WATERING UP AFTER CLEAR-CUTTING ON FORESTED WETLANDS OF THE ST-LAWRENCE LOWLAND

Clear-cutting on wetlands of the St. Lawrence lowlands raised the wate r table levels on seven of the eight studied sites encompassing five f orest types and four soil subgroups. Water table levels dropped 3 cm a fter clear-cutting on the eighth site, which was the wettest site with precut water levels within the top 10 cm. This was explained by evapo ration from exposed water surfaces. The magnitude of the water table r ises increased with the depth of the precut water table. The seasonal mean and maximum rise were respectively 20 and 52 cm on a poorly drain ed mineral soil which had the lowest precut water table levels. The sm allest rises, with means around 7 cm, were associated with high precut water table on bogs and on fens. The watering up was not reduced on f ens where a lateral how occurs. This study indicated that transition s ites between the bogs or Mns and the uplands were most susceptible to hydrologic changes after clear-cutting. Watering up was caused by redu ced evapotranspiration, of which the major component was interception. The rise of the water table observed in the clear-cut and the borderi ng forest indicates that clear-cutting in narrow strips is not an effe ctive solution to avoid water table rise. Silvicultural treatments to maintain interception and transpiration by leaving logging debris, sma ll trees, and preestablished regeneration would be more effective.