Spatial and Temporal Regeneration Dynamics in Ceriops tagal ( Perr.) C.B. Rob. (Rhizophoraceae) Mangrove Forests in Kenya

Mangrove forests consist of a mosaic of tree cohorts, each having a regeneration history that depends on episodic recruitment of seedlings based on the availability of a “regeneration niche”. The aim of this study was to assess seedling population structure in Ceriops tagal natural monospecific stands and one eight-year old reforested stand at Gazi Bay, Kenya, to determine performance in terms of growth and survival over a period of two years. Four natural monospecific stands (two on the western creek designated as site 1 and 2, and two on the eastern creek designated as site 4 and 5) were identified with the reforested stand (on the eastern creek) as site 3. The vegetation structure of these stands and natural regeneration were assessed by Linear Regeneration Sampling. Regeneration Class I (RCI) seedlings of known age were tagged and their growth parameters (diameter, height, leaf production and number of nodes) and survival rates monitored for a period of two years. The reforested stand had the same basal area (2.2 ± 0.1 m2 ha-1) as site 1 (2.1 ± 0.1 m2 ha-1), and the former’s mean height was also similar to that of western creek sites (1 and 2), but lower than the eastern creek sites (4 and 5). The western creek sites had the highest seedling mortality (61 % for both) and lowest growth rates recorded over the two year period compared to a mortality of 45 % for the eastern sites. An analysis of seedling growth over the two years as a function of measured environmental factors showed a weak interaction, with height above datum (HAD) and canopy cover showing the highest correlation of only 30 %. The high mortality on the western creek sites may be attributed to harsh environmental conditions due to the limited expanse of the intertidal area of the western creek mangroves compounded with anthropogenic pressure due to its proximity to human settlements. Overall, C. tagal forests at Gazi Bay seem to be more vulnerable to environmental stress because they occur at the upper intertidal area, which limits their long-term structural development. Of the four species common at Gazi, C. tagal is the least structurally developed with very low basal areas and mean heights, which are both important determinants of wood quality. These upper-shore forests therefore require management approaches distinct from those of other species to ensure that their natural regeneration and overall structural development are not unsustainably compromised.