Methane flux dynamics during mire succession.

Vegetation, temperature and hydrology are major factors controlling wetland methane (CH(4)) dynamics. In order to test their importance, we measured CH(4) emissions and environmental characteristics over 2 years from five mires representing a successional sequence, ranging in age from 178 to 2,520 years. We hypothesized CH(4) emissions to be higher from the sedge-dominated fens than from the older bog stage. The more constant hydrological conditions at later successional stages as a consequence of the thicker peat layer appeared to result in lower temporal variation in CH(4) emissions. Accordingly, the other controls, temperature and vegetation, had an effect on CH(4) emissions only when the water table was sufficiently high. The seasonal variation in CH(4) emissions was controlled by temperature only at the oldest study site, which had the lowest variation in water table. Within-season variation in emissions related to plant phenology was highest at the fen stage, which was dominated by aerenchymatous plants with a strong seasonal pattern, namely sedges and forbs. In contrast to our hypothesis, CH(4) emissions increased with mire age towards the bog stage. However, the trend did not emerge during a rainy growing season, due to a rise in CH(4) emissions at the younger stages. The results may imply two different mechanisms during mire succession: while old mires are able to avoid the perturbation associated with variation in the water table and maintain their function as CH(4) emitters, young mires are exposed to perturbation but are able to recover their function.