wise use of mires and peatlands - Peatland Ecology Research Group

80 VALUES AND FUCTIONS OF MIRES AND PEATLANDS(n) Regulation of regional and localclimatesMires have a specific microclimate, whichoften differs from that of their immediatesurroundings. Their microclimate ischaracterised by a greater variation intemperature, higher air humidity, greater fogfrequency and greater risk of night frostscompared with that of mineral soils. Thereare slight differences between the maximumtemperatures and pronounced differencesbetween the minimum temperatures of mirescompared with surrounding mineral areas 186 .Peatlands are by nature wet landscapes andare usually situated in terrain depressionsinto which cooler and heavier air flows(“Kaltluftseen”). This stimulates fog and dewformation 187 . As a consequence, the soils ofboth pristine and reclaimed peatlands aresignificantly cooler in summer than mineralsoils, and the air temperature is also lower.Forested tropical peatlands have lower meanand maximum temperatures than those thathave been deforested 188 .Mires and peatlands strongly depend on theprevailing climate. On the other hand, theyalso influence the regional and local climatethrough evapotranspiration and associatedalteration of heat and moisture conditions.This influence is larger in warmer or drierclimates and smaller when the regional climateis colder or more humid. In areas withextensive peatlands the regional climate isconsequently more humid and cool 189 .Drainage of mires in the boreal zone leads toa reduction in the minimum temperatures anda shortening of the yearly frost-free period, aprocess that is reversed by subsequentafforestation 190 .(o) Regulation of catchment hydrologyTraditionally, peatlands were generally seenas reservoirs or “sponges” storing waterduring wet periods and releasing it slowlyduring ensuing dry spells. In this way theywere believed to reduce flooding followinghigh precipitation and sustain water flowduring times of low precipitation andconsequently to have a “buffering” effect oncatchment hydrology. This traditional viewcan, however, no longer be upheldunconditionally 191 .With respect to the hydrologic reservoir orwater storage function of peatlands it isnecessary to distinguish between a static anda dynamic storage component 192 . The staticcomponent is the water in the permanentlywater-saturated peat layers (the catotelm) andthe water that is physically or chemicallybound into the uppermost peat layers whichare periodically exposed above the waterlevel. Depending on their thickness andextent, peatlands may have a very large staticwater store as undrained peat consists of 85- 95% water. In general this water either doesnot move or moves only slowly and thereforescarcely participates in the annual watercycling and regional water regulation.The dynamic storage component consists ofthe rapidly exchangeable water volumes inand over the uppermost peat and vegetationlayers (the acrotelm), or, in drained peatlands,the uppermost soil. It is composed of(drainable) soil pore water, water cushions inand under the peat, water in peatland hollowsand pools, and inundation water. Thesedifferent fractions imply that different miretypes may have completely different dynamicstorage characteristics.As peat accumulation requires high waterlevels at the mire surface during most of theyear, the dynamic storage capacity of mostmire types is limited. In times of abundantwater supply the available storage is rapidlyfilled and the surplus water drains quickly.Peat-covered areas therefore generally showconsiderable surface run-off, directlyconsequent on precipitation, and littlebaseflow 193 .