Digital terrain modelling to investigate the effects of sea level rise on mangrove propagule establishment

Abstract

We studied the vegetation structure dynamics of mangroves, in order to contribute to an overall view on the conditions of propagule establishment from the moment they detach from the parental tree. Microtopographical measurements, quantitative data on vegetation, and propagule counts were collected in Gazi Bay (Kenya) and utilized in a ‘geographical information systems’ (GIS) environment where all modelling took place based on digital terrain modelling (DTM). Suitability maps were created for propagules of Rhizophora mucronata and Ceriops tagal to analyse the dispersal possibilities (through stranding or self-planting) for the present situation, and for cases of degradation and sea level change. The GIS-analyses take into account the available information derived from the field data, but alterations that go hand in hand with degradation and/or sea level changes (e.g. erosion patterns, rates of sediment supply, wave action) were not considered, since our main focus was the behaviour of propagules. We found that the study area has the potential to successfully rejuvenate at present; however, increasing anthropogenic pressure may have severe consequences on propagule dispersal within this mangrove stand, mainly through the loss of aerial root masses, as these were shown to provide stranding areas for propagules. A relatively modest rise in sea level within a time span of 20 yr could affect the distribution pattern and the specific proportion of the juvenile vegetation layer, leading to notable floristic modifications from a regional point of view.