Sporophyte production and spore dispersal phenology in Sphagnum:: the importance of summer moisture and patch characteristics

Sporophyte production in Sphagnum was followed annually in 80 permanent 1-m(1) plots at the bog expanse of a pristine mire during 1993-1999 and in 60 plots in peat pits abandoned 50 years ago during 1996-1999 in east central Sweden. The nine most abundant Sphagnum species produced sporophytes, with mean annual production ranging from 0.64 to 20 sporophytes/dm(2) of cover among species. An estimated mean of 16 million Sphagnum spores/m(2) mire area was produced annually at both mires. At the pristine mire, sporophyte production was positively related to the amount of precipitation the previous summer, suggesting that gametangium formation is especially sensitive to summer droughts. At the wetter peat pits, the amount of precipitation during spring in the year of sporophyte formation appeared more important, probably by positively affecting male gamete dispersal and fertilization. Larger patches had a higher probability of producing sporophytes at least once, showing areas with both sexes present among dioecious species, and thus giving an indication of clone size. Only slight differences in sporophyte production were found between the two mires, apart from effects of hydrological conditions and patch size. This indicates numerous colonizations at the peat pits. Summer droughts affected sporophyte maturation negatively by drying out sporophytes prematurely. Spore release phenology differed among species by up to a month and lasted from the beginning of July until the end of August. The early timing of spore dispersal in the most drought-sensitive, lawn-inhabiting sphagna should reduce the risk of sporophytes drying out prematurely during summer droughts.