Long-term effects of forest management on nutrient cycling in spruce-fir forests

Abstract

Nutrient dynamics and soil disturbance have been monitored since 1979 on paired control and intensively managed watersheds in central Maine, USA. The mature spruce-fir stand was harvested by whole-tree clearcutting in 1981. Triclopyr was aerially applied to most of the same watershed 4 years after harvest to control hardwoods competing with conifer regeneration. In 1991, a series of plots on the treated watershed were precommercially thinned to stimulate growth of selected crop trees. Then in 1993, nitrogen fertilizer was applied to a subset of thinned plots to further stimulate crop tree growth.

Disturbance induced changes in soil solution chemistry differed by soil drainage class. Soil solution NO₃-N concentrations increased from near zero before harvest to 20 mg/l the second growing season following harvest on moderately well drained soils, more than two times corresponding values observed on somewhat poorly drained soils. Net nutrient losses resulting from harvest were <5% of total capital for the nutrients studied. Nitrate-N concentrations on poorly drained soils did not differ from those on the uncut watershed. This same temporal pattern was repeated following application of triclopyr in 1985, although peak concentrations were not as high as those following harvest. Elevated concentrations of nutrients in soil solution following both harvest and herbicide application disappeared within 3 years, concurrent with regeneration.

Precommercial thinning (PCT) reduced stem density approximately 12-fold, increasing crop tree DBH for both balsam fir (4 cm on control plots versus 6 cm on PCT plots) and red spruce (4 cm on control plots versus 5.5 cm on PCT plots). Although 34 Mg/ha of thinning slash remained after PCT (pre-harvest forest floor dry weight averaged 64 Mg/ha), soil solution chemistry was not affected and nutrients were apparently conserved following treatment. Soil solution nitrate and cation concentrations increased following N fertilization. The fertilizer treatment further increased balsam fir DBH (8 cm on fertilized plots) but not red spruce.

Soil disturbances associated with harvesting may be problematic. Compaction and rutting from the road system and repeated trips of the feller-forwarder were still highly evident. Much of this area remains out of production and likely will be characterized by poor tree establishment and reduced growth through the current rotation.