The role of permanent site factors in the assessment of soil treatment effects : A case study with a site preparation trial in jack pine plantations on glacial outwashes

In silvicultural trials, the confounding influence of permanent soil properties is assumed to be minimal. A covariance analysis using total elemental concentrations of parent material (geochemistry) and soil particle size distribution (texture) was used to understand the role that these variables could play in tree growth and foliar nutrient status, and in the growth response to site preparation of 16- to18-yr-old jack pine plantations growing on seemingly homogeneous glacial outwash deposits. Three treatments were tested in a replicated design: direct plantation, and site preparation with TTS or with Bracke. The combination of particle size distribution and soil geochemistry explained the site quality index (SQI) in control plots (R2 = 0.94) better than soil texture alone. In all plots, SQI was strongly related to K and Mg concentrations in foliage and in humus and, to a lesser degree, to foliar N concentrations. A covariance analysis using soil texture and geochemical composition indicated that while si...