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

Abstract

Mangrove forests consist of a mosaic of tree cohorts, each having a regeneration historythat depends on episodic recruitment of seedlings based on the availability of a “regenerationniche”. The aim of this study was to assess seedling population structure in Ceriops tagalnatural monospecific stands and one eight-year old reforested stand at Gazi Bay, Kenya, todetermine performance in terms of growth and survival over a period of two years. Four naturalmonospecific stands (two on the western creek designated as site 1 and 2, and two on the easterncreek designated as site 4 and 5) were identified with the reforested stand (on the eastern creek) assite 3. The vegetation structure of these stands and natural regeneration were assessed by LinearRegeneration Sampling. Regeneration Class I (RCI) seedlings of known age were tagged and theirgrowth parameters (diameter, height, leaf production and number of nodes) and survival ratesmonitored for a period of two years. The reforested stand had the same basal area (2.2 ± 0.1 m2ha-1) as site 1 (2.1 ± 0.1 m2 ha-1), and the former’s mean height was also similar to that of westerncreek sites (1 and 2), but lower than the eastern creek sites (4 and 5). The western creek sites hadthe highest seedling mortality (61 % for both) and lowest growth rates recorded over the two yearperiod compared to a mortality of 45 % for the eastern sites. An analysis of seedling growth overthe two years as a function of measured environmental factors showed a weak interaction, withheight above datum (HAD) and canopy cover showing the highest correlation of only 30 %. Thehigh mortality on the western creek sites may be attributed to harsh environmental conditionsdue to the limited expanse of the intertidal area of the western creek mangroves compoundedwith anthropogenic pressure due to its proximity to human settlements. Overall, C. tagal forestsat Gazi Bay seem to be more vulnerable to environmental stress because they occur at the upperintertidal area, which limits their long-term structural development. Of the four species commonat 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 thereforerequire management approaches distinct from those of other species to ensure that their naturalregeneration and overall structural development are not unsustainably compromised.