Mapping mangrove outplant sites and mangrove outplant performance in relation to local environmental factors on Bonaire, Caribbean Netherlands

Samenvatting

Mangrove forests are essential habitats for species that are of commercial significance and provide various ecosystem services, including coastal protection. Yet, the health and size of mangrove forests are facing a global decline, raising concerns. To protect these ecosystems from further decline, understanding how to restore degraded forests is essential. Knowledge of planting methods and suitable locations can be used to improve the success of restoration activities. Mangrove Maniacs, an NGO, has been active in outplanting mangrove trees in Bonaire since 2020. However, precise documentation on outplanting details like areas, dates, numbers, and species of mangroves that are outplanted is sometimes missing. While the growth and survival of a sub-sample is monitored, there is currently no island-wide standardized long-term monitoring program. Therefore, the aim of this internship was two-fold, first, to identify the locations, species, outplant types, and densities of mangroves on Bonaire; second, to develop a long-term mangrove outplant monitoring program. Fourteen mangrove outplanting sites were identified on Bonaire, with 11 along the southwest coast (SW), 2 in Lac Bay, and 1 in Lagun. A total of 5,141 Rhizophora mangle trees and 8 Laguncularia racemosa trees were estimated to be outplanted in SW, with a survival rate of 29.6% and 100%, respectively. In Lac Bay, approximately 200 R. mangle trees were outplanted, with a survival rate of 27.5%. No data was available on initial outplanting densities at Lagun, but 48 living and 525 dead Avicennia germinans trees were counted, suggesting a survival rate of 8.4%. Additionally, a long-term monitoring program was developed to examine mangrove outplant performance and survival over time in a standardized way, while also providing insight into the role of local environmental variables on mangrove outplant performance. For this purpose, 1 to 5 permanent monitoring plots were assigned within each of the identified outplanting sites. In each of these plots, the environmental condition (i.e. water depth, salinity, dissolved oxygen concentration, pH, sediment layer thickness and organic content) were assessed and biotic measurements (i.e. tree species, tree height, stem thickness, number of living outplants, number of living leaves) were carried out. Only data from SW and R. mangle was used to analyse the influence of environmental variables on performance. Environmental conditions varied significantly among the SW sites, showing differences in dissolved oxygen, temperature, pH, sediment layer thickness, and organic content. Conductivity negatively correlated with R. mangle stem height, whereas water depth showed a positive correlation. Moreover, a positive relationship was observed between the distance to the nearest tree and the number of living leaves. Distance to the nearest tree and dissolved oxygen concentration both showed a positive relationship with the proportion of living R. mangle trees, while conductivity and sediment organic matter content showed a negative relationship with the proportion of living R. mangle trees. Based on these results, we recommend strategic planting of R. mangle along the SW coast at sites with optimal environmental conditions (i.e. low conductivity and sediment organic content, high dissolved oxygen level, minimum water level of ~10 cm). It is also advisable to avoid planting A. germinans in dry hypersaline sites in Lagun and to reduce salinity levels in the backwaters of Lac Bay to increase outplant survival. Finally, to allow monitoring of outplant survival and performance, no new mangroves should be planted within the assigned long-term monitoring plots