Establishment, growth and survival of natural regeneration after clearcutting and drainage on forested wetlands

Natural regeneration may be disrupted by the rise of the water table in sur face layers after clearcutting forested wetlands. A study was initiated on eight forested wetlands that were successively clearcut and drained 3 years later. The objectives were (1) to assess conifer and deciduous regeneratio n on waterlogged clearcut sites, (2) to determine the effect of water table level changes after clearcutting and drainage on the growth rate of advanc e regeneration, and (3) to determine if sphagnum moss growth rate was influ enced by clearcutting, drainage and microrelief. 3 years after strip clearc utting, the area based inventory showed that softwood regeneration was abun dant in the clearcut, but 84% of the seedlings were smaller than 30 cm and vulnerable to suppression from the competition. Indeed, the opening of the forest cover promoted massive invasion of pioneer species such as trembling aspen (Populus tremuloides Michx.) and white birch (Betula papyrifera Mars h.). Black spruce (Picea mariana (Mill.) B.S.P.) and red spruce (Picea rube ns Sarg.) had difficulties maintaining their presence after clearcutting. T he ratio hardwood stems:softwood stems changed from 1.1 in the forest to 3. 8 in the clearcut. Balsam fir (Abies balsamea (L.) Mill.), larch (Larix lar icina (Du Roi) K. Koch) and cedar (Thuja occidentalis L.) represented 93% o f the softwood regeneration. 3 years after clearcutting, balsam fir advance regeneration had a significa ntly lower growth rate in the middle of the clearcut than near the edges of the clearcut. 2 years after drainage, seedling growth at 10 m from the dra inage ditch was significantly greater than at 70 m from ditch. No significa nt differences were found between mineral and organic soil types, but heigh t growth was positively correlated to C : N ratios of the individual site t ypes. Abundant competition on richer sites limits the development of softwo od regeneration. On a short term basis, competition more than watering-up s eems a threat to softwood regeneration. Sphagnum growth rates measured with the crank-wire method showed no effect of clearcutting or drainage, but a higher growth rate in hollows (3.2 cm per year) compared to hummocks (2.0 c m per year). These growth rates indicate that sphagnum should not affect re generation but could overgrow yearlings of slow growing species. (C) 2000 E lsevier Science B.V. All rights reserved.