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AbstractThe first objective of this present study was to determine the natural regeneration potential of reforested mangrove stands at Gazi Bay, Kenya in terms of successive regeneration classes and assess contribution of previously recruited saplings to the overall structural stand development of the plantations. The second objective was to assess seedling establishment and population structure of seedlings of a known age to determine the average population of these seedlings, which reaches the sapling stage and thus subsequently contributes to stand complexity. Regeneration classes (RCs) were categorized into three classes based on height i.e. RCI < 40cm, RCII < 40.1 – 150cm and RCIII > 150cm but with dbh of less than 2.5cm. For the first objective, the study was conducted in two reforested monospecific mangrove stands (Sonneratia alba and Rhizophora mucronata), while for the demographic study, four natural and one reforested Ceriops tagal stands were used. All the replanted stands were 8 yr old. For environmental parameters, respective bare and natural sites were used as controls for the first study, while for vegetation structure and seedling/sapling recolonisation, the respective natural sites above were used as controls. Environmental parameters were not measured in the second study, due to lack of an appropriate bare control. The criteria for selection of these control sites was based on physical proximity, site history and inundation class. Interstitial water salinity and temperature were generally similar among the S. alba forest sites, while salinity differed significantly among the three sites of the R. mucronata forest with the bare site having the highest salinity (56 ± 2), and the natural site had the lowest (36 ± 0.4). Temperature was similar among the reforested and natural sites of the R. mucronata forest, which was significantly lower than that in the comparable bare site. pH, NO-3 and PO-4 concentrations generally didn’t differ significantly among sites within the two forests. Moisture content was highest in natural sites and lowest in bare sites within mangrove types. Organic matter content significantly differed among sites within mangroves types with the bare sites having the lowest, while the natural sites had the highest. Clay content was similar among the reforested and natural sites within the R. mucronata forest, whereas the bare site had the lowest. For the S. alba forest, the natural site had a higher clay content than both the bare and reforested sites which had a similar content. R. mucronata was the most dominant species within both its reforested (97%) and natural (84%) sites and this species also had the highest stem density of 3,022 ± 228 stems ha-1 and 1,502 ± 191 stems ha-1 in the reforested and natural sites respectively. The R. mucronata reforested stand had four adult mangrove species (the same species richness as the natural stand), which contributed to a higher complexity index than the S. alba reforested stand, which was monospecific for the adult tree species. For juvenile recolonisation, species richness and type were similar among the R. mucronata sites (reforested and natural), although the total juvenile density was significantly higher (p < 0.000) in the natural stand (7,390 ± 660) as compared to the reforested stand (2,048 ± 667). The converse was true for the S. alba stands where the reforested stand had a higher juvenile density (5,704 ± 647) than the respective natural stand (1,008 ± 194). Results therefore suggest that mangrove reforestation has modified site conditions and encouraged recolonisation of new/nonplanted mangrove species. Previously recruited seedlings have also grown into adult trees, which contributed to the higher species diversity and complexity index in the R. mucronata reforested stand, which was initially monospecific.
Publisher or UniversityWestern Indian Ocean Marine Science Association
Series : NrWIOMSA/MARG-I/2009 –05;