Ten‐Year Responses of Soil Quality and Conifer Growth to Silvicultural Treatments

Abstract

The development of sustainable forestry practices and credible certification systems relies on continuous monitoring of indicators. In the present study, carried out at the Petawawa Research Forest (Ontario, Canada), we evaluated the impacts of three intensive silvicultural treatments: scarification, fertilization, and herbicide treatment, applied alone or in combination—on indicators of organic layer quality, foliar nutrition, and tree growth—10 yr after establishment of eastern white pine ( Pinus strobus L.) and white spruce [ Picea glauca (Moench) Voss] plantations. We compared these 10‐yr results with measurements made 3 to 4 yr after plantation establishment. In both 1989 and 1996, the herbicide treatment had the greatest effect on organic layer quality. In 1996, compared with the no‐treatment control, herbicide application reduced organic C mass by 46%, total N mass by 15%, and acid phosphatase activity by 64%. These negative effects were offset when herbicide was applied in combination with fertilizer. The negative impact of herbicide on microbial biomass C noted in 1990 was no longer evident in 1996. In herbicide‐treated plots, the nitrate‐dominated cycle observed 1989–1990 was replaced by an ammonium‐dominated cycle in 1996. Although herbicide application negatively affected soil quality, it increased tree growth and generally improved foliar nutrition; thus organic layer and tree responses were not correlated. The indicators used were sensitive to changes in the ecosystem over time and signaled soil impacts that could have consequences for long‐term productivity.