No abstract

Mangroves are intertidal ecosystems that are particularly vulnerable to climate change. At the low tidal limits of their range, they face swamping by rising sea levels; at the high tidal limits, they face increasing stress from desiccation and high salinity. Facilitation theory may help guide mangrove management and restoration in the face of these threats by suggesting how and when positive intra-and interspecific effects may occur: such effects are predicted in stressed environments such as the intertidal, but have yet to be shown among mangroves. Here, we report the results of a series of experiments at low and high tidal sites examining the effects of mangrove density and species mix on seedling survival and recruitment, and on the ability of mangroves to trap sediment and cause surface elevation change. Increasing density significantly increased the survival of seedlings of two different species at both high and low tidal sites, and enhanced sediment accretion and elevation at the low tidal site. Including Avicennia marina in species mixes enhanced total biomass at a degraded high tidal site. Increasing biomass led to changed microenvironments that allowed the recruitment and survival of different mangrove species, particularly Ceriops tagal.

ABSTRACT1. This study reports above-ground biomass of 5 and 8 years old mangrove plantations in Kenya. Trees with stem diameter greater than 5.0 cm inside 100 m 2 sample plots were harvested, and then separated into stems (trunks), branches, leaves and prop roots.2. Mean above-ground biomass was calculated at 20.25 t dry matter ha À1 for Rhizophora mucronata Lam., 11.7 t dry matter ha À1 for Avicennia marina (Forsk.) Vierh., 6.7 t dry matter ha À1 for Sonneratia alba Sm. and 3.7 t dry matter ha À1 for Ceriops tagal (Perr.) C. B. Robinson. In A. marina and R. mucronata, stems (52.19%) and prop-roots (30.28%), respectively, accounted for the highest proportion of the above-ground dry weight. While in S. alba and C. tagal, branch biomass represented the highest percentage of biomass, 48.20% and 43.62%, respectively.3. The total above-ground biomass of R. mucronata was best estimated from regression equations using a combination of height and diameter above stilt root as the independent variables. For A. marina, C. tagal and S. alba there was no simple correlation found between the above-ground biomass and tree height or stem diameter.4. Comparison of the regression models with those developed elsewhere gave different biomass values in these plots, further reinforcing the need for the use of site-specific allometric equations for biomass estimation.

Mangrove reforestation projects often suffer from low sapling survival, especially after transplanting saplings from nurseries to reforestation areas. This may be due to the sediment conditions at the target site, the planting strategy or failure to reestablish ecosystem processes. We examined experimentally the influence of environmental context, species richness and identity, sapling height and position on sapling survival and environmental variables linked to ecosystem functioning at deforested sites in Gazi Bay, Kenya. At site 1, a high shore location, 32 plots (36 m 2 ) were planted with 8 treatments: all possible combinations of Avicennia marina, Bruguiera gymnorrhiza, and Ceriops tagal and an unplanted control (total: 3390 saplings; 4 plots/treatment). At site 2, a low shore location, the influence of sapling height, sapling position and sediment depth were tested by planting with 697 Sonneratia alba in a single monospecific plot (341 m 2 ). After *2 years, there were significant differences in survival among the three species at site 1 with Bruguiera gymnorrhiza recording the lowest survival rate (29%). Survival was correlated with salinity (a strong effect) and height above chart datum (a weaker effect) at site 1. Sapling position did not significantly affect survival at either site. There was thus no evidence that early survival of transplanted saplings is influenced by the species mix in which they are grown, or by their position in the plot. Rather the tolerance of individual species to salinity was the key to their survival at the high tidal site. Species mix also had no significant effects on environmental variables in the plots. The former presence of a species at a site does not guarantee it will succeed there again if environmental degradation has exceeded species' tolerance.

Enhanced species richness can stimulate the productivity of plant communities; however, its effect on the belowground production of forests has scarcely been tested, despite the role of tree roots in carbon storage and ecosystem processes. Therefore, we tested for the effects of tree species richness on mangrove root biomass: thirty-two 6 m by 6 m plots were planted with zero (control), one, two or three species treatments of six-month-old Avicennia marina (A), Bruguiera gymnorrhiza (B) and Ceriops tagal (C). A monoculture of each species and the four possible combinations of the three species were used, with four replicate plots per treatment. Above- and belowground biomass was measured after three and four years' growth. In both years, the all-species mix (ABC) had significant overyielding of roots, suggesting complementarity mediated by differences in rhizosphere use amongst species. In year four, there was higher belowground than aboveground biomass in all but one treatment. Belowground biomass was strongly influenced by the presence of the most vigorously growing species, A. marina. These results demonstrate the potential for complementarity between fast- and slow-growing species to enhance belowground growth in mangrove forests, with implications for forest productivity and the potential for belowground carbon sequestration.