Growth of hybrid poplar as affected by dandelion and quackgrass competition

A pot experiment was conducted in a growth chamber to investigate the effects of dandelion (Taraxacum officinale) and quackgrass (Elymus repens) on the growth of hybrid poplar (Populus deltoides x Populus x petrowskyana var. Walker). Single hybrid poplar seedlings were grown in pots either alone (SHP) or with four or eight dandelion plants per pot or with one or three quackgrass plants per pot in two soils collected from sites previously managed for alfalfa and pasture near Meadow Lake, Saskatchewan, Canada. Hybrid poplar and weed species were harvested approximately 7 and 14 weeks after planting. Approximately 14 weeks after planting, hybrid poplar shoot biomass in the SHP treatment was 28 g for the pasture soil and 22 g for the alfalfa soil. Corresponding shoot biomass for hybrid poplar grown with the dandelion and quackgrass treatments varied from 0.54 to 0.81 g and 0.3 to 3.66 g, respectively. Other hybrid poplar growth parameters including stem height, root collar diameter and fresh root biomass were similarly reduced by competition with the weed species. During the growing period, soil solution N and K concentrations decreased several-fold in both soils; however, the magnitude of decrease was comparatively higher in the weed treatments. Nitrogen, P and K uptake by hybrid poplar was greater in the SHP treatment in both the soils (337-425, 38-49 and 396-463 mg pot-¹, respectively) compared to the weed treatments (4-28, 0.4-6.2 and 0.6-54.0 mg pot-¹, respectively) by the end of the experiment. The presence of quackgrass and dandelion severely affects the growth of hybrid poplar by causing intense below-ground competition for nutrients.