Long-term effects of site preparation and postplanting vegetation control onPicea glaucasurvival, growth and predicted yield in boreal British Columbia

Abstract

The 19Á20-year effects of mechanical site preparation, windrow burning, chemical site preparation, and postplanting vegetation control on survival and growth of planted white spruce are reported from two boreal sites in British Columbia, Canada. Survival differed between treatments at both sites, but was relatively good (]77%) even in untreated plots. Current data regarding the proportion of spruce that were physically overtopped by vegetation and previous results from related soils and vegetation studies suggest that lasting reductions in tall shrub and aspen abundance were more important to spruce growth than early microenvironmental effects associated with manipulating the rooting environment. At Inga Lake, postplanting vegetation control produced a 13-fold increase in spruce volume over the control after 19 years, which was statistically equivalent to increases resulting from fine mixing, plow-inverting and windrow burning site preparation treatments. At Iron Creek, chemical site preparation and plow-inverting quadrupled spruce volume, whereas mounding, patch scarification and disc trenching were ineffective. Growth and yield simulations using treatment-specific site index curves for Inga Lake suggested that rotation length could be shortened by 12Á16 years through the use of site preparation or postplanting vegetation control. However, untreated areas, due to the relatively good survival of white spruce at age 19, were predicted to produce equivalent volume if left to grow to mean annual increment culmination age.

Yield Implications of Site Preparation Treatments for Lodgepole Pine and White Spruce in Northern British Columbia

“…In boreal forests white spruce establishment is often limited by severe vegetation competition and unfavorable soil conditions, therefore treatments that affect these factors have proven to increase growth and survival of spruce (e.g., [8,26,28,48]). Both mechanical site preparation treatments and the removal of competing vegetation, by applying herbicide or fire, result in a shift in the plant community from tall shrubs (e.g., green alder and willow), as in the untreated, to mainly grasses and forbs [22,28,49].…”

“…For the delta hinge treatment the projected growth was not modified from age 20. For the untreated plots, Boateng et al [28] indicate that the overtopped white spruce is expected to be taller than competing vegetation at stand age 26. Consequentially, growth increments may differ from age 26.…”

“…Soils are fine clayey to fine loamy basal till, with a 2-4 cm forest floor at the time of the treatment. The site where the trial was located was never harvested but regularly burned until the 1950s which resulted in a willow-dominated vegetation community that was sheared in winter 1987 [28].…”

“…In each plot 48 trees were assessed and monitored. The treatments represented are: 1) control, trees were planted without site preparation, 2) burned windrow, trees planted in well burned areas free of slash, 3) Madge, trees planted in well-mixed layer of surface organic matter and mineral soil, 4) bedding plow, trees were planted in roughly mixed mineral soil and chunks of forest floor, 5) breaking plow, trees were planted deeply into berms of mineral soil overlying inverted forest soil, 6) Delta disk trenching hinge-planting, trees planted in mineral soil at the edge of the berm, 7) vegetation control, three years after shearing and planting, herbicide was applied followed by six manual cuttings [28]. Site preparation treatments were applied in July to October 1987 and the site was planted at the end of May to first week of June 1988 with interior spruce (PSB 313 2+0).…”

Aspen and white spruce productivity is reduced by organic matter removal and soil compaction

“…Competitive vegetation can be more important than soil physical characteristics for early spruce growth (Brais 2001, MacAdam andKabzems 2006). Twenty-year white spruce volume growth was equivalent between vegetation control alone and the top three mechanical site preparation treatments, which had decreased soil density and improved nutrient availability on boreal sites (Boateng et al 2009). …”