• Wagner, Robert G
  University of Maine School of Forest Resources
  
• Noland, Thomas L
  
• Mohammed, Gina H
  

Optimum timing and duration of herbaceous vegetation control during development the early of forest plantations can be assessed using critical-period analysis. Critical periods are being developed for black spruce (Picea mariana (Mill.) B.S.P.), jack pine (Pinus banksiana, Lamb.), eastern white pine'(Pinus strobus L.), and red pine (Pinus resinosa Ait.) seedlings in the Great Lake/St Lawrence forest type Ontario, Canada. Six patterns of herbaceous vegetation control were examined: 3 consecutive years, first year only, first 2 years, second and third years, third-year only, and no control. Critical-period-analysis indicated that herbaceous vegetation control is important immediately after planting for both tolerant and intolerant conifer species. Stem diameter gains also were proportional to the number of years of herbaceous vegetation control.