Plant biomass and production and CO 2 exchange in an ombrotrophic bog

Summary1 Above‐ground biomass was measured at bog hummock, bog hollow and poor‐fen sites in Mer Bleue, a large, raised ombrotrophic bog near Ottawa, Ont., Canada. The average above‐ground biomass was 587 g m −2 in the bog, composed mainly of shrubs and Sphagnum capitula. In the poor fen, the average biomass was 317 g m −2 , comprising mainly sedges and herbs and Sphagnum capitula. Vascular plant above‐ground biomass was greater where the water table was lower, with a similar but weaker relationship for Sphagnum capitula and vascular leaf biomass. 2 Below‐ground biomass averaged 2400 g m −2 at the bog hummock site, of which 300 g m −2 was fine roots (< 2 mm diameter), compared with 1400 g m −2 in hollows (fine roots 450 g m −2 ) and 1200 g m −2 at the poor‐fen site. 3 Net Ecosystem Exchange (NEE) of CO 2 was measured in chambers and used to derive ecosystem respiration and photosynthesis. Under high light flux (PAR of 1500 µmol m −2 s −1 ), NEE ranged across sites from 0.08 to 0.22 mg m −2 s −1 (a positive value indicates ecosystem uptake) in the spring and summer, but fell to –0.01 to –0.13 mg m −2 s −1 (i.e. a release of CO 2 ) during a late‐summer dry period. 4 There was a general agreement between a combination of literature estimates of photosynthetic capacity for shrubs and mosses and measured biomass and summer‐time CO 2 uptake determined by the eddy covariance technique within a bog footprint (0.40 and 0.35–0.40 mg m −2 s −1 , respectively). 5 Gross photosynthesis was estimated to be about 530 g m −2  year −1 , total respiration 460 g m −2  year −1 , and export of DOC, DIC and CH 4 10 g m −2  year −1 , leaving an annual C sequestration rate of 60 g m −2  year −1 . Root production and decomposition are important parts of the C budget of the bog. Root C production was estimated to be 161–176 g m −2 year −1 , resulting in fractional turnover rates of 0.2 and 1 year −1 for total and fine roots, respectively.