Plant population patterns in a glacier foreland succession: pioneer herbs and later-colonizing shrubs



Glacier forelands provide valuable sites for the study of patterns and processes of primary succession The Storbreen glacier foreland, south-central Norway, has previously been the subject of a suite of community-level approaches Here the focus is population-level studies of six key pioneer plants Arabis alpina, Deschampsia alpina, Oxyria digyna, Poa alpina, Saxifraga cespitosa and Trisetum spicatum and four heath shrubs Betula nana, Empetrum hermaphroditum, Phyllodoce caerulea and Salix complex (Salix glauca and S lanata) Size-frequency distributions are used as indicators of population phenomena

The optimal conditions for growth and fecundity of Arabis alpina and Saxifraga cespitosa appear to be almost immediately upon deglaciation (< 7 yr), although both achieve their maximum cover on slightly older terrain A similar pattern was noted for the grasses, although their optima come at slightly later times Population phenomena in Oxyria digyna could not be related to terrain age, indicating a differing role of this species within the succession

Among the shrubs a number of demographic and behavioural patterns are identifiable as successional features, and other patterns are explicable in relation to changing histories of disturbance and site moisture relationships Establishment and build-up in Empetrum hermaphroditum and Salix complex do not appear to be environmentally constrained, in contrast to Betula nana and Phvllodoce caerulea Betula nana is the slowest species to colonize, but occasional early colonists appear healthy and fecund, pointing to a strong environmental sieve at the point of establishment The population patterns varied considerably among the shrub species, the most striking common feature being in mode of establishment Establishment of ramets by vegetative means occurs increasingly on older ground within the foreland (maximum terrain age 230 yr) but only in sites of the mature heath outside the foreland is it the characteristic mode of establishment In exception to this pattern, vegetative establishment of Phyllodoce caerulea is very rare in all conditions studied Discussion focuses upon the importance of population phenomena, seed dispersal and life-history characteristics in successional change It is concluded that processes involved in species turnover include both allogenic and autogenic elements and that although the two classes are difficult to separate in practice, autogenic factors become more significant whilst species-environment relations become tighter in later phases of vegetation development