Seasonal dynamics of biomass increase and shoot elongation in five co-occurring boreal forest bryophytes

The seasonal patterns of length and biomass increase in five co-occurring b ryophytes (Dicranum majus, Hylocomium splendens, Plagiochila asplenioides, Ptilium crista-castrensis and Rhytidiadelphus loreus), were studied through out one growing season in a boreal spruce forest by growing shoots in exper imental assemblages at two densities at consecutive four-week intervals. Se asonal trends in the mean relative growth rate (RGR) were found, with highe st RGR in spring and early summer (0.0025-0.005 g g(-1) day(-1)) and a low or negative RGR in autumn. This temporal variation could best be explained by the variation in precipitation, but interactions with other environmenta l factors were important, especially in spring and autumn. The five species responded differently to the environmental conditions, with the result tha t their relative rank in RGR changed from period to period. However, the ef fect of density on RGR varied between periods, indicating that the impact o f macroenvironmental conditions depends on shoot density. Mean rate of leng th increase (LGR) peaked in early summer (0.12-0.20 mm day(-1)) with low le ngth increases in spring and late in the season. LGR varied among species, showing differences in both maximum length growth rate and temporal pattern s of growth. Length increase did not follow the temporal patterns of biomas s increase, and an evident uncoupling of biomass and length increase was fo und in all species. The cumulative seasonal increase in length and biomass varied among species with a twofold and ninefold variation among species in cumulative length and biomass increase respectively. Further, an effect of shoot density on cumulative biomass increase was found, with the higher bi omass increase at the lower shoot density.