.
The local people, both from the village of Lovokampy and from surrounding villages use the
mangrove trees for a variety of uses including firewood and fencing materials (Frontier-
Madagascar, unpublished data). Semesi and Howell (1989) state that both A. marina and C. tagal
are often used for these purposes and this could account for the low proportion of adults in relation
to the seedlings for these two species. As there was such a high percentage of seedlings it would
seem that regeneration of both A. marina and C. tagal is possible.
The percentage of adult B. gymnorrhiza is only slightly greater than that of juveniles (saplings and
seedlings) suggesting a healthy population. While the populations of L. racemosa, and R.
mucronata have a much lower percentage of juveniles which would indicate that regeneration is
unlikely. Only one adult of X. granatum was observed so the future of this species in the mangal
seems very unlikely, especially when considered in conjunction with the high salinity of the mangal
(see below). It seems likely that this individual was seeded from the Lavadanora mangal.
As shown by Figure 26 A. marina has the average tallest height of an adult of all the species in the
mangal with a mean height of 4.0m. R. mucronata had a mean height of 2.8m, L. racemosa had a
mean height of 1.7m, C. tagal had a mean height of 1.6m, B. gymnorrhiza had a mean height of
1.4m while the X. granatum tree had a height of 1.1m. None of these species have reached their
potential height. The trees can reach 13m, 20m, 3m, 7m, 18m and 10m respectively (Semesi,
1996). The low average heights achieved in this mangal could be attributed to the fact that the
mangal was cut off from inflow of seawater for a significant amount of time, therefore stunting the
mangrove trees. For example C. tagal occurs as short stunted trees in very saline environments. In
addition to this A. marina, the dominant species present is extremely suitable for coppice (regular
cutting to form new shoots) (Semesi, 1996), which the locals regularly chop for use as firewood and
building materials (Frontier-Madagascar, unpublished data) contributing to the general low height
of this species. Casual observations during the survey showed that A. marina was utilised both in
terms of grazing and chopping. Chopping appeared to be spread throughout the mangal rather than
concentrated in one area, with two exceptions of extensively cleared areas on the outskirts of the
mangal where many stumps were observed. Trees chosen for harvest tended to be those that had
died after the creek blocking (1990-1995) (Frontier-Madagascar, unpublished data).
The Lovokampy mangal has an overall high soil salinity, which varies between 4 and 155+ ppt as
shown in Table 9. The quadrats, which had low soil salinity measurements were near the
freshwater source that emerges from the bottom of the cliff. The rest of the mangal, less influenced
by this freshwater had much higher salinity. An explanation for this is most likely to be the mouth
of the creek being blocked at intervals throughout the years (Frontier-Madagascar, unpublished
data), culminating in high soil salinity due to evaporation. A. marina is dominant and widespread
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