Limits to seaward expansion of mangroves: Translating physical disturbance mechanisms into seedling survival gradients

Mangroves are valuable coastal habitats that are globally under pressure due to climate change and coastal development. Small-scale physical disturbance by tidal inundation and wave-induced sediment dynamics has been described as the main bottlenecks to mangrove seedling establishment on exposed tidal flats. However, such biophysical bottlenecks remain poorly studied.Mangrove progradation at our study site at the Firth of Thames (New Zealand) has been described as rapid but disturbance limited. For this site, we apply a mechanistic model according to the 'Window of Opportunity' (WoO) concept, which analyses real time series of external forcing for disturbance free periods. The model was parameterized with manipulative experiments on seedling stability and monitoring data. The modelled inundation free periods for initial anchoring were validated by a caging experiment with loose and tethered propagules.Although the time series of external forcing derived from monitoring data are simplified, our model confirms the absence of mangrove progradation due to failed recruitment events on the tidal flat since 1997. The model also shows, that a temporary reduction in external forcing would lead to a sudden progradation of the mangrove forest.WoO dynamics, where vegetation establishment requires temporarily benign conditions, may be of general importance to other ecosystems with stochastic external forcing. Understanding the biophysical interactions between vegetation and geomorphic processes is key to better manage and protect disturbance-driven ecosystems in times of changing wind pattern and accelerated sea-level rise.