Grapsid crabs can play a considerable role in the predation of mangrove propagules and possibly are a threat to the regeneration of mangroves, whether natural or artificial. Experiments consisting of artificial plantation of mangrove juveniles were set up in Gazi Bay (Kenya). Grapsid crabs, particularly Neosarmatium meinerti in the most landward fringe and N. smithii and Sesarma guttatum in the middle fringe of the mangrove forest, were found to be a regeneration-limiting factor. Almost 100% (n = 40) of the landward plantations were cleared. There was a slight initial preference for Rhizophora propagules lying horizontally on the mangrove floor, which simulates the position of a stranding propagule. In Mida Creek (Kenya) the snail Terebralia palustris was observed predating mangrove propagules. This study shows that crabs may affect the regeneration potential of mangroves, and snails might also be a factor in predation. A need to actively search for ways to protect re-afforestation plots from predators of mangrove tree juveniles is necessary.

In June 1990 a research project sponsored by DAN IDA and A WF was carried out by botany students of Nairobi University to investigate the regeneration strategy of mangroves at Gazi bay and Mida creek.Statistical analysis of 449 quadrats (5 x 5 m) sampled along 35 line transects in 4 mangrove forests showed that mangrove seedlings follow the same distribution pattern in the intertidal zone as their parent trees. In other words, mangrove seedlings mainly develop within a well-defined species specific zone. These distribution zones for the various mangrove trees and their seedlings are defined in terms of elevation above the mean low water level of spring tides.Through assigning mangrove seedlings in the intertidal zone to the categories (i) 'fixed' or 'not-fixed', and (ii) 'covered' or 'not-covered', evidence was found that propagule dispersal followed both the 'self-planting' theory and the 'stranding' theory. The 'self-planting' theory appeared to be the major mechanism of propagule dispersal in 'undisturbed' mangrove forests, whilst the 'stranding theory' proved to be predominant in colonizing 'over-exploited' and cleared mangrove forests.It is concluded that re-afforestation of mangrove seedlings in the intertidal zone will be most successful when the seedlings are planted in their specific distribution zones under fixed conditions.

SUMMARYCation-exchange processes of plant tissue are often hypothesized to affect ion uptake. Fixation of cations on exchange sites could either withdraw cations from further uptake, or concentrate them for the absorption sites. For a number of bryophyte taxa (Atrichum undulatum (Hedw.) P. Beauv., Homalothecium sericeum (Hedw.) B.S.&G., Leucohryum glaucum (Hedw.) Angstr., Mniiim hornum Hedw., Plagiomnium undulatum (Hedw.) T. Kop., Polytrichum formosum Hedw.), these contrasting hypotheses were tested by studying cation-exchange properties and soil preference of populations with respect to soil acidity and soil exchangeable cations (Al, H, Fe, Mn, Ca, Mg, K). Acidiphilous and acidicline taxa (preference for soils with pH < 5) with relatively lower cation-exchange capacities tolerate high exchangeable aluminium levels and low calcium levels. Neutrocline taxa (preference for soils with pH > 5) with higher cation-exchange capacity avoid aluminium in the substrate and thrive on calciumrich substrates. Cation-exchange properties therefore do not protect mosses against potentially toxic cations such as aluminium through a sequestration mechanism. However cation exchange sites may increase the availability of cations. High cation-exchange capacity was indeed shown to favour fixation of aluminium over calcium ions and hence a low capacity is a prerequisite for taxa adapted to acid soils.

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Summary Measurements were carried out of the gas exchange properties (namely, photosynthesis, stomatal conductance and transpiration rates), water use efficiency and water relations of two mangrove species, Rhizophora mucronata and Ceriops tagal at Gazi Bay, Kenya. Rhizophora mucronata had significantly higher photosynthetic rates than C. tagal. Internal CO2 concentrations were higher during the wet season than the dry season in both species. Gas exchange properties were correlated positively with photon flux density in both species. Leaf water potentials were highest during the morning and lowest at midday and were also highest in the lower canopy leaves in both species. The two mangrove species had conservative water use. Management potential for the East African mangroves based on the results of this study is suggested.