Water table changes following precommercial thinning on post-harvest drained wetlands

Precommercial thinning is a silvicultural treatment commonly used to reduce the number of stems in densely regenerated stands. When applied to organic or mineral wetland sites, this treatment drastically eliminates a great portion of the canopy, which is responsible for maintaining evapotranspiration processes. The water table is susceptible, therefore, to rise in those sites where evapotranspiration is a significant regulator of the hydrological balance. The objective of this study was to investigate and quantify water table rise after precommercial thinning on post-harvest, drained forested wetlands, and to relate this rise to factors influencing evapotranspiration processes. The water table rose after precommercial thinning regardless of soil type, the distance to the drainage ditch, or the year of observation. In the second year after thinning on mineral soils, canopy openings were invaded by abundant and vigorous ground vegetation. Water table rise was significantly lower at that time than for the preceding year, indicating a tendency for faster hydrological recovery on mineral soil sites. The water table rise was related to the basal area removed, indicating the importance of woody vegetation in regulating water table depth in forested wetlands. Seasonal patterns in water table depth further illustrated the role of vegetation in the hydrological balance of such sites. Light to moderate thinning practices, therefore, appeared to be the most appropriate recommendation to avoid watering-up in young forested wetland stands.