Growth and disturbance in the spatial dynamics of an endangered seashore grass Arctophila fulva var. pendulina

The shores of the Bothnian Bay in the northern Baltic Sea are characterised by relatively rapid isostatic land uplift (ca. 6.9 mm year−1) continuously exposing virgin land for plants to colonise. Autogenic and allogenic changes in soil and plant communities will eventually make these exposed sites unsuitable for the early colonisers. The course of the primary succession in the waterfront is often delayed and shaped by frequent disturbances by ice scouring, water level movements and suffocation of extant vegetation by floating plant debris. Hence, we hypothesise that disturbances are the key processes underlying the local short-term dynamics of the plant populations of the seashore meadows. We monitored formation, growth, shrinkage and mortality of patches of a critically endangered early successional grass species Arctophila fulva var. pendulina for four consecutive years at the Liminka Bay in Finland. The data allow estimation of quality, magnitude and frequency of the wintertime disturbance and the growth during the summer. We formulated and parameterised a spatially explicit simulation model to predict the fate of the population in the area. Simulations with the fitted parameter estimates predicted that the area of A. fulva population will considerably decrease in the next 30 years. A parameter sensitivity analysis showed the probability of a new patch formation to be the most important factor for the population persistence, whereas changes in the dispersal distance, for example, are less important. The best way to enhance the persistence of the population is to promote the new patch formation. This could be done by transplanting rhizome fragments of the species to the suitable open areas in the shallow water where they are not immediately displaced by the later successional species.