Site quality, or the ability of land to grow trees, is an important component for identifying the most appropriate locations for establishing plantations of fast growing tree species to meet societys demands for timber and other environmental benefits. The goal of this thesis project was to predict site quality for poplars using soil and site information in Saskatchewan, Alberta and Quebec and to examine the effect of site quality on competition control in hybrid poplar plantations in Saskatchewan.<p>The first study examined factors affecting trembling aspen productivity in the boreal shield ecozone of Quebec on till and fluvial parent materials using general map data and measured soil and site information. Relationships with productivity were stronger using measured soil and site variables for individual parent materials (R2>0.6) than using general map data only (R2<0.25). Including biological variables, such as overstory species composition, had a major impact on site quality with conifer dominance negatively impacting the growth of trembling aspen.<p>The second study examined the factors affecting trembling aspen productivity in the boreal transition ecoregion of Saskatchewan on three different soil parent materials: fluvial, lacustrine and till. Relationships with productivity were stronger using soil and site variables for individual parent materials (R2 0.48-0.58) than using agricultural capability classes or other soil properties for all plots combined (R2<0.2). For fluvial and lacustrine sites, increasing clay content and nutrient availability (e.g. pH and total N) were positively related to productivity while tree productivity was negatively related to poor drainage for till sites. <p>The third study examined the factors affecting site quality for a single hybrid poplar clone in industrial plantations in Alberta at both the local scale (between plantations) and the microsite scale (within plantations). At the local scale, foliar P and Cu concentrations, soil water availability and drainage, and Ca and Mn in the C horizon were related to hybrid poplar productivity. There were also curved relationships with productivity and soil texture in the B horizon and pH of the A horizon, indicating an optimal range for poplar growth. At the microsite scale, soil texture was the best predictor of productivity with different relationships at each site depending on where the sites were in relation to the optimal soil texture.<p>The final study examined the response of hybrid poplar plantations in Saskatchewan to interspecific competition control on a range of site productivities. Competition control greatly increased tree growth with the greatest benefit being on the best quality sites. Both water and nutrients were highly competed for between trees and weeds. In the weed-free plots, tree growth was positively related to the amount of silt and clay in the soil and foliar P concentrations. This series of studies has demonstrated that it should be possible to predict poplar productivity reasonably well using only soil and site information within limited areas across Canada. However, the important drivers of productivity varied between the regions studied and between site groupings, such as by parent material, within local areas. This information can now be used to help land managers make better decisions regarding the establishment and management of plantations of fast growing tree species, notably hybrid poplar plantations.

Site quality, or the ability of land to grow trees, is an important component for identifying the most appropriate locations for establishing plantations of fast growing tree species to meet societys demands for timber and other environmental benefits. The goal of this thesis project was to predict site quality for poplars using soil and site information in Saskatchewan, Alberta and Quebec and to examine the effect of site quality on competition control in hybrid poplar plantations in Saskatchewan.<p>The first study examined factors affecting trembling aspen productivity in the boreal shield ecozone of Quebec on till and fluvial parent materials using general map data and measured soil and site information. Relationships with productivity were stronger using measured soil and site variables for individual parent materials (R2>0.6) than using general map data only (R2<0.25). Including biological variables, such as overstory species composition, had a major impact on site quality with conifer dominance negatively impacting the growth of trembling aspen.<p>The second study examined the factors affecting trembling aspen productivity in the boreal transition ecoregion of Saskatchewan on three different soil parent materials: fluvial, lacustrine and till. Relationships with productivity were stronger using soil and site variables for individual parent materials (R2 0.48-0.58) than using agricultural capability classes or other soil properties for all plots combined (R2<0.2). For fluvial and lacustrine sites, increasing clay content and nutrient availability (e.g. pH and total N) were positively related to productivity while tree productivity was negatively related to poor drainage for till sites. <p>The third study examined the factors affecting site quality for a single hybrid poplar clone in industrial plantations in Alberta at both the local scale (between plantations) and the microsite scale (within plantations). At the local scale, foliar P and Cu concentrations, soil water availability and drainage, and Ca and Mn in the C horizon were related to hybrid poplar productivity. There were also curved relationships with productivity and soil texture in the B horizon and pH of the A horizon, indicating an optimal range for poplar growth. At the microsite scale, soil texture was the best predictor of productivity with different relationships at each site depending on where the sites were in relation to the optimal soil texture.<p>The final study examined the response of hybrid poplar plantations in Saskatchewan to interspecific competition control on a range of site productivities. Competition control greatly increased tree growth with the greatest benefit being on the best quality sites. Both water and nutrients were highly competed for between trees and weeds. In the weed-free plots, tree growth was positively related to the amount of silt and clay in the soil and foliar P concentrations. This series of studies has demonstrated that it should be possible to predict poplar productivity reasonably well using only soil and site information within limited areas across Canada. However, the important drivers of productivity varied between the regions studied and between site groupings, such as by parent material, within local areas. This information can now be used to help land managers make better decisions regarding the establishment and management of plantations of fast growing tree species, notably hybrid poplar plantations.