Competitive effects of conspecific and herbaceous (weeds) plants on growth and branch architecture of Populus xeuramericana cv. Eugenei

The influence of tree density (2 x 3, 1 x 2, and 0.5 x 1 m) and weed control (weedy vs. weed free) on the growth and branching architecture of Populus xeuramericana cv. Eugenei was examined in an experimental plantation in southwestern Michigan, U.S.A. In the presence of weeds, poplars were shorter (p < 0.0001), had a smaller basal diameter (p < 0.0001), initiated fewer branches (p = 0.004), and had fewer living branches (p = 0.002), shorter branches (p < 0.0001), and branches that spread outward (p < 0.0001). Increasing tree density decreased poplar height (p < 0.0001), basal diameter (p < 0.0001), branch length (p = 0.0062), and the number of living branches (p < 0.0001). At high densities, branches spread outward at their origin (p = 0.0015) and then curved sharply upward (p = 0.002). At the highest planting density there was no difference in branching in plots with and without weed control. However, poplars in the weedy treatment were smaller than those in the weed-free treatment (p < 0.0001). Our results demonstrate that poplar growth and architecture can be influenced by competitors of very different stature. Weeds, in fact, have a greater effect on growth than conspecifics. As a result, poplars grown in the presence of weeds are stunted relative to poplars grown in the absence of weeds, even at the highest tree density where the competitive effects of poplars reduced weed biomass.