On the southern slopes of Vatnajökull ice cap in SE-Iceland, numerous outlet glaciers descend into a mild and humid lowland environment. They all reached maximum extent at the end of the Little Ice Age (LIA) ca 1870-1890 and have been retreating since, at greatly accelerated speed in recent years, even >100 m/yr.

We consider these outlet glaciers as a replicated natural experiment which provides a unique opportunity to test hypotheses on ecosystem development and the importance of deterministic vs. stochastic factors in determining successional rates and direction. If succession is a deterministic process, vegetation development should follow the same pattern at all glaciers. Although the abiotic environment is comparable at all sites, neighbouring species richness and degree of isolation varies. We propose that differences in successional patterns can be explained by stochastic factors related to the seed rain composition and distance to seed sources.

We analysed vascular plant communities at three seral stages (deglaciated in 2001-2005, 1945 and LIA maximum) at six outlet glaciers. Some are close to birch forests and rich species pools while others are bordered by species-poor vegetation dominated by graminoids.  We measured cover and species composition of vascular plants and the cover of cryptogams. Seed rain and soil seed banks were estimated at all sites.

Preliminary results indicate extremely rapid colonization of deglaciated terrain. They also suggest differences in functional group composition between glaciers and variable colonization of bryophytes and mountain birch which may be acting as ecological engineers directing ecosystem development along different pathways.