Chemical site preparation influences productivity, composition, and structure of boreal mixedwoods in Ontario, Canada

We examined the effects of chemical site preparation on jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) B.S.P.) plantations 14 years after application on boreal mixedwood sites in Ontario, Canada. The experimental treatments were four rates of hexazinone (0, 1, 2, and 4 kg (a.i.) ha−1 formulated as Velpar-L®) and two planting years (immediately and 1 year after herbicide application) in a split-plot design. At the individual-tree level, this study supported findings from previously published short-term results, i.e., increased crop tree diameter and volume growth with increasing hexazinone rates. At the stand level, volume of planted conifers did not increase with hexazinone rate for jack pine, but increased significantly for both container and bareroot black spruce. Planting immediately after hexazinone treatments resulted in greater crop tree and stand volume for both container jack pine and black spruce, but not bareroot black spruce. Total stand volume did not change with hexazinone rate or planting year, but stand composition shifted from hardwood-dominated to mixedwoods for both black spruce stock types while the compositional shift was less apparent in the jack pine plantations. An importance value used to describe height structure did not differ with hexazinone rate or planting year for jack pine. For black spruce plantations, importance values for non-crop conifer and hardwood decreased with increasing hexazinone rate, indicating a non-stratified, single-storied canopy structure on sites with high hexazinone rates and a stratified, two-storied structure on sites with lower hexazinone rates at early stages of stand development.