Millar Western Thinning Trial


To evaluate the impact of sludge application without incorporation on thinned forest stands


The Millar Western Thinning Trial was established in 1998 in the eastern portion of a 20 ha thinned forest block adjacent to Highway 43 and approximately 13 km northwest of Whitecourt.  Activities for the thinning trial included:

  • The Aero-Spread® unit was used to apply Millar sludge on eight plots at application rates ranging from 4.5 t/ha to 100 t/ha in June, 1998.
  • Baseline and post-application soil samples were collected in the control and sludge applied plots.
  • Upstream and downstream surface water samples were collected from a creek downslope of the treatment area. 
  • Tree measurements were taken prior to application and from 2000 to 2002.  
  • Equipment evaluation for spreading sludge in the forest in terms of cost effectiveness and practicality.
  • During the sludge application process different ground speeds and machine openings or rates of sludge discharge were used.  

Results and Conclusions

  • Tree measurements in 2001 indicated that both mean diameter and height were slightly greater for the sludge amended than the control treatments in the non-thinned forest however the reverse was true for most of the plots in the thinned forest. 
    • It was speculated that these results were due to the more diverse and dense understory cover in the thinned areas utilizing more of the nutrients from the sludge than the cover in the non-thinned area thereby resulting in less nutrients being available for tree growth. 
  • Differences in density and diversity of the understory cover were observed in 2001, however no differences were observed in 2005 compared to previous years.
  • Results from this trial indicated that there was a significant effect on the groundcover vegetation in thinned sludge amended areas when compared to non-amended areas or non-thinned areas that received sludge. 
    • The openness of the thinned sludge amended areas allowed the understory herbaceous cover to flourish and utilize the sludge, thus making it unavailable for the trees. 
    • Vegetative assessments indicated that the control areas had ground cover comprised mainly of mosses, bearberry, bunchberry, and minor amounts of grasses and the sludge amended areas were dominated by bracted honeysuckle, raspberry, and there was no evidence of any reduction in the density of these species seven years after sludge application.
  • It was determined that the Aero-Spread® unit could achieve:
    • A maximum discharge rate for the MW sludge of 14 m3 sludge in 8 minutes.
    • A spread distance of approximately 15 to 20 m in a relatively uniform pattern. 
    • Travel distance of 40 to 250 m depending on discharge rate and ground speed.