Effect of forest drainage on the carbon balance of a mire: a case study

Natural mires generally act as sinks for CO2 and are sources of CH4 for the atmosphere. However, drainage may transform these ecosystems into net sources of CO2, but the increase in growth of the tree stand and consequent fixation of carbon after drainage may be expected to have a compensating effect. The post-drainage change in the peat carbon stores was determined using a method based on the bulk density and carbon content profiles measured along a transect running from the undrained part to the drained part of the mire. The difference between the undrained and drained peat carbon stores, averaged over the 30 yr period after drainage, indicated that the accumulation of carbon had been 35 g C m-2 yr-1 greater in the undrained site. As the long-term accumulation rate of the undrained site had been 21 g C m-2 yr-1, the drained part had lost 14 g C m-2 yr-1 from the pre-drainage carbon store. Total accumulated biomass in the tree stand was estimated from stem wood development using the growth and yield tables for this site type. The stem wood volumes were transformed into total biomass values using published stem-volume/total-biomass ratios. If no harvesting operations were to occur during the first 300 yrs, the tree stand would produce an average carbon store of approximately 11 kg C m-2. The application of a standard thinning regime would reduce this storage by ca 50%. Expressed as an average over 300 yrs, drainage increased the total carbon store of the mire if no tree harvesting was done and the store remained unchanged if standard harvesting procedures were used. 24 refs, 3 figs