Effects of forest floor mosses on elemental cycling in spruce forests

Mosses are abundant components of understory vegetation in coniferous forests. The poikilohydric life style of bryophytes implies that active phases in moist state alternate with inactive phases in dry state, which requires a range of physiological adaptations, such as the accumulation of sugars and antioxidants. Re-wetting of desiccated bryophytes during intensive summer rain events, however, may cause considerable leakage of intracellular moss metabolites, leading to a flush of labile carbon (C) compounds washed into the soil.

In the presented study we investigated (1) what amounts of C and nutrients are leached from forest floor mosses over a growing season; (2) how C leaching from mosses is related to the climatic conditions at the field site; (3) how moss layers alter the chemical composition of the canopy throughfall. We collected leachates under four species of forest floor mosses in two montane spruce forests differing in climate over a 4-months growing season.

Our results showed that total fluxes of dissolved organic C (DOC) by the moss leachates were comparable at the two field sites, irrespective of climatic conditions, although C concentrations were higher in moss leachates at the drier site. Surprisingly, C leaching from mosses was rather stable compared to the significant temporal variation in DOC concentration in canopy throughfall. Furthermore, we found that moss layers significantly altered the chemical quality and elemental composition of the canopy throughfall, and that this effect depended on the moss species, field site and season.

Our study demonstrates that moss leachates represent a significant soil C input and relevant labile C source for soil microorganisms besides root exudates and litter leachates, and that forest floor mosses play an important role in elemental cycling of montane spruce forests.