Seasonal bryophyte productivity in the sub-Arctic: a comparison with vascular plants

1. Arctic ecosystems are experiencing rapid climate change, which could result in positive feedbacks on climate warming if ecosystem carbon (C) loss exceeds C uptake through plant growth. Bryophytes (mosses, liverworts and hornworts) are important components of Arctic vegetation, but are currently not well represented in terrestrial C models; in particular, seasonal patterns in bryophyte C metabolism compared to vascular plant vegetation are poorly understood. 2. Our objective was to quantify land-surface CO2 fluxes for common sub-Arctic bryophyte patches (dominated by Polytrichum piliferum and Sphagnum fuscum) in spring (March-May) and during the summer growing season (June-August) and to develop a simple model of bryophyte gross primary productivity fluxes (PB). We use the model to explore the key environmental controls over PB for P. piliferum and S. fuscum and compare seasonal patterns of productivity with those of typical vascular plant communities at the same site. 3. The modelled total gross primary productivity (RPB) over 1 year (March-November) for P. piliferum was c. 360 g C m) 2 ground and for S. fuscum c. 112 g C m) 2 ground, c. 90% and 30% of total gross primary productivity for typical vascular plant communities (RPV) over the same year. In spring (March-May), when vascular plant leaves are not fully developed, RPB for P. piliferum was 3 u RPV. 4. Model sensitivity analysis indicated that bryophyte turf water content does not significantly affect (March-November) RPB for P. piliferum and S. fuscum, at least for periods without sustained lack of precipitation. However, we find that seasonal changes in bryophyte photosynthetic capacity are important in determining RPB for both bryophyte species. 5. Our study implies that models of C dynamics in the Arctic must include a bryophyte component if they are intended to predict the effects of changes in the timing of the growing season, or of changes in vegetation composition, on Arctic C balance.