Forty years of spruce-fir stand development following herbicide application and precommercial thinning in central Maine, USA

We examined the development of a 33-year experiment in spruce-fir stands that received nine herbicide treatments (applied aerially in 1977), with and without precommercial thinning (PCT) (applied in 1986). We tested two commonly held assumptions about the long-term effects of herbicide and PCT in mixedwood stands managed for softwoods: (i) herbicide release produces stands dominated by softwoods and (ii) PCT promotes both softwood dominance and merchantable volume. All herbicides were effective at releasing balsam fir (Abies balsamea (L.) Mill.) and spruce (Picea spp.) from overtopping shrubs and hardwoods that had naturally regenerated following clearcut harvesting in 1970. Glyphosate (Roundup), triclopyr amine (Garlon 3A), 2,4,5-T, and a mixture of 2,4,5-T and 2,4-D herbicide treatments were all effective at shifting long-term species composition to softwood dominance after 30 years, irrespective of the rates applied. Herbicide-treated stands that did not receive PCT were overstocked with softwoods for at least 24 years and produced less than half of the merchantable softwood volume by 40 years than herbicide-treated stands that received PCT. Intolerant hardwoods dominated untreated stands for all 33 years of the experiment. When unsprayed plots received PCT, however, they produced stands at 40 years that were compositionally and structurally indistinguishable from those that had received both herbicide and PCT treatments. Results from this experiment clearly demonstrated that early herbicide application, regardless of type and rate of herbicide, created softwood-dominated mixedwood stands over the long term and that PCT more than doubled merchantable softwood volume within 25 years of application.