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

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Abstract
The seasonal patterns of length and biomass increase in five co-occurring bryophytes (Dicranum majus, Hylocomium splendens, Plagiochila asplenioides, Ptilium crista-castrensis and Rhytidiadelphus loreus), were studied throughout one growing season in a boreal spruce forest by growing shoots in experimental assemblages at two densities at consecutive four-week intervals. Seasonal trends in the mean relative growth rate (RGR) were found, with highest 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 environmental factors were important, especially in spring and autumn. The five species responded differently to the environmental conditions, with the result that their relative rank in RGR changed from period to period. However, the effect of density on RGR varied between periods, indicating that the impact of macroenvironmental conditions depends on shoot density. Mean rate of length increase (LGR) peaked in early summer (0.12-0.20 mm day(-1)) with low length increases in spring and late in the season. LGR varied among species, showing differences in both maximum length growth rate and temporal patterns of growth. Length increase did not follow the temporal patterns of biomass increase, and an evident uncoupling of biomass and length increase was found 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 biomass increase at the lower shoot density.