Success of hybrid poplar plantations will rely on the efficient management of nutrients and weeds. Relatively little is known about the root uptake characteristics of hybrid poplar and weeds, their belowground interactions and particularly, the quantitative understanding of nutrient uptake using mechanistic models under weed-competing conditions. Therefore, the objectives of this study were to investigate the effects of dandelion and quackgrass on the growth of hybrid poplar, to establish their root uptake characteristics and to quantify their nutrient uptake using the soil supply and nutrient demand (SSAND) model. In a pot study, hybrid poplar stem height, root collar diameter, shoot and root biomass, root length, and N, P and K uptake significantly decreased in the presence of dandelion and quackgrass weeds. Similar weed competition effects on growth of hybrid poplar were also observed in the field at the Pasture and Alfalfa sites where hybrid poplar was grown with and without weeds for 50, 79 and 100 days. In a hydroponic experiment, Imax values for NH4-N, NO3-N, P and K varied significantly among hybrid poplar seedlings and dandelion and quackgrass weed species and was greatest for dandelion followed by hybrid poplar and then quackgrass. The Km values were lowest for quackgrass compared to the other plant species for all of the nutrients. Simulation results from the SSAND model for the pot study showed that N uptake was underpredicted in hybrid poplar by 58 to 73%, depending upon soil type and weed treatment. Incorporation of N mineralization as a model input improve the hybrid poplar N uptake predictions by 24 and 67% in the Pasture and Alfalfa soil, respectively, when grown without weeds. SSAND model underestimated P uptake by 84-89% and overestimated K uptake by 28 to 59% for hybrid poplar depending upon the soil type and weed treatment. In the field, N uptake by hybrid poplar was in close agreement to measured N uptake in the control treatment. N uptake was greatly underestimated for both hybrid poplar and weeds in the weed treatment. Including changing water content greatly improves the N uptake by hybrid poplar and weeds in weed treatments. Results from this study suggest weed control is an essential practice to establish successful hybrid poplar plantations. Also, SSAND model can be an effective tool for predicting the nutrient uptake under two plant species competing environment if all the processes of nutrient supply are adequately described in the model.

Quackgrass is the most serious perennial grassy weed of wheat in Manitoba. Field experiments and surveys investigating the nature and extent of interference in monocultures and mixtures of quackgrass and wheat were conducted over the years 1987 to 1989 at Portage La Prairie, Manitoba, Canada. Intraspecific and interspecific interference between spring wheat and quackgrass was investigated in the field using an additive series design consisting of five replacement series proportions at total stand densities of 75, 150, and 300 plants sqM. A revised synthetic no-interaction analysis determined that wheat was superior to quackgrass in both intraspecific and interpecific interference and that niche dedifferentiation was large. Quackgrass reproductive variable were less sensitive to interspecific interference than were vegetative variables. Surveys of commercial fields of spring wheat infested with quackgrass were conducted using a dynamic stratified random sampling design in which systematic samples were taken at approximately 30, 60 and 93 days after planting. Wheat yield loss, as a percentage of weed-free yield, (Yw%) was related to spring quackgrass shoot counts/m-2 (Qs) by a rectangular hyperbolic model of the form Yw% = 98.7(1-0.433(Qs)/100(1+(0.433(Qs)/193.7))). Wheat kernel weight was the wheat yield component most influenced by quackgrass infestation. In quackgrass populations the majority of new rhizome production occurred during wheat senescence and biomass partitioning to heads increased as quackgrass infestation increased. Allometric models of the relationship between quackgrass parts were site specific and generally became more accurate the later the sampling date. A set of models relating spring quackgrass infestation to yield losses in hard red spring wheat, flax, and polish canola were combined with allometric models in a multi-year spreadsheet (Lotus 1-2-3, v 3.1) model. Simulations run using the multi-year model demonstrated the potential of a spreadsheet model of assisting in weed control decisions. / October 1995

Success of hybrid poplar plantations will rely on the efficient management of nutrients and weeds. Relatively little is known about the root uptake characteristics of hybrid poplar and weeds, their belowground interactions and particularly, the quantitative understanding of nutrient uptake using mechanistic models under weed-competing conditions. Therefore, the objectives of this study were to investigate the effects of dandelion and quackgrass on the growth of hybrid poplar, to establish their root uptake characteristics and to quantify their nutrient uptake using the soil supply and nutrient demand (SSAND) model. In a pot study, hybrid poplar stem height, root collar diameter, shoot and root biomass, root length, and N, P and K uptake significantly decreased in the presence of dandelion and quackgrass weeds. Similar weed competition effects on growth of hybrid poplar were also observed in the field at the Pasture and Alfalfa sites where hybrid poplar was grown with and without weeds for 50, 79 and 100 days. In a hydroponic experiment, Imax values for NH4-N, NO3-N, P and K varied significantly among hybrid poplar seedlings and dandelion and quackgrass weed species and was greatest for dandelion followed by hybrid poplar and then quackgrass. The Km values were lowest for quackgrass compared to the other plant species for all of the nutrients. Simulation results from the SSAND model for the pot study showed that N uptake was underpredicted in hybrid poplar by 58 to 73%, depending upon soil type and weed treatment. Incorporation of N mineralization as a model input improve the hybrid poplar N uptake predictions by 24 and 67% in the Pasture and Alfalfa soil, respectively, when grown without weeds. SSAND model underestimated P uptake by 84-89% and overestimated K uptake by 28 to 59% for hybrid poplar depending upon the soil type and weed treatment. In the field, N uptake by hybrid poplar was in close agreement to measured N uptake in the control treatment. N uptake was greatly underestimated for both hybrid poplar and weeds in the weed treatment. Including changing water content greatly improves the N uptake by hybrid poplar and weeds in weed treatments. Results from this study suggest weed control is an essential practice to establish successful hybrid poplar plantations. Also, SSAND model can be an effective tool for predicting the nutrient uptake under two plant species competing environment if all the processes of nutrient supply are adequately described in the model.

Quackgrass is the most serious perennial grassy weed of wheat in Manitoba. Field experiments and surveys investigating the nature and extent of interference in monocultures and mixtures of quackgrass and wheat were conducted over the years 1987 to 1989 at Portage La Prairie, Manitoba, Canada. Intraspecific and interspecific interference between spring wheat and quackgrass was investigated in the field using an additive series design consisting of five replacement series proportions at total stand densities of 75, 150, and 300 plants sqM. A revised synthetic no-interaction analysis determined that wheat was superior to quackgrass in both intraspecific and interpecific interference and that niche dedifferentiation was large. Quackgrass reproductive variable were less sensitive to interspecific interference than were vegetative variables. Surveys of commercial fields of spring wheat infested with quackgrass were conducted using a dynamic stratified random sampling design in which systematic samples were taken at approximately 30, 60 and 93 days after planting. Wheat yield loss, as a percentage of weed-free yield, (Yw%) was related to spring quackgrass shoot counts/m-2 (Qs) by a rectangular hyperbolic model of the form Yw% = 98.7(1-0.433(Qs)/100(1+(0.433(Qs)/193.7))). Wheat kernel weight was the wheat yield component most influenced by quackgrass infestation. In quackgrass populations the majority of new rhizome production occurred during wheat senescence and biomass partitioning to heads increased as quackgrass infestation increased. Allometric models of the relationship between quackgrass parts were site specific and generally became more accurate the later the sampling date. A set of models relating spring quackgrass infestation to yield losses in hard red spring wheat, flax, and polish canola were combined with allometric models in a multi-year spreadsheet (Lotus 1-2-3, v 3.1) model. Simulations run using the multi-year model demonstrated the potential of a spreadsheet model of assisting in weed control decisions.

Quackgrass is the most serious perennial grassy weed of wheat in Manitoba. Field experiments and surveys investigating the nature and extent of interference in monocultures and mixtures of quackgrass and wheat were conducted over the years 1987 to 1989 at Portage La Prairie, Manitoba, Canada. Intraspecific and interspecific interference between spring wheat and quackgrass was investigated in the field using an additive series design consisting of five replacement series proportions at total stand densities of 75, 150, and 300 plants sqM. A revised synthetic no-interaction analysis determined that wheat was superior to quackgrass in both intraspecific and interpecific interference and that niche dedifferentiation was large. Quackgrass reproductive variable were less sensitive to interspecific interference than were vegetative variables. Surveys of commercial fields of spring wheat infested with quackgrass were conducted using a dynamic stratified random sampling design in which systematic samples were taken at approximately 30, 60 and 93 days after planting. Wheat yield loss, as a percentage of weed-free yield, (Yw%) was related to spring quackgrass shoot counts/m-2 (Qs) by a rectangular hyperbolic model of the form Yw% = 98.7(1-0.433(Qs)/100(1+(0.433(Qs)/193.7))). Wheat kernel weight was the wheat yield component most influenced by quackgrass infestation. In quackgrass populations the majority of new rhizome production occurred during wheat senescence and biomass partitioning to heads increased as quackgrass infestation increased. Allometric models of the relationship between quackgrass parts were site specific and generally became more accurate the later the sampling date. A set of models relating spring quackgrass infestation to yield losses in hard red spring wheat, flax, and polish canola were combined with allometric models in a multi-year spreadsheet (Lotus 1-2-3, v 3.1) model. Simulations run using the multi-year model demonstrated the potential of a spreadsheet model of assisting in weed control decisions.