Evaluating feathermoss growth: a challenge to traditional methods and implications for the boreal carbon budget

1 Accurate assessment of net primary production is vital for understanding carbon (C) cycling, both regionally and globally. However, this requires effective methods of measurement that acknowledge the unique characteristics of the subject or area being monitored. 2 Feathermosses dominate the ground layer of boreal upland ecosystems and play a vital role in soil C accumulation, accounting for up to 50% of total photosynthesis. Feathermoss growth is both apical and lateral, with branches of determinate length at maturity produced at consistent frequencies along the stem. 3 Traditional methods of estimating annual production of feathermosses underestimate total plant production because they do not account for lateral growth of the previous year's immature branches. We present a conceptual model of feathermoss growth using Pleurozium schreberi that includes apical and lateral annual growth. From this model, we provide a modified method to more accurately estimate P. schreberi production. The top 3 cm of 10 P. schreberi plants from each of five bog peatlands in Alberta, Canada, were collected. For each plant, distance from the stem apex to branch insertion, branch length and dry mass, and dry mass of 3-mm stem sections were measured and used to define model parameters that, due to lack of significant variability among the sites, can be applied regionally and possibly globally throughout the boreal zone. An additional 20 plants were collected from a sixth site for testing the accuracy of our modified method. 4 Assessment of our method showed an insignificant mean difference between observed and calculated production values. Furthermore, comparison of our method with traditional methods showed a c. 25% underestimation of annual production by the latter. Traditional methods underestimate annual biomass production of P. schreberi by c. 73 g m-2, accounting for more than 14 Tg C year-' across the boreal region. 5 Our study shows that accounting for species-specific growth characteristics when estimating ground layer production has a substantial impact on boreal C budget assessments and therefore the terrestrial C cycle.