Aspen canopy removal and root trenching effects on understory vegetation

Thin trees within forests: effects on understory plants

A replicated, controlled study in 2000-2002 in boreal forest in Alberta, Canada, (Powell & Bork 2006) found that removal of trembling aspen Populus tremuloides canopies increased the biomass of understory vegetation and cover of herbaceous species. Biomass  (kg/ha) of understory vegetation was higher in partial  (1,300-2,200) and complete removal plots (2,100-2,700) than control plots (700-850) at the parkland site and differed between all treatments at the boreal site (control: 400-750; partial removal: 1,100-1,150; complete removal: 2,100-2,900). Cover of non grass herbaceous plants at the boreal site (control: 29-45%; partial removal: 33-38%; complete removal: 46-66%) and of grasses at the parkland site (control: 8-20%; partial removal: 15-37%; complete removal: 52-79%) was higher in complete than in partial removal and control plots. Cover of tall shrubs (>1 m) at the boreal site was lower in partial (3-8%) and complete removal (8-20%) than in control plots (15-42%). There was no difference between treatments for the following: cover of tall shrubs at the parkland site (control: 4-10%; partial removal: 5-8%; complete removal: 0-3%), low shrubs (<1 m) at the parkland (control: 25-31%; partial removal: 12-25%; complete removal: 17-38%) and at the boreal site (control: 24-51%; partial removal: 35-40%; complete removal: 46-52%), forbs at the parkland (control: 7-10%; partial removal: 3-4%; complete removal: 4-7%) and grasses at the boreal site (control: 1-3%; partial removal: 0-2%; complete removal: 2-9%). Three replicates of complete removal (all aspen canopies removed), partial removal (half of aspen canopy area removed) and control plots (10 × 10 m) were established in 2000 in a 'boreal' site (16,319 stems/ha) and a 'parkland' site (13,194 stems/ha). Data were collected in 2002.