wise use of mires and peatlands - Peatland Ecology Research Group

VALUES AND FUCTIONS OF MIRES AND PEATLANDS77As in agriculture, increased aeration of thepeat after forestry drainage results in fasterpeat mineralisation and a decrease in the peatcarbon store. In the boreal zone this aerationmay be accompanied by a decrease in peatpH and a lower peat temperature, which mayagain reduce the rate of peat mineralisation.As water-logging in mires generally preventsan economic level of wood production,peatland drainage aims to increase the woodyield (see § 3.4.1 [eb]). After drainage, forestvegetation (such as trees and shrubs) takesthe place of the original mire vegetation andthe peatland biomass carbon store (bothabove and below ground) increases quickly.This store would eventually reach a newequilibrium much higher than that of theformer mire vegetation. Before this stage isreached the wood is harvested and thebiomass store reduces substantially again.Peatland drainage for forestry also leads tochanges in the litter carbon store. The moistlitter in the mire’s acrotelm is generallyconsidered as part of the peat carbon storeas it gradually passes into the catotelm peat.The litter in a drained forest 181 is of differentquality and can be considered as a separatecomponent. The accumulation of litter leadsto an increase in the litter carbon store. Asthis litter accumulates under aerobicconditions, the litter carbon store eventuallyreaches an equilibrium and the netaccumulation stops. Depending on thepeatland type and the cutting regime of theforest, it might take centuries before thisequilibrium is reached.Peatland drainage for forestry therefore leadsto■■■a steady decrease of the peat carbon store,a rapid initial increase of the biomass carbonstore, the harvesting of which leads to asubstantial reduction, anda slow initial increase of the peatland littercarbon store which eventually, after somecenturies, reaches an equilibrium.Thus, the peatland carbon store, being thecombined effect of these processes, variesstrongly in time. In the first period afterdrainage, the increase in biomass and litterstores may strongly exceed the losses fromthe peat carbon store. As the biomass andlitter stores tend to an equilibrium, but thepeat carbon losses continue 182 , thecumulative carbon losses from peat oxidationprevail in the long run.With respect to gas exchange, the drainageof peatlands for forestry generally leads toan increase in carbon dioxide emissions, asubstantial decrease in methane emissionsand, depending on peatland type and type ofland use (fertilisation), to a sometimes drasticincrease in nitrous oxide emissions.Peat extraction: The effect of peat extractionand subsequent oxidation is similar to that ofburning fossil fuels 183 . The peat carbon storeis largely transformed into carbon dioxide.Efficient drainage in the extraction areas maymaintain high rates of carbon dioxideemissions while methane and nitrous oxideemissions remain fairly low.Integrated effects: Detailed national balanceswith respect to peat carbon stores andradiative forcing are available only forFinland. In Finland both undisturbed andforestry drained peatlands currently have apositive carbon balance, the former becauseof peat accumulation, the latter because ofan increase in root biomass and litter carbon.Table 3/16 presents the integrated effects ofvarious greenhouse gases on radiativeforcing. The strongly time-dependent effectof undisturbed mires is striking, because ofthe decreasing effect of methane. Estimatingthe effects on a 500-year horizon is even morespeculative than doing so on a 100-yearhorizon, and does not take into accountchanges in hydrology and temperatureresulting from global climate change.Peatland fires: In many areas of the worldnatural fires ignited by lightning strikes were