Colonization of non-planted mangrove species into restored mangrove stands in Gazi Bay, Kenya

Recruitment of non-planted mangrove species into Rhizophora mucronata, Sonneratia alba and Avicennia marina reforested stands (all of them 5 years old) was investigated to assess possibilities for natural colonization. Corresponding bare (denuded or open without mangroves) and natural (relatively undisturbed) sites were used as controls. Interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover. Also, the bare sites were more sandy, whereas those with mangrove cover had more clay and silt. There was no apparent recruitment of non-planted mangrove species into the bare areas, but the reforested stands of S. alba, A. marina, and R. mucronata had 5400, 4000 and 700 recruits ha-1, respectively of different mangrove species. The results therefore suggest that mangrove reforestation has facilitated natural colonization of sites, most likely by altering local hydrodynamics

Spatial and temporal regeneration dynamics in <i>Ceriops tagal</i> ( 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

Structural development and productivity of replanted mangrove plantations in Kenya

Forest structure and productivity was investigated in a 12-year-old Rhizophora mucronata Lam. plantation at Gazi Bay, Kenya. Sampling was carried out in 22, 10 m Ã- 10 m quadrats laid along belt transects perpendicular to the waterline. Within each quadrat all trees with stem diameter greater than 2.5 cm were identified, position marked and counted. Vegetation measurements included tree height (m), canopy cover (%) and stem diameter measured at 1.3 m above the ground (D130); from which were derived basal area (m2/ha); stand density (stems/ha) and biomass (t/ha). Information regarding composition and distribution of juveniles was derived using linear regeneration sampling (LRS). The replanted forest had a stand density of 5132 stems/ha; with a mean canopy height and stem diameter of 8.4 ± 1.1 m (range: 3.0-11.0 m) and 6.2 ± 1.87 cm (range: 2.5-12.4 cm), respectively. The total juvenile density was 4886 saplings per hectare; 78.6% of which constituted the parental canopy. The standing biomass for the 12-year-old R. mucronata plantation was 106.7 ± 24.0 t/ha, giving a biomass accumulation rate of 8.9 t/(ha year)

Spatial variations in macrobenthic fauna recolonisation in a tropical mangrove bay

Recolonisation by crab species and sediment-infauna taxa (at class level) in artificially regenerated mangrove stands of Avicennia marina, Rhizophora mucronata and Sonneratia alba (5 yr old) were studied using respective bare sites (open without mangroves or denuded) and natural sites (relatively undisturbed) as controls. The controls were chosen based on site history, physical proximity and tidal inundation class in reference to the particular reforested mangrove stand and samples randomly taken. A number of environmental variables were measured; interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover, with the natural sites having the highest content. The bare sites were generally sandier, whereas the areas with mangrove cover had higher proportions of clay and silt. Generally, there was a higher crab density in the reforested sites than in the bare sites, whereas crab species diversity (Shannon diversity index) did not vary from one site to another for any of the mangrove species. In terms of crab species composition, the reforested sites were more similar (Sørensen similarity coefficient) to the natural sites and less to the bare controls. For sediment-infauna, the reforested sites had a significantly higher density than the respective bare controls, while the natural sites had the highest density. The number of sediment-infauna taxa in both the reforested and natural sites of all the mangrove species was similar and higher than in the comparable bare sites. The results suggest that the reforested sites are supporting more faunal recolonisation, and therefore becoming more akin to the natural mangrove sites in terms of the investigated functional indicators. The findings seem to support the use of artificial mangrove regeneration (in areas where natural regeneration has been impeded by physical conditions or otherwise) as an effective management tool in the restoration and conservation of the functional integrity of degraded mangrove habitats

Predation on propagules regulates regeneration in a high-density reforested mangrove plantation

Seedling recruitment underpins natural regeneration and contributes to the restocking of forest stands, which in the long term determines the structure and functioning of mangrove ecosystems. Propagule predation (among other factors) has been found to play an important role in determining seedling distribution patterns in many mangrove stands. The role of propagule predation in influencing the observed regeneration in a Rhizophora mucronata reforested stand (9 yr old) with various spontaneously established mangrove species was investigated by looking at the effects of pruning, position (planted/vertical or prone/horizontal on the forest floor to simulate stranding) and species (Rhizophora mucronata, Ceriops tagal and Bruguiera gymnorrhiza) found to be spontaneously growing in the plantation. Predation intensity was higher (p = 0.05) in the pruned part of the stand than in the unpruned part. Prone propagules were preyed on more (p = 0.05) than the planted category, while R. mucronata was the species least preyed on compared to C. tagal and B. gymnorrhiza, suggesting that predation favors the recruitment of R. mucronata into this conspecific stand. Significant differences among prone and planted propagules suggest that predation is intense during the stranding phase, and this may play a critical role in limiting seedling establishment and subsequent recruitment. Predation is suggested to be a regulator of competition in this high-density stand by eliminating or reducing potential competitors, alluding to a ‘mutual relationship’ between crab predators and mangrove trees

Trends and spatial variation in water and land footprints of meat and milk production systems in Kenya

Global consumption of livestock products is increasing steadily due to human population growth, poverty reduction and dietary changes raising the demand for already scarce freshwater and land resources. Here, we analyze the changes associated with direct and indirect use of freshwater and land for meat and milk production in three production systems in Kenya between the 1980s and 2000s. We use two resource use indicators, the water footprint (m3/year) and land footprint (ha), to assess changes in freshwater and land use for cattle, goats, sheep and camels in arid, semi-arid and humid production systems. We estimate actual water and land use using Kenya-wide data for yields, feed composition and feed conversion efficiencies. Our results show that the amounts of freshwater and land resources used for production are determined mainly by production volumes and feed conversion efficiencies. Total water and land footprints of milk production increased for goats, sheep and camels but decreased by half for cattle in arid and semi-arid production systems, in correspondence with similar changes in the total numbers of each livestock species. Green water and grazing land footprints dominated in all production systems due to the predominance of indirect use of water to support forage production. The per unit meat footprint for cattle increased significantly between the 1980s and 2000s in all production systems, due to adverse trends in feed conversion efficiency, while changes in the water and land footprints of other animal products were small, due to modest changes in all influencing factors. In contrast, national average footprints per unit of beef and milk show a modest decrease due to a relative shift of production to the more resource-efficient humid production system. Given the potential increase in demand for livestock products and limited freshwater and land availability, feed conversion efficiencies should be improved by rehabilitating degraded rangelands, adopting improved breeds and using appropriate feed compositio

Success rates of recruited tree species and their contribution to the structural development of reforested mangrove stands

In response to widespread global degradation of mangrove forests, many initiatives have been undertaken to reforest degraded sites. However, many of these replantings are monospecific, raising questions regarding habitat change and reduced ecological functions in mangrove plantations compared to natural mixed mangrove forests. The main objective of this study was to determine whether colonising mangrove juveniles can be recruited into the adult tree layer and thus enhance the structural complexity of previously reforested monospecific stands. Vegetation structure of originally monospecific replanted mangrove stands (Sonneratia alba and Rhizophora mucronata) that were 8 yr old was assessed. Instantaneous juvenile (seedling/sapling) diversity and abundance were determined. Adjacent natural stands were used as reference sites. The R. mucronata stand had the same species richness as its natural reference and thus had a higher Complexity Index due to recruitment of non-planted species into the adult tree population than in a previous assessment when it was monospecific. Juvenile species richness and diversity were similar between the R. mucronata stands (reforested and natural); however, the total juvenile density was significantly higher (p –1) than in the reforested stand (2048 ± 667 juveniles ha–1). In contrast, the adult vegetation layer of the S. alba reforested stand remained monospecific, suggesting 100% mortality of previously observed non-planted mangrove juveniles. Many replanted mangroves around the globe are monocultures and are thus likely to have low thresholds to as they do not benefit from the ‘portfolio effect’ provided in mixed species stands. Successful recruitment of non-planted species into replanted monospecific mangroves stands may therefore enhance ecosystem productivity and stability. However, more research needs to be conducted on traits of different mangrove species and their contribution to ecosystem productivity and sensitivities to environmental perturbations