Greenhouse gas flux measurements in a forestry-drained peatland indicate a large carbon sink

Biogeosciences, 8, 3203–3218, 2011 www.biogeosciences.net/8/3203/2011/ doi:10.5194/bg-8-3203-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences Greenhouse gas flux measurements in a forestry-drained peatland indicate a large carbon sink A. Lohila1, K. Minkkinen2, M. Aurela1, J.-P. Tuovinen1, T. Penttil¨ a3, P. Ojanen2, and T. Laurila1 1Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland 2Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 University of Helsinki, Finland 3Finnish Forest Research Institute, P.O. Box 8, 01301 Vantaa, Finland Received: 16 May 2011 – Published in Biogeosciences Discuss.: 21 June 2011 Revised: 20 September 2011 – Accepted: 21 October 2011 – Published: 8 November 2011 Abstract. Drainage for forestry purposes increases the depth of the oxic peat layer and leads to increased growth of shrubs and trees. Concurrently, the production and uptake of the greenhouse gases carbon dioxide (CO2), methane (CH4)and nitrous oxide (N2O) change: due to the accelerated decomposition of peat in the presence of oxygen, drained peatlands are generally considered to lose peat carbon (C). We measured CO2exchange with the eddy covariance (EC) method above a drained nutrient-poor peatland forest in southern Finland for 16 months in 2004– 2005. The site, classified as a dwarf-shrub pine bog, had been ditched about 35 years earlier. CH4and N2O fluxes were measured at 2–5-week intervals with the chamber technique. Drainage had resulted in a relatively little change in the water table level, being on average 40cm below the ground in 2005. The annual net ecosystem exchange was −870±100gCO2m−2yr−1in the calendar year 2005, indicating net CO2uptake from the atmosphere. The site was a small sink of CH4(−0.12gCH4m−2yr−1) and a small source of N2O (0.10gN2Om−2yr−1). Photosynthesis was detected throughout the year when the air temperature exceeded −3◦C. As the annual accumulation of C in the above and below ground tree biomass (175±35gCm−2)was significantly lower than the accumulation observed by the flux measurement (240±30gCm−2), about 65gCm−2yr−1was likely to have accumulated as organic matter into the peat soil. This is a higher average accumulation rate than previously reported for natural northern peatlands, and the first time C accumulation has been shown by EC measurements to occur in a forestry-drained peatland. Our results suggest that forestry-drainage may significantly increase the CO2uptake rate of nutrient-poor peatland ecosystems. Correspondence to: A. Lohila ([email protected]) 1 Introduction One-third of the European peat soil area is located in Finland (Montanarella et al., 2006), where more than half of the original wetland area of 100000km2has been drained, mostly for forestry (Lappalainen, 1996). This constitutes 34% of the global area of forestry-drained peatlands; the rest of the peatlands ditched to improve forest growth are located in Russia (26%), Sweden (11%), other Northern European countries (23%), North America (3%) and China (0.5%) (Minkkinen et al., 2008). The drainage of peatlands has been suggested to lead to rapid aerobic decomposition of organic matter and, consequently, to high carbon dioxide (CO2)emissions and to a gradual depletion of peat carbon (C) pool (e.g. Turetsky and Louis, 2006). At the same time, methane (CH4)emissions typically cease, in some cases even leading to CH4uptake by the forest soil (Minkkinen et al., 2007b). Nitrous oxide (N2O) emissions may increase in minerotrophic peatlands, but not in the ombrotrophic ones (Martikainen et al., 1993; Regina et al., 1996). Direct measurements of net ecosystem exchange (NEE) on peatlands converted to agricultural use have shown high decomposition rates of peat and large C losses due to drainage (Lohila et al., 2004; Veenendaal et al., 2007). Peat C loss was also observed in an afforested agricultural site 30 years after the forest had been established (Lohila et al., 2007), whereas C uptake on an afforested peatland in Scotland was reported by Hargreaves et al. (2003). In contrast, a moderately rich fen in Canada with naturally generated tree cover and relatively deep water table (30–70cm) was found to act as a large annual CO2sink, although this site had not been managed (Syed et al., 2006; Flanagan and Syed, 2011). To our knowledge no NEE measurements including tree canopy have been made previously in forestry-drained peatlands without agricultural history. Studies on peat subsidence and bulk density have indicated a trend that nutrient-rich peatlands tend to lose peat Published by Copernicus Publications on behalf of the European Geosciences Union. 3204 A. Lohila et al.: Forestry-drained peatland is a carbon sink C following drainage while nutrient-poor ones may still sequester C (Minkkinen and Laine, 1998; Minkkinen et al., 1999). In forestry-drained peatlands, the nutrient status has been shown to regulate post-drainage tree stand growth (Keltikangas et al., 1986). Stand volume, in turn, regulates water table depth through transpiration (Sarkkola et al., 2009), and water table depth is the main control over the organic matter decomposition in peatlands (e.g. Silvola et al., 1996). Measurements of CO2efflux by the chamber method have supported the idea of higher soil respiration rates in well-drained nutrient-rich sites in contrast to lesswell-drained nutrient-poor sites (Silvola et al., 1996; Minkkinen et al., 2007a; Ojanen et al., 2010). We measured NEE above a drained peatland forest in southern Finland with the micrometeorological eddy covariance (EC) technique, which is the only viable method to monitor biosphere-atmosphere CO2exchange of a whole forest ecosystem. Earlier, Pihlatie et al. (2010) have reported greenhouse gas (GHG) fluxes measured at this site during April–June 2007, in particular the short-term dynamics of N2O and CH4and the effect of soil thawing on them. Here we report a full year of data on CO2exchange of a forestrydrained peatland drained 35 years before the start of our EC measurements. These data cover a 16-month period from September 2004 to December 2005. In addition, annual dynamics and balances of CH4and N2O fluxes are presented based on soil chamber measurements. 2 Material and methods 2.1 Site presentation Measurements were conducted at Kalevansuo-peatland, located in the municipality of Loppi, in southern Finland (60◦3804900 N, 24◦2102300 E; elevation 123m). The site, originally classified as a dwarf-shrub pine bog, was drained in 1969 with open ditches at approximately 40m intervals (Fig. 1). Drainage has resulted in a slightly lowered water table (on average 40cm below the ground) and has increased the growth of the natural tree stand. The topography of the site was flat. Within 160m of the EC measurement mast the peat depth, measured from the 33 sample plots, varied from 1.3 to 3.0m, being deepest in the middle (Fig. 2). The average peat depth (±standard deviation, SD) was 2.2±0.5m. The physical and chemical properties of the peat soil (surface humus and the layers 0–10cm and 10–20cm below the humus) were determined from eight samples taken evenly around the measurement mast. The ash content was determined as loss on ignition (550◦C) and the C and N concentrations using LECO CHN-2000. Element concentrations (Al, Ca, Fe, K, Mg, Mn, P, B, Cu, Mn, Zn) were measured by an ICP Iris emission spectrometer in dry-ashed material. The bulk density of the peat varied from 0.08 to 0.11gcm−3at the depths of 0–10cm and 10–20cm, respectively (Table 1).     1 2 3 4 5 6 © Maanmittauslaitos, lupa nro 351/MML/11 Fig. 1. Map of the Kalevansuo drained peatland showing the location of the EC measurement mast (black circle) and the chamber measurement plots (stars). The black solid lines indicate the borders of the sectors (1–6) used in the footprint analysis. The dashed circle shows the distance of 200m from the mast. The main drainage ditch that empties towards the south-east and the smaller ditches that empty into the main ditch have been indicated by the gray solid lines. The two ponds, Kalaton and Kalevanlammi, located on the borders of the site are indicated in the map together with their elevation from the sea level. The CN-ratio was high, varying from 34 to 41, the lowest values being measured at a depth of 0–10cm. The concentrations of most of the measured elements were highest in the humus layer and lowest deeper in the peat (Table 1). The pH of the peat was 5.0 (Pihlatie et al., 2010). The concentration of soil NO− 3was negligible, whereas the NH+ 4concentration in the peat varied between 5 and 15mgNkg−1and that of dissolved N between 75 and 225mgNkg−1soil (Pihlatie et al., 2010). Basic tree stand variables for the biomass calculations (diameter, height) were measured from 33 plots (500m2)in eight radial transects extending 160m from the EC mast (Fig. 2). Biomasses were estimated with the functions of Repola (2008, 2009) and Laiho and Fin´ er (1996) for aboveground and belowground components, respectively. The tree stand consisted of a dominant Scots pine (Pinus sylvestris) stand of 835 stems ha−1, and an understorey of pubescent birch (Betula pubescens) trees (<500 stems ha−1)that were mainly found on the ditch banks. Scots pine constituted 63% of the number of trees taller than 1.3m and 98% of the stand volume. The pine stand had a dominant height of 15m, a basal area of 17m2ha−1, and the annual stem volume growth was 5.5m3ha−1yr−1. The annual increment of Biogeosciences, 8, 3203–3218, 2011 www.biogeosciences.net/8/3203/2011/ A. Lohila et al.: Forestry-drained peatland is a carbon sink 3205 Table 1. Soil characteristics and element concentrations at the surface peat at Kalevansuo. Soil layer Bulk density2Ash content % C (%) N (%) C/N Al1Ca1Fe1K1Mg1Mn1P1B1Cu1Mn1Zn1 (kgm−3) Humus – 2.4 49 1.2 41 210 4000 260 3100 880 260 1000 4.6 5.5 260 63 0–10cm 81±9 2.9 50 1.5 34 800 2400 1000 470 460 56 680 1.9 4.7 56 56 10–20cm 106±8 2.0 51 1.4 38 740 1300 950 140 260 5.2 450 0.73 1.4 5.2 16 1total amount of the element in mgkg−1 2±denotes standard deviation  -150 -100 -50 0 50 100 150 distance [m] -150 -100 -50 0 50 100 150 100 150 200 250 300 344° 29° 119° 74° 209° 164° 254° 299°   20 m distance [m] 60m 100m 140m 40m 80m 120m 160m  Fig. 2. Peat depth (cm), interpolated as a contour plot, around the EC flux measurement mast (at (0,0)) at Kalevansuo. The white circles show the locations of the tree stand and peat depth measurement points (n=33), with the directions of lines and the distances of points from the EC mast indicated. The lines 344◦and 164◦ were perpendicular to the drainage ditches, while the lines 74◦and 254◦were parallel to the ditches. The distances from the centre of the plot on lines 119◦, 209◦and 299◦follow those depicted for line 29◦, and the distances of the points on lines 164◦, 254◦and 344◦ follow those depicted for line 74◦. live tree stand biomass was 3.52t of dry mass ha−1, corresponding to about 175gCm−2yr−1(see Appendix B2 for uncertainty analysis). During the summertime maximum biomass, the all-sided leaf area index (LAI) of the needles estimated from the needle mass, was 5m2m−2. The field layer was dominated by Ledum palustre, Vaccinium uliginosum, V. vitis-idaea, V. myrtillus, Empetrum nigrum, Calluna vulgaris, Eriophorum vaginatum and Rubus chamaemorus. The bottom layer was dominated by forest-mosses Pleurozium schreberi, Dicranum polysetum, Aulacomnium palustre and Polytrichum strictum with some peat-mosses like Sphagnum angustifolium, S. magellanicum and S. russowii on the wetter spots. Mosses cover about 90% of the land surface at Kalevansuo. The one-sided LAI of the field layer varied from 0.1 to 0.6m2m−2during the course of the growing season (Badorek et al., 2011). 2.2 Measurements of the CO2exchange and meteorological variables The turbulent fluxes of CO2, H2O, sensible heat and momentum were measured with the eddy covariance technique on top of a 21.5m telescopic mast (at 17.5m from April 2005). Fluctuations of wind velocity components were measured with a sonic anemometer/thermometer (SATI-3SX, Applied Technologies, Inc.) and those of CO2concentration with a closed-path infrared CO2/H2O analyzer (LI-7000, LI-COR, Inc.). The heated inlet tube (3.1mm Bevaline IV) for the LI7000 was 17m in length, and a flow rate of 6lmin−1was used. CO2-free synthetic dry air was used as a reference gas. From February 2005 onwards, the mean CO2concentration ([CO2]) was also observed at a height of 4m with a LI-820 CO2analyzer. Both analyzers were calibrated monthly with two known [CO2] (0 and 421ppm). The fluxes were calculated on-line as 30min averages as described by Pihlatie et al. (2010), taking into account the density fluctuations related to the water vapour flux (Webb et al., 1980). The results were corrected for systematic flux losses using the transfer function method of Moore (1986), including the losses due to autoregressive running mean filtering and the imperfect high-frequency response of the measurement system. For the former, the transfer function presented by Moore (1986) was used, while for the latter an empirical transfer function representing the overall system response was determined from the field data using the sensible heat flux as a reference (with a half power frequency of 1.6Hz for the CO2flux); for details, see Laurila et al. (2005). Idealized cospectral distibutions (Kaimal and Finnigan, 1994) were assumed. The flux-loss corrections were carried out as a post-processing step and implemented as look-up tables of wind speed and stability dependent correction factors. The storage flux of CO2was estimated from the [CO2] data measured at the top of the mast and at the height of 4m by assuming that these represent [CO2] from 8m to the measurement height and from the ground to 8m, respectively. The storage term was calculated with the central difference www.biogeosciences.net/8/3203/2011/ Biogeosciences, 8, 3203–3218, 2011 3206 A. Lohila et al.: Forestry-drained peatland is a carbon sink method from the mean concentration during the subsequent and preceding 30min periods and added to the measured turbulent flux (hereafter NEE refers to the sum of turbulent and storage fluxes). However, before 21 February 2005, the storage flux was calculated from the [CO2] measured at the top of the mast only, with the assumption that it represented the whole air column below that height. In this paper, we use the convention that a positive value of NEE indicates a flux from the ecosystem to the atmosphere. Supporting meteorological measurements were conducted at the study site as follows: air temperature (Tair)and relative humidity (RH) (Vaisala HMP230) at 2m and at the top of the mast, soil temperature (PT100) at depths of 0.05, 0.15 and 0.30m in a moist hollow, and at 0.05m in a hummock, soil moisture (ThetaProbe ML2x, Delta-T Devices Ltd.) at depths of 0.07 and 0.30m, soil heat flux (HFP01) at a depth of 0.15m, net radiation (Kipp&Zonen NR Lite), global and reflected global radiation (LI-200SZ), photosynthetic photon flux density (PPFD) and reflected PPFD (Licor LI-190SZ) at the top of the mast, and the water table level (WTL) (PDCR 830). The data were acquired using a Vaisala QLI 50 and Campbell CR10X1 sensor collectors and stored as 30min averages. The snow depth and the precipitation data, as well as the Tair data in cases of data collection failure at the Kalevansuo site were collected from four nearby weather stations operated by the Finnish Meteorological Institute (Maasoja/station code 0309, Mutila/1302, Nurmij¨ arvi/1321 and Jokioinen/1201, at distances of 19–53km, on average 29.5km, from the site). For climatic comparisons, long-term data were used from stations 0309, 1201 and 1302. To cover the spatial variation, manual measurements of WTL were carried out at bimonthly intervals from eight perforated plastic pipes, which were located in four directions within a distance of ca. 50m from the EC mast. The half-hourly water table data were then calibrated against the average WTL from these manual measurements. 2.3 CO2flux data screening Longer gaps in the CO2flux data were mainly caused by power failure, sensor malfunction and freezing of the anemometer. The longest gaps took place on 2–13 April 2005, 7–11 August 2005, 21 August–5 September 2005 and 9–24 November 2005. The remaining 30 min data records were screened according to the following criteria: (1) [CO2]>350ppm, (2) number of spikes in the vertical wind speed (w) and [CO2] raw data <180, (3) variances of the raw data σ2 Tair <3K2,σ2 CO2<50ppm2and σ2 w<3m2s−2 and (4) −40◦C< Tair <40◦C. In order to estimate how well the measured fluxes represent the Scots pine stand around the measurement mast, a source area analysis was carried out with a micrometeorological footprint model. The relative source weight functions (flux footprints) were calculated for each 30min averaging period using the footprint model of Kormann and Meixner 3 Friction velocity [m s -1 ] 0.025 0.075 0.125 0.175 0.225 0.275 0.325 0.375 0.425 0.475 0.525 0.575 0.625 0.675 0.725 0.775 NEE [mg CO 2 m -2 s -1 ] -0.05 0.00 0.05 0.10 0.15 0.20 Fig. 3. Average night-time CO2flux (±standard error) in the growing season 2005 plotted against the friction velocity. Data were grouped into u∗classes of 0.05ms−1. The u∗limit below which the data was rejected from the further analysis, 0.1ms−1, is indicated with the dashed line. (2001). The horizontal dimension of the stand was defined as six circular sectors (Fig. 1), and the modelled (cross-windintegrated) footprint was accumulated over the radius of the sector corresponding to the observed wind direction. If this cumulative footprint was larger than 70%, the flux data of that period were considered sufficiently representative of the peatland forest and accepted for further analysis. Owing to the high surface roughness, the source areas were concentrated relatively close to the measurement mast with the typical distance of the footprint maximum being less than 30m. During suppressed turbulence (typically summer nights), part of the CO2produced by ecosystem respiration may accumulate near the surface and be advected below the measurement height, in which case the measured vertical flux is likely to underestimate NEE. Therefore we rejected the NEE data from further analysis if the friction velocity (u∗)was below 0.10ms−1(Fig. 3). This threshold was defined as the upper limit of the lowest u∗class with a mean NEE value significantly different from that of the next class. The difference between these mean values (NEEiand NEEi+1)was considered significant if the 95% confidence intervals, defined as (NEEi+1−NEEi)±2qSE2 i+1+SE2 i(1) where SEiis the standard error of NEEi, did not cross zero. By applying the footprint and u∗criteria, an additional 17% of the data were discarded. After filtering the data according to the quality criteria shown above, altogether a total of 10959 flux values (47.3%) of the 30min periods during the whole measurement period (11 September 2004–31 December 2005) were accepted and used in the further analysis. Of these, night-time data accounted for 49% (n=5361). Biogeosciences, 8, 3203–3218, 2011 www.biogeosciences.net/8/3203/2011/ A. Lohila et al.: Forestry-drained peatland is a carbon sink 3207 4 Wind direction [deg] 0 45 90 135 180 225 270 315 360 Number of accepted flux values, all data 0 500 1000 1500 2000 2500 3000 0 200 400 600 800 1000 All data May-Sept. 2005 Number of accepted flux values, summer Fig. 4. Number of the accepted CO2flux data separated into different wind direction sectors at 45◦intervals. Data are shown for the periods of September 2004–December 2005 (white bars) and May–September 2005 (gray bars) (note different y-axes). Most of the data were related to the south-western and southern wind directions, whereas the contribution of other directions was more evenly distributed (Fig. 4). During the growing season, most of the data originated from the southwest, and the least data from the north-east and east. Energy balance closure at the site was 91% (y= 0.91x+ 11.2Wm−2, where xis net radiation and yis the sum of sensible and latent heat flux, R2=0.86), indicating a relatively small imbalance as compared to typical values in forests, 10–30% (Wilson et al., 2002). We did not adjust the measured NEE with the energy balance. 2.4 Gap-filling of NEE In order to gap-fill the CO2flux time-series, which is required to calculate the CO2balances, and for the analysis of seasonal patterns of the flux components, the measured CO2 flux was partitioned by means of empirical equations into gross primary production (GPP) and total ecosystem respiration (Rtot).Rtot was modelled as a function of temperature using the Arrhenius-type model of Lloyd and Taylor (1994): Rtot =Rref ×expE0×1 Tref −1 Tair −227.13 (2) where Rref is equal to Rtot at Tair = 283.15K, Tref = 56.02K, and E0is the temperature sensitivity of the respiration (in K). Air temperature measured at 2m height was used for Tair (in K). Tair was selected since a full time-series was easily available and a slightly higher correlation was obtained for Tair than for soil temperature. NEE was modelled using PPFD and the modelled Rtot as input values to the following equation: NEE = GPP+Rtot (3) NEE = fVPD ×α×PPFD×GPmax α×PPFD+GPmax +Rtot (4) where αis the apparent quantum yield and GPmax is the full daylight asymptotic value of NEE after subtracting the modelled Rtot.fVPD is a unitless function that was estimated based on the linear relationship between the measured NEE (with PPFD >1000µmolm−2s−1)and atmospheric vapour pressure deficit (VPD) in July. fVPD was given a value of 1 when VPD<10hPa, and a value of 0.2 when VPD>25hPa. These are within the typical limits of observed VPD effects in Scots pine forests (Mills et al., 2010). The parameter fitting procedure is presented in more detail in Appendix A. The daily, seasonal and annual CO2balances were calculated from the full time-series consisting of measured and gap-filled half-hourly CO2flux data. The error analysis of the CO2balance and tree C accumulation estimates is described in Appendix B. 2.5 Chamber measurements of CH4and N2O fluxes Chamber measurements were carried out in 2–5-week intervals between 7 July 2004 and 17 September 2005. The CH4and N2O fluxes were measured with static chambers at 16 sampling points located in four directions (plots 1– 4) each at a distance of about 50m from the EC mast (Fig. 1). Round, metallic chambers (diameter = 0.315m, height = 0.30m) equipped with a fan for mixing the air in the chamber headspace were used. During the measurement, the chamber was placed in a 0.02m deep collar that had been carefully installed on the soil to ensure sealing but not to cut any root connections. In winter, when the snowpack was deeper than 0.10m, fluxes were measured with the help of an 0.10m deep collar inserted on the snowpack prior to measurement. Four air samples (20ml) were drawn into syringes at 10min intervals (5, 15, 25, 35min). The samples were analysed within 24h using a gas chromatograph with flame ionisation and electron capture detectors in the laboratory of the Finnish Forest Research Institute, Vantaa Unit. Gas fluxes were calculated from the change of the gas concentration over time (slope estimated by linear regression). All data were used, regardless of the goodness of fit of the regression, as in most cases the fluxes were very small and close to the detection limit (below 0.4mgm−2day−1). Fluxes exceeding 0.4mgm−2day−1had minimum/mean R2 values of 0.63/0.91 (CH4)and 0.67/0.90 (N2O). Four measurements were deleted as a result of chamber sealing problems in winter. As the site was well drained, no ebullition events were observed, so the data represents diffusive or/and plant-mediated gas fluxes. During each chamber measurement, the soil temperatures at 0.05 and 0.30m were recorded and the WTL relative to each sampling point was measured from perforated water wells. Annual CH4and N2O balances were calculated as the mean of the 16 plots for each measurement day. Mean fluxes between these days were linearly interpolated and seasonal fluxes were then integrated from these interpolated daily fluxes. www.biogeosciences.net/8/3203/2011/ Biogeosciences, 8, 3203–3218, 2011 3208 A. Lohila et al.: Forestry-drained peatland is a carbon sink 3 Results 3.1 Meteorological conditions and water table level The measurement period from September 2004 to December 2005 was slightly warmer and wetter than average: at the three nearby weather stations (0309, 1302 and 1201) the mean annual temperature during the period varied from 5.0 to 5.5◦C as calculated for consecutive 365-day periods during the measurement period, whereas the mean annual long-term (1971–2000) temperature was 4.4◦C (Drebs et al., 2002). Annual precipitation during the measurement period ranged from 615 to 701mm, which is similar or slightly higher than the long-term average of 627mm. In 2005, annual mean temperature and precipitation were 5.2◦C and 626mm, respectively. Daily mean temperatures varied from −16 to 22◦C. The first snow appeared on the ground on 18 November 2004, but by 20 January 2005, the snow-pack had melted twice during warm spells. Thereafter, snow was present until mid-April with the maximum depth of 37cm being measured on 14 February. The largest deviations from the longterm monthly mean temperatures were observed in January, March and November 2005 (Fig. 5). The monthly precipitation exceeded the 30-year mean between May and August and also in January and November 2005. In contrast, September and October 2005 were especially dry. During the study period, the WTL ranged from −31 to −50cm, being deepest in mid-October 2005. 3.2 Seasonal dynamics and factors affecting the CO2 exchange The seasons were defined according to meteorological conditions. The start of the autumn season took place when the daily mean air temperature dropped permanently below 10◦C, and after that did not exceed 10◦C for more than two consecutive days. In 2004, when autumn started on 29 September, the amplitude of the diurnal NEE cycle was still rather high, but began to decline soon after this (Fig. 6). In 2005, the autumn period began on 12 October. The start of winter was defined as the first day when the daily mean temperature was less than 0◦C, and that the temperature stayed below 0◦C for at least five consecutive days. The winter seasons, which began on 17 November and 16 November in 2004 and 2005, respectively, were characterized by a low amplitudein NEE,which variedfrom −0.005to 0.1mgm−2s−1 and averaged 0.025mgm−2s−1. We defined the start of the spring season as the first day when the daily mean temperature exceeded 0◦C, and remained above that value for at least 10 consecutive days. The start of the spring, 31 March, likely coincided with the rapid increase in the CO2uptake, although the exact date is unknown due to a gap in the measurement data. Summer was specified to begin when the daily mean temperature exceeded 10◦C for five consecutive days, and afterwards did not drop below that value for more 1 2 3 4 5 6 7 8 9 10 11 12 Precipitation [mm] 0 20 40 60 80 100 120 140 160 180 Month of year Air temperature [ O C] -10 -5 0 5 10 15 20 2004 2005 1971-2000 a) b) Fig. 5. (a) Monthly mean temperature and (b) monthly cumulative precipitation during the study period and in 1971–2000. The bars indicate the range between the minimum and maximum values. 2004 2005 NEE [mg CO 2 m -2 s -1 ] -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 AWSp Su WA Su Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 6. Time series of the accepted half-hourly measurements of the CO2exchange at Kalevansuo in September 2004–December 2005. Positive values indicate emission of CO2from the ecosystem to the atmosphere. Letters indicate meteorologically defined seasons (Su = summer, A = autumn, W = winter, Sp = spring). than five consecutive days. This took place on 10 May 2005. The highest uptake rates of about −0.85mgm−2s−1were observed at the end of July, at about the same time as the maximum night-time respiration of about 0.45mgm−2s−1. Although NEE was mainly positive in winter, periods of small uptake of CO2were detected occasionally, particularly between November 2004 and mid-February 2005. We found that when temperatures fell below −3◦C, CO2fluxes Biogeosciences, 8, 3203–3218, 2011 www.biogeosciences.net/8/3203/2011/ A. Lohila et al.: Forestry-drained peatland is a carbon sink 3209 Air temperature [ o C] -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Day Night NEE [mg CO2 m-2 s-1] a) Time of day 23:00 03:00 07:00 11:00 15:00 19:00 23:00 NEE [mg CO 2 m -2 s -1 ] -0.04 -0.02 0.00 0.02 0.04 T air -3 o 0.00 0.02 0.04  T air -3 o  b) Fig. 7. (a) Average (±standard error) nocturnal and day-time (PPFD<1 and PPFD>1µmolm−2s−1, respectively) NEE plotted against air temperature in winter. The data are grouped into 12 classes with equal amount of observations (n=170 and n=93 in night and day, respectively). (b) Half-hourly medians (±25th and 75th percentiles) of measured NEE in winter when air temperature is below −3◦C (upper graph) and above −3◦C (lower graph). measured during the daytime and night-time were similar, increasing with higher temperature (Fig. 7a). When the air temperature exceeded −3◦C, the difference between the day and night fluxes increased rapidly due to CO2uptake by the trees and ground vegetation during the daytime. When Tair was lower than −3◦C, the diurnal variation in NEE disappeared, whereas with higher temperatures, a clear pattern with CO2uptake peaking at noon was observed (Fig. 7b). Irradiance is generally recognized as the most important variable causing diurnal variation in growing season NEE also. This was also observed at the Kalevansuo peatland forest. However, a significant correlation between VPD and the 30min NEE was observed in May–August (Fig. 8). The correlation was strongest in July with the deepest regression slope and the highest coefficient of determination being observed then. However, since temperature and VPD are likely to show a positive correlation, it is possible that the correlation between NEE and VPD can be attributed to increased respiration rather than decreased photosynthesis. To determine the impact of Tair and VPD on ecosystem processes, we grouped the July NEE observations at full radiation (PPFD>1200µmolm−2s−1)into temperature classes of 1◦C. In each class, a linear correlation between NEE and VPD was observed (Fig. 9). However, the correlation was not explained by the temperature (Fig. 9). 3.3 CO2balance In 2005, annual Rtot was 2750gCO2m−2, GPP was −3620gCO2m−2and annual NEE was −871±100gCO2m−2(−238±27gCm−2yr−1), where ±denotes the uncertainty estimate (see Appendix B1). If calculated using a moving 365-day window over the whole period (2 September 2004–31 December 2005), the annual balance ranged from −806 to −898gCO2m−2yr−1, the highest NEE (lowest net CO2uptake) being observed during an annual period starting on 2 September 2004. In autumn 2004, the daily CO2balance varied from −5 to 10gm−2day−1, the ecosystem being a sink during warm and sunny days and a source during overcast days (Fig. 10). After mid-November (start of the winter), photosynthesis ceased, and the daily balance turned positive. Wintertime NEE was on average 2.0gCO2m−2day−1. During the winter months, the lowest average NEE was measured in March (1.6gCO2m−2day−1). This was attributed to the low respiration rate following the lowest monthly mean temperature, rather than to the occurrence of CO2uptake, which was only observed during a few days. At the beginning of April and during the spring season, the forest turned to a CO2sink. With a few exceptions, the ecosystem acted as a CO2sink until mid-September. During this period, positive daily CO2 balances were only observed during cloudy days, e.g. 8–11 August 2005, when the precipitation sum exceeded 50mm. In 2005, the length of the sink period was approximately 210 days, ending at the beginning of November, two weeks after the end of summer and two weeks before the start of winter. Again, a strong reduction in photosynthesis coincided exactly with the beginning of the winter season. The contribution of the wintertime photosynthesis on the seasonal and annual balances was estimated by calculating the balance by different methods. First, the CO2balance for the winter period (17 November 2004–30 March 2005) was calculated as a sum of the modelled respiration (Eq. 2). The result, 286gCO2m−2, represents a situation where no photosynthesis occurred during winter. If calculated as a sum of the measured and modelled values, then gap-filling with the modelled respiration data results in a balance of 279gCO2m−2. In this estimate, winter photosynthesis is www.biogeosciences.net/8/3203/2011/ Biogeosciences, 8, 3203–3218, 2011 3210 A. Lohila et al.: Forestry-drained peatland is a carbon sink a) MAY -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 VPD [hPa] 0 5 10 15 20 25 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 NEE [mg CO2 m-2 s-1] -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.0 0.2 0.4 0.6 0.8 1.0 b) JUNE c) JULY d) AUGUST b 0 = -0.400 b 1 = 0.009 r ² = 0.169 b 0 = -0.607 b 1 = 0.019 r ² = 0.347 b 0 = -0.755 b 1 = 0.024 r ² = 0.613 b 0 = -0.624 b 1 = 0.012 r ² = 0.094 f VPD Fig. 8. Half-hourly NEE measured during high irradiance (PPFD>1000µmolm−2s−1)plotted against vapour pressure deficit (VPD) in May–August 2005. The parameter values of the linear regression (NEE = b0+b1×VPD) are shown. In (c) the line shows the fVPD function used in Eq. (4), defined according to the regression for the July data. 10 6 8 10 12 14 16 NEE [mg CO 2 m -2 s -1 ] -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 12 13 14 15 16 17 18 19 -0.6 -0.5 -0.4 -0.3 -0.2 20.0 20.5 21.0 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 NEE [mg CO 2 m -2 s -1 ] 6 8 10 12 14 16 18 20 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 22.0 22.5 23.0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 a) r ²=0.45 b) r ²=0.01 c) r ²=0.81 d) r ²=0 24.0 24.5 25.0 -0.6 -0.5 -0.4 -0.3 -0.2 e) r ²=0.72 f) r ²=0.04 10 12 14 16 18 20 22 -0.5 -0.4 -0.3 -0.2 -0.1 26.0 26.5 27.0 -0.5 -0.4 -0.3 -0.2 -0.1 g) r ²=0.75 h) r ²=0.02 VPD [hPa] T air [ o C] Fig. 9. Measured half-hourly NEE (PPFD>1200µmolm−2s−1) plotted against VPD (left panel) and air temperature (right panel) in temperature ranges 20–21◦C(a, b), 22–23◦C(c, d), 24–25◦C(e, f) and 26–27◦C(g, h). Parallel plots represent the same NEE data. accounted for in that part of the data where accepted records exist. However, since the gap-filling was done with Eq. 2, which ignores photosynthesis, NEE is potentially overestimated when Tair >−3◦C. If the gaps were filled using Eq. (4) that includes the radiation-response, the wintertime CO2balance was 264gm−2. This method represented the best estimate and was used in the calculation of the wintertime and annual balances presented in this paper. 3.4 CH4and N2O fluxes between the soil and the atmosphere The site was a small sink of CH4 (−0.35±0.58mgCH4m−2day−1; mean ±SD) and a small source of N2O (0.30±0.25mgN2Om−2day−1). N2O fluxes were very similar at all 16 sampling points (Fig. 11): no statistical differences were detected (ANOVA; p=0.358). In contrast, CH4fluxes varied between the sampling points (p < 0.001), and ranged from sample points with a small consumption to points with small emissions (Fig. 11). Surprisingly, the highest consumption (−1.0±0.5mgm−2day−1)of CH4was measured at plot 2, which also had the highest WTL, whereas Biogeosciences, 8, 3203–3218, 2011 www.biogeosciences.net/8/3203/2011/ A. Lohila et al.: Forestry-drained peatland is a carbon sink 3211 2004 2005 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan NEE, GPP,  TOT [g CO 2 m -2 day -1 ] -30 -20 -10 0 10 20 Precipitation [mm] 0 10 20 0 600 1200 1800 2400 Temperature [ o C] -20 -10 0 10 20 Snow depth [cm] 0 10 20 30 WTL [cm] -50 -40 -30  TOT GPP NEE PPFD [mol m -2 day -1 ] PPFD  AIR  SOIL Rain Snow depth WTL Fig. 10. Daily NEE (top black bars), modelled Rtot and GPP (lines in the upper panel), snow depth (light gray bars), water table level (WTL; open circles), daily cumulative photosynthetic radiation (PPFD, dark gray bars), daily air temperature (thick line in the bottom), daily soil temperature at 5cm depth (thin line in the bottom), and precipitation (black bars in the bottom) at Kalevansuo peatland forest from September 2004 to December 2005. The snow depth and precipitation data represent the average of the four nearest weather stations. the highest emissions (0.02±0.40mgm−2day−1)were measured at plot 1 with the deepest water levels. Emissions showed no correlation with the plant community (forest mosses/Sphagnum), nor with the microtopography (hummock/non-hummock) (Fig. 11). CH4consumption and N2O emissions peaked at midsummer, although some high N2O emissions were also detected in winter (Fig. 12). The highest CH4emissions were detected in autumn 2005 at plots 3 and 4. The seasonal and annual fluxes were integrated from the interpolated fluxes (Fig. 12). The growing season (May– September) fluxes were on average −58mgCH4m−2and 47mgN2Om−2and the annual fluxes (August 2004–July 2005) were −120mgCH4m−2and 100mgN2Om−2. When converted to global warming potential values using a time horizon of 100 years (Forster et al., 2007), the annual emissions were −3 and 30gCO2eq.m−2. 4 Discussion 4.1 Annual uptake of CO2exceeds peat decomposition In this paper we have reported a significant CO2sink in a drained peatland forest in southern Finland. Ombrotrophic treed peatlands, such as the Kalevansuo site presented in this study, constitute over 10% of the total area (5.4–5.7 million ha) of peatlands ever drained for forestry in Finland (Minkkinen et al., 2002), and the site type (dwarf-shrub type) is the single most commonly drained one in southern Finland. Such sites therefore have general importance in regard to their impacts on global-scale GHG balances and climate, for example. The annual net CO2uptake at our site (240±30gCm−2) was very likely higher than the amount of C accumulated in the tree biomass (175±35gCm−2), which indicates that the forest floor, i.e. the ground layer vegetation and the underlying peat must also have constituted a C sink. In general, the mineralisation rate of peat should increase after drainage, since lowering of the water level increases the aeration of the surface peat (Laine et al., 2006; Nyk¨ anen et al., 1995; www.biogeosciences.net/8/3203/2011/ Biogeosciences, 8, 3203–3218, 2011 3218 A. Lohila et al.: Forestry-drained peatland is a carbon sink Saarnio, S., Morero, M., Shurpali, N. J., Tuittila, E.-S., M¨ akil¨ a, M., and Alm, J.: Annual CO2and CH4fluxes of pristine boreal mires as a background for the lifecycle analysis of peat energy, Boreal Environ. Res., 12, 101–114, 2007. 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