|Mangrove’s anchorage beyond opportunity. Physical-plant properties and soil characteristics are vital for mangrove seedling stability within the muddy inter-tidal realm.|
van Rees, F. (2019). Mangrove’s anchorage beyond opportunity. Physical-plant properties and soil characteristics are vital for mangrove seedling stability within the muddy inter-tidal realm.
. Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 30 pp.
Background and aims: Mangrove restoration successes are very much determined by the physical/environmental conditions of the hydrodynamic system. At the coast of Demak, Java, Indonesia, seedlings in fluid mud die in their first year and full recruitment fails. Unable to link this to research-based causes (e.g. seedlings unable to grow at unsuitable depth, duration and frequency of tidal flooding), this study aims to elaborate on the link between soil strength and seedling anchorage, which has not been studied before.Methods: Hydrodynamic forces and critical erosion were mimicked by manually dislodging Avicennia ssp. and R. mucronata from the field. The findings were compared with the measured hydrodynamic forces in the field. In addition, a field experiment was conducted in mesocosm in a sheltered environment, so that soil water content could be altered and its effect on dislodgement force could be measured. We further analyzed the effect of physical plant properties in different soils, using artificial seedling mimics. To gain insight in the underlying mechanisms that drive seedling stability, we examined the link between dislodgment force, critical erosion and physical-plant properties and soil characteristics by the use of step-wise regression.Key results: Avicennia ssp. stabilizes itself significantly faster with longer stem lengths than R. mucronata, with rates of 143 and 70.9 N/meter stem length in sand (p=5E-28) and fluid mud (p=1E-21), respectively. Yet, with maximum horizontal orbital velocities up to 0.42m/s of the highest wave, wave forces are two orders of magnitude lower than dislodgement forces. Draining fluid mud (decreasing the soil water content with 7%) increases dislodgement forces significantly with a factor 3.3. More erosion was needed to dislodge seedlings (of any species) from sand, compared with dislodging seedlings from fluid mud. In terms of underlying mechanisms, dislodgement forces of Avicennia ssp. is positively related to the number of roots (p=9.26e-04), while dislodgement forces of R. mucronata is positively related to total root diameter (p=0.0020).Conclusions: We concluded that hydrodynamic force from waves alone is unable to dislodge seedling of any kind from any soil. Draining fluid mud increases seedling anchorage even more. We found that R. mucronata is more resistant to erosion compared with Avicennia ssp., the latter being still vulnerable in the first month in fluid mud. Water content appears to affect critical erosion depth and dislodgement force the most, which is in agreement with findings of an increase in stability in drained soils. The number of roots is crucial for dislodgement forces, while the resistance against erosion is mainly influenced by how deep plant elements recite in the soil.Recommendations: This research has shown that larger seedlings are more stable than smaller ones. Larger seedlings can repel hydrodynamics drag forces and erosion even in fluid mud. Planting larger seedlings in these unstable soils can be beneficial for restoration, provided that they are planted at the right depth, and have the right duration and frequency of tidal flooding.Key words: mangrove forests, restoration, dislodgement force, drainage, critical erosion depth, soil properties, fluid mud, Avicennia marina, Avicennia alba, Rhizophora mucronata