TREEFALL DISTURBANCE MAINTAINS HIGH BRYOPHYTE DIVERSITY IN A BOREAL SPRUCE FOREST

(1) The effect of disturbance on diversity patterns of cryptogams has received little attention. We compared bryophyte diversity in thirty-seven patches formed by uprooting with that of the undisturbed forest floor in a north Swedish Picea abies forest. (2) Bryophyte diversity and species richness were significantly higher in patches with soil disturbance compared with undisturbed forest floor. Species richness and diversity were highest at intermediate patch ages, but disturbed patches had an altered vegetation composition for at least 100 years. (3) Four causes of high bryophyte diversity in disturbed patches are proposed: (i) uprooting creates space for bryophyte colonization that is free from potential competitors; (ii) disturbed patches have high habitat heterogeneity; (iii) within-patch disturbance continues long after patch formation through erosion from tip-up mound; and (iv) the small patch size implies a short distance to potential sources of bryophyte diaspores which should increase the chance of establishment. (4) Both early and late successional bryophytes establish shortly after an uprooting has occurred. This suggests that no facilitation by early species is necessary before late successional species invade. (5) Results show that treefall disturbances are important for both the persistence of colonists and the maintenance of high bryophyte diversity in boreal-forest ecosystems. Consequently, the studied system has clear parallels with the gap-phase systems widely recognized in temperate and tropical forests.