Autosuccession in alpine vegetation: Testing the concept on an altitudinal bioclimatic gradient, Jotunheimen, southern Norway

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Specific tests of autosuccession (equivalent to non-replacement change in species composition) are made, in which pioneer communities on roadside verges and areas of patterned ground disturbed by cryoturbation are compared with mature communities on a bioclimatic gradient from sub-alpine woodland (850 m a.s.l) to high-alpine fjellfield (2200 m a.s.l). Autosuccession is quantified for the first time using community similarity coefficients and indices of pioneer persistence and importance, which measure nominal- or ordinal-scale differences in species composition between 65 paired pioneer and mature communities. Linear relationships to altitude, with coefficient and index values of ~90–100% in the upper part of the high-alpine belt to ~10–20% in the sub-alpine zone, indicate a continuum from autosuccession to relay succession (the latter characterised by high species turnover and replacement change). Values based on ordinal-scale data are generally ~10% lower than those based on nominal-scale data and use of pioneer sites from roadside verges result in a ~20% offset relative to pioneer sites from sorted circles (the latter comparison reflecting the effect of substrate differences). Autosuccession appears to be characteristic only at altitudes >2000 m a.s.l. in the upper high-alpine belt. Replacement change increases in importance as a constituent of mixed-mode succession through the conventional mid- and low-alpine belts. Spatial variation in the nature of primary succession along the bioclimatic gradient supports a geo-ecological model of succession with predominantly allogenic controls (climatic stress and high levels of substrate disturbance by cryoturbation) at high altitudes and increasing autogenic controls (biological interactions and substrate stability) at lower altitudes.