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

Clear-cutting on wetlands of the St. Lawrence lowlands raised the water table levels on seven of the eight studied sites encompassing five forest types and four soil subgroups. Water table levels dropped 3 cm after clear-cutting on the eighth site, which was the wettest site with precut water levels within the top 10 cm. This was explained by evaporation from exposed water surfaces. The magnitude of the water table rises increased with the depth of the precut water table. The seasonal mean and maximum rise were respectively 20 and 52 cm on a poorly drained mineral soil which had the lowest precut water table levels. The smallest 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 fens where a lateral how occurs. This study indicated that transition sites between the bogs or Mns and the uplands were most susceptible to hydrologic changes after clear-cutting. Watering up was caused by reduced evapotranspiration, of which the major component was interception. The rise of the water table observed in the clear-cut and the bordering forest indicates that clear-cutting in narrow strips is not an effective solution to avoid water table rise. Silvicultural treatments to maintain interception and transpiration by leaving logging debris, small trees, and preestablished regeneration would be more effective.