Mitä tapahtuu ojitettujen soiden turvevarastoille muuttuvien ympäristötekijöiden sekä metsityksen ja avohakkuiden vaikutukset

摘要

Climate change is likely to affect the large carbon (C) stocks of northern peatlands. These C reservoirs may further be affected by human-induced forestry activities and changes in land use. Possible responses of peatland C storages to these changes have significant uncertainties mainly because of the difficulties in predicting peat decomposition rates in changing conditions. This study aims at revealing the effects of abiotic drivers, especially soil temperature and water table level (WL), on peat decomposition rate indicated by heterotrophic peat soil respiration (RPEAT) in drained forested peatlands. Furthermore it aims to describe the changes in RPEAT following clearfelling in forestry-drained peatlands and afforestation of former agricultural organic soil croplands. For this, RPEAT was estimated using chambers to measure CO2 efflux from trenched litter-free plots, at nine afforested organic soil cropland sites and one forestry-drained site with clearfelling treatment. This study revealed that within the studied sites soil temperature was the main driver of RPEAT. It was also apparent that the old peat storage in these sites was rather resistant to the short-term changes in WL conditions; i.e. fluctuations of WL caused only minor changes in RPEAT. The study also demonstrated that in low water level conditions there were mechanisms that could hinder RPEAT. Excessive WL drawdown (>61cm ) was observed to reduce RPEAT and furthermore, in low water level conditions the temperature sensitivity of RPEAT was reduced. These findings suggest that climate change and the associated increase in temperature would have the potential to substantially increase soil C release from drained peatlands. This C release may, however, be constrained, if warming is accompanied by changes in evapotranspiration, precipitation regimes, or the frequency of extreme events (e.g. droughts) that would severely affect WL and surface soil moisture conditions. Changes in environmental conditions following clearfelling caused only small absolute changes in RPEAT. Following clearfelling, a decrease in evapotranspiration raises water table level which in turn decreases the volume of aerated peat layers. Furthermore the soil surface is exposed to direct solar radiation which causes excessive dryness in surface soil. These factors are capable of compensating and overruling the effect of increased soil temperatures on RPEAT following clearfelling. Below the logging residue piles, however, considerable increase in RPEAT was observed. This indicates that human induced forestry activities could potentially cause significant C release from the oldest and largest C stocks in these ecosystems This C release may, however, be avoided if logging residue is removed from the site. Further investigations on longevity of the effect of logging residues on CO2 emissions as well as on the sources of CO2 under logging residues are required to confirm these findings. The estimated RPEAT from the afforested organic soil croplands varied widely between the sites (from 750 to 2000 g CO2 m–2 a–1). It appeared that afforestation has the potential to reduce the extremely high soil CO2 effluxes of actively cultivated peat soils. Despite this, agricultural history has obvious effects on peat properties and observed RPEAT, with the result that these soils remain sources of CO2.