Microtopography creates small-scale refugia for boreal forest floor bryophytes during clear-cut logging

Abstract

The biotic response of ecological systems to disturbances has traditionally been explained by attributes of the disturbance event itself, such as its intensity, the distribution of traits within a community related to resistance (e. g. physiological, morphological or life-history) or their interaction. Another less investigated mechanism explaining variation in response to disturbance is microtopographic heterogeneity, which might modify survival rates unevenly. We tested the hypothesis that forest floor microtopography creates small-scale refugia for bryophytes following conventional clear-cut logging by comparing a) survival of transplanted bryophytes and b) compositional changes of forest floor bryophytes among three different positions: on the northern side of boulders and stumps and on unsheltered forest floor. The investigation was carried out as a before-and-after study in 12 Swedish boreal forests (eight stands subjected to clear-cutting and four reference stands). Significantly more bryophyte transplants survived where they were sheltered by boulders and stumps (30 and 29% respectively) compared to on level forest floor (10%) and less compositional changes occurred in sheltered microtopographic positions than on level forest floor. Shelter from boulders and stumps increased survival from both microclimatic stress and mechanical disturbance but not from burial by logging residues. Our findings provide evidence that microtopography can modify initial responses to disturbances by creating small-scale refugia. Further studies are needed to determine whether this phenomenon is commonly occurring among other organisms and in other ecosystems and to what extent scattered in-situ survivors can increase the recovery rate following disturbances.