Questions: How does vegetation first establish on newly-formed lava substrates? Do very small (cm) and meso-scale (m) variations in the physical environment influence this process and subsequent vegetation development?

Location: Mount Hekla, southern Iceland (64°00′ N, 19°40′ W).

Methods: Data on vegetation structure and the incidence of ‘safe sites’ suitable for colonisation were collected from high and low points on the surfaces of lava flows emplaced during the 1991 and 2000 A.D. eruptions of Mount Hekla. Effects of flow age and meso-topographic position on vegetation structure (moss cover, patch density, stem length) were assessed by two-way analyses of variance. The distributions of colonisation events and available safe sites were analysed using point pattern techniques.

Results: Rapid colonisation of the lava surface was observed, despite stressful environmental conditions. The 1991 and 2000 flows differed significantly in vegetation structure, but there were no significant differences in moss cover, patch density and stem length between ‘high’ and ‘low’ sites.

Conclusions: Colonisation events are invariably associated with small-scale irregularities on the surface of the lava. The colonisation process appears to be spatially random. Development of the moss ‘carpet’ proceeds by vertical thickening and lateral growth and coalescence of moss patches that establish in ‘safe sites’. This process is rapid, with close to 100% of available safe sites exploited within 20 years. Topographic position makes no difference to the very early stages of vegetation development and cannot be used to ‘forecast’ the later stages of development.

Nomenclature: Purvis et al. (1994); Hansen & Anderson (1995); Kristinsson (2005); Smith (1996, 2004).