Eastern white pine response to release 30 years after partial harvesting in pine mixedwood forests

Exploitive logging and wildfires during the past 150 years have created larger areas of pine mixedwood forests. A study was set up in pine mixedwood forests within the Petawawa Research Forest, near Chalk River, Ontario, Canada to investigate increasing the growth, yield and quality of eastern white pine (Pinus strobus L). Understorey pine was released from an overstorey of mainly intolerant hardwoods by careful partial harvesting in two-storied stands. Growth and development of white pine were assessed at three stand densities (initial white pine basal areas of 7, 11.5 and 16 m2 ha−1 in 1971) in both partially harvested and uncut control plots. A randomized design with five replicates was used. Plots were re-measured 10, 20 and 30-years after treatment. A total volume of 123 m3 ha−1 of mainly intolerant hardwoods was harvested in 1971 from treated areas, yet total stand volumes in 2001 were similar to those in control plots. After partial harvesting, the rate of stand development was increased and much of what would have been natural hardwood mortality was used for pulpwood. The growth of the immature released white pine was increased significantly at all three initial basal areas. At the highest initial pine basal area, the periodic annual increment of pine was greatest during the 30-year period after partial harvesting. During the last decade, the annual increment of pine has declined, but in the partially harvested plots it still ranged between 6.1 and 8.3 m3 ha−1 year−1 being highest at the medium initial pine basal area. The total increment within the control plots was very low, especially during the last decade because of high losses to mortality of both aspen and white birch. The white pine increment was lower in the control plots at about half that in the partially harvested plots. The approach investigated here allowed for the effective use of the shorter-lived intolerant hardwoods while increasing the growth and development of white pine. This method could be applied to increase pine productivity, and possibly the extent of white pine in areas where pine is less common today than in earlier times. White pine is a fire-adapted species and with the effective forest fire suppression programs of today, silvicultural techniques are needed to maintain, enhance and in some cases, restore white pine forests.