Crown release as a potential old-growth restoration approach in northern hardwoods

The question of whether crown release might hasten the development of large trees in second-growth northern hardwoods was examined by measuring 6- to 10-year growth response to a wide range of past thinning treatments in seven stands in northern Wisconsin. Percent increase in basal area growth after thinning was linearly correlated with percent plot basal area removed and with percent crown perimeter release of individual trees. Trees on untreated plots showed a mean 7% growth decline, while mean response on treated plots ranged from a 21% increase for trees given 25% crown perimeter release to an 88-107% increase for trees given full release. A basal area increment model calibrated with the data suggests that 30 cm DBH sugar maples (Acer saccharum Marsh.) given full crown release would reach 50 cm DBH (the mean size of canopy trees in old-growth stands) in 46-49 years, compared with 92 years with no treatment. In addition to accelerating the development of large trees, crown release has the potential for enhancing foliar height diversity and increasing the number of canopy gaps, standing snags, and fallen logs, all of which occur only to a limited extent in existing second-growth, even-aged stands.