Abundance and composition of plant biomass as potential controls for mire net ecosytem CO 2 exchange Anna M. Laine, Jill Bubier, Terhi Riutta, Mats B. Nilsson, Tim R. Moore, Harri Vasander, and Eeva-Stiina Tuittila Abstract: We compared the amount and composition of different aboveground biomass (BM) fractions of four mires with their net ecosystem CO 2 exchange (NEE) measured by eddy covariance. We found clear differences in response of green bi- omass (GBM) of plant functional types (PFTs) to water table (WT), which resulted in larger spatial variation in GBM within a mire than variation between mires. GBM varied between mires from 126 ± 7 to 336 ± 16 g·m –2 (mean ± SE), while within mire variation at largest was from 157 ± 17 to 488 ± 20 g·m –2 (mean ± SE). GBM of dominant PFTs appeared to be better in explaining the peak growing season NEE than the total BM or GBM of a mire. The differences in photosyn- thetic capacity between PTFs had a major role, and thus a smaller GBM with different species composition could result in higher NEE than larger GBM. Vascular plant GBM, especially that of sedges, appeared to have a high impact on NEE. Eleven PFTs, defined here, appeared to capture well the internal variation within mires, and the differences in GBM between communities were explained by the water table response of PFTs. Our results suggest the use of photosynthesizing BM, sep- arated into PFTs, in modelling ecosystem carbon exchange instead of using just total BM. Key words: bog, fen, microtopography, net ecosystem exchange, peatland, plant functional type, water table. Résumé : L’auteure a comparé la quantité et la composition des différentes fractions de la biomasse (BM) épigée dans qua- tre tourbières, incluant la mesure de l’échange net de CO2 par l’écosystème (NEE), à l’aide de la covariance de fluctuation. Elle a observé des différences nettes de réaction de la biomasse verte (BMV) des types fonctionnels de plantes (TFP) à la nappe phréatique, lesquelles conduisent à une variation plus importante de la BMV à l’intérieur d’une tourbière qu’entre les tourbières. La BMV varie entre les tourbières de 126 ± 7 à 336 ± 16 g·m –2 (moyenne ± SE), alors qu’àl’intérieur des tour- bières la plus grande va de 157 ± 17 à 488 ± 20 g·m –2 (moyenne ± SE). La BMV du TPF dominant semble la meilleure pour expliquer le pic de le NEE de la saison de croissance, que la BM ou la BMV d’une tourbière. Les différences de capa- cité photosynthétique entre les TFP jouent un rôle majeur, et ainsi une plus petite BMV avec différentes espèces peut conduire à des résultats de NEE plus élevés qu’une BMV plus grande. La BMV des plantes vasculaires, surtout pour bien capter la variation interne dans une tourbière, et les différences de BMV entre les communautés s’expliquent par la réaction de la nappe phréatique des TPF. Les résultats suggèrent d’utiliser la BM photosynthétique, distribuée selon les TPF, pour la modélisation de l’échange de carbone par les écosystèmes, plutôt que la seule BM. Mots‐clés : tourbière ombrotrophe, tourbière minérotrophe, microtopographie, échange écosystémique net, tourbière, type fonctionnel de plantes, nappe phréatique. [Traduit par la Rédaction] Introduction Mires have a dual effect on atmospheric greenhouse gas concentrations. They assimilate atmospheric carbon dioxide (CO 2 ) in photosynthesis and release CO 2 and methane (CH 4 ) in respiration. Mires have stored a significant amount of car- bon into peat over the last millennia (Gorham 1991; Turunen Received 9 February 2011. Accepted 6 September 2011. Published at www.nrcresearchpress.com/cjb on 16 December 2011. A.M. Laine. Department of Biology, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland; Peatland Ecology Group, Department of Forest Science, University of Helsinki, P.O. Box 27, FI 00014 University of Helsinki, Finland. J. Bubier. Environmental Studies Program, Mount Holyoke College, Clapp Laboratory 50 College Street, South Hadley, MA 01075, USA. T. Riutta. Peatland Ecology Group, Department of Forest Science, University of Helsinki, P.O. Box 27, FI 00014 University of Helsinki, Finland; University of Oxford, School of Geography and the Environment, Environmental Change Institute, South Park Road, OX1 3QY, UK. M.B. Nilsson. Department of Forest Ecology and Management, SLU, SE-901 83 Umeå, Sweden. T.R. Moore. Department of Geography, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 2K6, Canada. H. Vasander. Peatland Ecology Group, Department of Forest Science, University of Helsinki, P.O. Box 27, FI 00014 University of Helsinki, Finland. E.-S. Tuittila. Peatland Ecology Group, Department of Forest Science, University of Helsinki, P.O. Box 27, FI 00014 University of Helsinki, Finland; School of Forest Sciences, University of Eastern Finland, PL 111, 80101 Joensuu, Finland. Corresponding author: Anna M. Laine (email: anna.m.laine@helsinki.fi). 63 Botany 90: 63–74 (2012) doi:10.1139/B11-068 Published by NRC Research Press Botany Downloaded from www.nrcresearchpress.com by MOUNT HOLYOKE COLLEGE LIBRARY on 01/23/12 For personal use only.