Land-use changes have contributed to increased atmospheric CO 2 concentrations. Conversion from natural peatlands to agricultural land has led to widespread subsidence of the peat surface caused by soil compaction and mineralization. To study the net ecosystem exchange of carbon (C) and the contribution of respiration to peat subsidence, eddy covariance measurements were made over pasture on a welldeveloped, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water table fluctuated between 0.02 m in winter 2002 to 0.75 m during late summer and early autumn 2003. Peat soil moisture content varied between 0.6 and 0.7 m 3 m À3 until the water table dropped below 0.5 m, when moisture content reached 0.38 m 3 m À3 . Neither depth to water table nor soil moisture was found to have an effect on the rate of nighttime respiration (ranging from 0.4-8.0 lmol CO 2 m À2 s À1 in winter and summer, respectively). Most of the variance in night-time respiration was explained by changes in the 0.1 m soil temperature (r 2 5 0.93). The highest values for daytime net ecosystem exchange were measured in September 2002, with a maximum of À17.2 lmol CO 2 m À2 s À1 . Grazing events and soil moisture deficiencies during a short period in summer reduced net CO 2 exchange. To establish an annual C balance for this ecosystem, non-linear regression was used to model missing data. Annually integrated (CO 2 ) C exchange for this peat-pasture ecosystem was 45 AE 500 kg C ha À1 yr À1 . After including other C exchanges (methane emissions from cows and production of milk), the net annual C loss was 1061 AE 500 kg C ha À1 yr À1 .

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m–2 y–1 (265 kg C ha–1 y–1) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO2, while during the rest of the year there is net production of CO2. The average daytime assimilation rates ranged between – 0.2 and – 0.5 mg CO2 m–2 s–1 (– 45 and –11.3 μmol CO2 m–2 s–1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these factors, LAI and the soil temperature codetermine the net exchange of CO2. The total nocturnal respiration during the growing season lies within the same order as the average daytime net assimilation rate. Temperature was found to be the main factor controlling soil respiration, with a Q10 of 4.8.

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Observations of sensible and latent heat¯ux above forest downwind of a forest edge show these¯uxes to be larger than the available energy over the forest. The enhancement averages to 56 W m À 2 , or 16% of the net radiation, at fetches less than 400 m, equivalent to fetch to height ratios less than 15. The enhancement of turbulent energy¯uxes is explained by advection and increases with the difference in temperature and humidity of the air over the upwind area as compared to the forest. The relatively high temperature and humidity of the upwind air are not caused by high surface heat¯uxes, but are explained by the relatively low aerodynamic roughness of the upwind surface. Although the heat uxes over forest are enhanced, the momentum¯uxes are almost adjusted to the underlying forest. The different behaviour of heat and momentum¯uxes is explained by absorption of momentum by pressure gradients near the forest edge. It is concluded that fetch requirements to obtain accurate surface¯uxes from atmospheric observations need to be more stringent for scalar¯uxes as compared to momen-tum¯uxes. 232W. Klaassen et al.Increased heat¯uxes near a forest edge 241

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