The success of eco-engineering mangrove restoration in a high energy area, at gazi bay, kenya

Abstract

Mangroves and their ecosystem offer a range of globally recognized benefits, yet they continue to be lost and degraded. Efforts to restore lost mangroves using conventional methods in high-energy areas result in low success rates due to the removal of seedlings via wave’s action. This study assessed the efficacy of using modified Riley Encasement Methods (REMs) in the restoration of mangroves in high-energy areas at Gazi Bay, Kenya. Prior to the mangrove planting experiment, mapping for mangrove cover change, and assessment of vegetation and soil conditions were done. Global Positioning System (GPS) device was used to record the study site coordinates. Landsat images were systematically sampled using these coordinates from the United States Geological Survey (USGS). Vegetation and soil baseline data were collected in 49 square plots of 100 m2 ; established along belt transects perpendicular to the shoreline. The mangrove vegetation structural data that was collected included; species composition, tree height (m), and stem diameter (cm); from which the importance value index (IV), basal area (m2 ha-1 ) and standing density (stems ha-1 ) were derived. Sediment cores were made in the center of each square plot for carbon and grain size analysis. Mangrove (Rhizophora mucronata) planting was done using a randomized complete block design (RCBD) in which the planting area was divided into three sections. On each section, one block measuring 7 m by 5.5 m was established, resulting to three blocks namely A, B, and C. Within each block, treatments that involved use of bamboo and different-sized PVC pipes were randomly assigned locations. Monitoring involved assessment of survival and growth parameters including shoot growth, number of leaves, number of internodes, number of branches and leaf area. Statistical data analysis was done using SPSS version 26.0, GRADISTAT computer program and Microsoft Excel 2019. The findings of this study were that the mangrove forest was highly degraded recording relatively low proportions of silt and clay (3.03 ± 0.17%), soil organic matter (6.33 ± 0.24) and soil organic carbon (5.52 ± 0.10). Following repeated measures of Analysis of Variance (ANOVA) and a post-hoc Tukey's Honest Significant Difference (HSD) test (p < 0.05), the results of the planting experiment revealed significant variations in survival and growth rates among treatments. Seedlings grown within PVC encasements recorded significantly higher survival rates (43%) compared to those in the bamboo (1%) and control groups (4%). These findings suggest that PVC pipes were efficient in supporting and protecting seedlings from external forces. The study highlights the potential of adopting the encasement technique in mangrove restoration. These findings are particularly relevant to environmental conservation policies, climate change mitigation strategies, and coastal community development programs. Current mangrove restoration policies should consider the potential of eco-engineering techniques in addressing challenges facing mangrove restoration in high energy sites.