Phenology of rhizophora mucronata LAMK, avicennia marina (FORSSK.) VIERH. and Sonneratia alba SM in natural and reforested mangrove forests at Gazi Bay, Kenya
This study was conducted at Gazi Bay (4° 25'S, 39° 3' E) in the southern Kenyan coast, 50 km from Mombasa. The aim of the study was to investigate the phenology of three principal mangrove species; Rhizophora mucronata Lamk., Avicennia marina (Forsk.) Vierh. and Sonneratia alba Sm in natural and reforested stands. The study was conducted in the period between January 2005 and December 2006. Direct shoot observations to establish the phenology of A. marina, R. mucronata, and S. alba was carried out in randomly selected plots measuring lOx 10m in monospecific natural and reforested stands. Shoots in randomly selected trees were tagged and observations on leaf emergence, leaf fall, appearance of buds, flowers and propagules were monitored on a fortnight basis. Rhizophora mucronata and S. alba were observed to have continuous growth throughout the study period. However, their reproduction was seasonal though there was an overlap on the reproductive cycle of R. mucronata. The reproduction cycle from bud initiation to propagule fall was estimated to take 17-18,6-7 and 4-5 months in R. mucronata, A. marina and S. alba respectively. Propagule maturity took 11-12 months in R. mucronata and 3-4 months in A. marina and S. alba. Leaf longevity of the three species decreased in the following order R. mucronata > A. marina> S. alba. Mean leaf longevities of 13, 11 and 9 months were recorded for the three species respectively. The phenology of A. marina in Gazi Bay which is disjunctly zoned with landward, midland and seaward A. marina trees was observed to be clearly seasonal and was influenced by its location on the intertidal zone. This species was observed to be highly adaptable and flexible in terms of growth. Landward and midland trees showed a unimodal pattern of leaf emergence and fall whereas the seaward trees showed a bimodal pattern. Bud initiation in A. marina started in November irrespective of the zone but flowers appeared earlier (December) in the landward trees and in January for the seaward trees. However, peak propagule fall in all the zones was in April and May, though fruits could still be observed up to July in the seaward natural forest. Assessment on litter productivity of the three species was conducted by use of litter traps of 0.25 m2 trap mouth randomly placed in the same plots used for phenological observations. Litter was then dried in the oven at 80°C until constant weight was obtained and later sorted into leaves, stipules, twigs, reproductive parts (buds and flowers) and propagules and weighed. Results on litter production resembled those of direct shoot observation. Avicennia marina showed seasonal litter production as opposed to R. mucronata and S. alba. Rhizophora was the most productive of the three species with mean annual total litter production rates of 8.94-11.02 t ha'i yr' for the Kinondo sites followed by S. alba (7.85-10.15 t ha' y(1 ) and A. marina (2.51-5.44 t ha' yr' ). Leaves contributed the largest percentage (>60%) to total litter fall for all species. At Kinondo the reforested thinned site produced more leaf litter (9.52±0.9 t ha'i yr') than the reforested unthinned and natural sites (8.69±0.9; 8.03±0.8 t ha' yr'). Viability tests of A. marina fruits and R. mucro nata propagules was done by planting various sizes of propagules in nursery with the aim of establishing which seed sizes can be incorporated or utilised in the propagation of seedlings for planting purposes, especially in months when propagule availability is limiting. Results indicated that seed size did not influence initial seed germination. However, subsequent growth and development of the saplings was directly related to the initial seed/propagule size. In A. marina seeds with a diameter of 21-25 mm and above gave the best growth performance and are thus recommended for raising seedlings for reforestation. Propagules of R. mucronata that were observed to have more vigour in early growth and establishment were those with a length of 31-40 cm and above. However, during periods of critical seed availability 21-30cm propagules can also be used. This study recommends the use of the generated information in mangrove forest management by forest managers. However, long term monitoring outside a thesis concept is recommended if effects of climate change on world mangroves are to be properly understood.