Regeneration of four boreal bryophytes: colonization of experimental gaps by naturally occurring propagules

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Abstract
The feather moss community in upland black spruce stands is dominated by Pleurozium schreberi, with small pure colonies and mixtures of ubiquitous and apparently persistent minor species. Previous studies indicated that this spatial pattern is not due to microsite conditions. This study tested the hypothesis that the minor species persist by dynamic occupation of small disturbance-type gaps. Colonization of experimental gaps (10 cm diameter, three substrate types) was tracked over 2 years to determine (i) how Pleurozium and the three minor species colonize gaps and (ii) how characteristics of the gaps affect colonization. All species colonized by encroachment of surrounding vegetative shoots. Availability of propagules, i.e., species composition of the surrounding bed, had the greatest impact, but regeneration of two species was affected by substrate: Ptilium growth was reduced on spruce needles, whereas that of Ptilidium was increased. Pleurozium schreberi displayed the most rapid growth in terms of shoot encroachment regardless of gap characteristics. By virtue of its abundant propagules and rapid growth, Pleurozium is most likely to colonize randomly located disturbance-type gaps, hence maintaining its dominance. Differentiation in regeneration characteristics does not account for the persistence of the minor species in the community. Keywords: Pleurozium schreberi, Ptilium crista-castrensis, Dicranum polysetum, Ptilidium ciliare, gap dynamics, community structure.