Comparative study of the disjunct zonation pattern of the grey mangrove Avicennia marina (Forsk.) Vierh. in Gazi Bay (Kenya)

The disjunct zonation pattern of Avicennia marina (Forsk.) Vierh. , frequently observed along the Kenyan coast as a landward and a seaward zone, has been studied on a morphological level and complemented with preliminary genetic data. The objective was to describe the two zones in-depth in order to provide a background ecological setting that may be used in explanations on the emergence of such a bimodal zonation. The vegetation structure in the two zones is described by means of the Braun-Blanquet releve and PCQ-methods, root- and leaf-morphological characteristics, and environmental variables to provide a background ecological setting. The two zones show considerable differences in physiognomy of A. marina trees, and indicate that the differences may constitute an environmental response. Results show that in the landward A. marina zone, tree heights, leaf sizes, density, and total length of pneumatophores tend to be smaller as compared to the same attributes of the trees in the seaward zone. Differences in leaf stomatal density and in above- and below-ground pneumatophore length, the latter closely related to the effect of inundation, were observed. This suggests a considerable adaptability and plasticity with respect to environmental conditions. The hypothesis for the genetic analysis was that mangrove fringes as close as 105 m from one another constitute a same genetic pool. Preliminary genetic analysis using 48 amplified DNA fragments (RAPD - Randomly Amplified Polymorphic DNA) from 37 landward and seaward trees revealed significantly different frequencies for four DNA fragments, indicating that there might be less contact and genetic interchange between the two zones than within each zone. Whether this phenomenon, coupled to the different environmental conditions, can lead to further divergence cannot be answered yet. Of all mangrove zonation patterns described worldwide by various authors, that of Avicennia marina (Forsk.) Vierh. provides an incentive for particular analysis because of its disjunct nature (Watson, 1928; Macnae, 1968, 1969; Johnstone, 1983; Smith, 1987a, 1992). In Kenya this species displays a disjunct zonation pattern across the intertidal zone (Dahdouh-Guebas et al., 2002). Avicennia marina is present in the most landward as well as seaward fringe of the mangrove forest at certain locations along the Kenyan coast. The same pattern has been reported for other specific regions of the world; however, it has been described visually or superficially without intensive sampling or in-depth comparisons. Walter and Steiner (1936) described the higher tidal A. marina trees as “exceptions” of seaward specimens. Other authors (e.g., Macnae, 1968; Johnstone, 1983; Smith, 1992) reported a “double zonation” of A. marina, but did not go beyond stating this observation or mentioning as personal observations that tree height or leaf size varies between the upper and lower intertidal zones. This paper provides a more rigorous comparison of the landward and seaward A. marina fringe on a morphological level (such as tree height, leaf size, root density, etc.), as an ecological setting framing the emergence of the two disjunct zones. We also provide preliminary findings on genetic differentiation between the observed zones. Rather than aiming at a generalization from