An experimental test of the relationship between small scale topography and seedling establishment in primary succession

In infertile environments, the spatial scale and distribution of favourable microsites may be an important determinant of vegetation patterns. Such patterns may be persistent although the association and causality may only be detectable during initial establishment. In this study we investigated experimentally how spatial variation on two different scales and species-specific traits affected seedling survival at an early successional site on Skei{eth}arársandur, a 1,000 km² homogeneous glacial outwash plain in SE-Iceland. Seedlings of eight native species were transplanted into six different micro-topographical combinations: three types of microsites (lee side of small stones and cushion plants, and control), located within two topographical features (shallow depressions and surrounding flats). Seedling survival was then recorded. Only 11 % of transplanted seedlings survived through the second winter, however seedlings that survived past the second growing season were likely to persist. Survival rates varied by species and were positively linked to seed size. Seedling survival was only weakly associated with spatial variation. The strongest association found was that survival was sometimes higher on flats compared to depressions. Sand accumulation in depressions might lower seedling survival there. We conclude that early plant establishment at the site, and the emergent vegetation mosaic, is most likely produced by the interaction of stochastic factors, such as the sand storms that intermittently rage across the plain and species-specific properties like seed size. However, in better-vegetated areas of Skei{eth}arársandur depressions often have higher moss and vascular plant cover than nearby flats, suggesting that moss may control vegetation patterns seen later in succession.