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The effects of six mechanical site preparation treatments, two stock-type treatments, and early chemical release on survival and growth of planted white spruce (Picea glauca (Moench) Voss) were studied in the BWBSmw1 biogeoclimatic zone of northeastern British Columbia. After 20 years, spruce height and diameter were larger in all mounding treatments than in the control. Early results suggested better spruce performance on large than small mounds, but after 20 years, growth was equally good on small mounds as on mounds with 20 cm mineral capping. Spruce planted on hinge positions in the Bräcke patch and blade scarification treatments did not survive or grow well. Early chemical release improved spruce growth equally as well as the mounding treatments. Twenty year spruce survival averaged 71% in the 14 and 20 cm mound treatments, 60% in the early chemical release treatment, and ≤35% in the Bräcke patch and blade scarification treatments. A large stock type was also planted in untreated ground and, after 20 years, had similar survival and growth as the standard stock type. Differences in survival had a large effect on basal area at age 20 years. Trend analysis showed that treatments diverged into two distinct groups with regard to spruce size during the 20 year span of the study.Résumé : Les effets de six traitements de préparation mécanique de terrain, de deux traitements avec différents types de plants et de l'application précoce d'un herbicide chimique ont été étudiés sous l'angle de la survie et de la croissance de plants d'épinette blanche (Picea glauca (Moench) Voss) dans la zone biogéoclimatique BWBSmw1 du nordest de la Colombie-Britannique. Vingt ans après l'application des traitements, la hauteur et le diamètre des épinettes étaient plus grands dans tous les traitements de mise en buttes que dans le traitement témoin. Les résultats préliminaires indiquaient que la performance de l'épinette était meilleure sur les grandes buttes que sur les petites, mais après 20 ans, la croissance s'est avérée aussi bonne sur les petites buttes que sur les buttes recouvertes de 20 cm de sol minéral. Les épinettes plantées à des positions charnières des parcelles scarifiées au Bräcke et des parcelles scalpées n'ont pas eu de bons taux de survie ou de croissance. L'application précoce d'un herbicide chimique a eu pour effet d'augmenter la croissance de l'épinette autant que dans le cas des traitements de mise en buttes. Le taux moyen de survie de l'épinette après 20 ans était de 71 % dans les traitements de mise en buttes de 14 et 20 cm, de 60 % dans le traitement d'application précoce d'herbicide chimique et inférieur ou égal à 35 % dans les traitements de scarifiage au Bräcke et de scalpage. Des plants de grande taille ont aussi été plantés sur un terrain non traité et avaient après 20 ans des taux de survie et de croissance semblables à ceux des plants standard. Une analyse de tendance a montré que les traitements se divisaient en deux groupes en ce qui concerne la taille des épinettes au cours de la période de 20 ans pen...

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.

Site preparation and vegetation control can be used to mitigate climate change effects on early plantation growth in boreal forests. In this study, we explored growth of lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and white spruce ( Picea glauca (Moench) Voss) in relation to climate and site preparation using 20 years of data collected from studies in British Columbia. Results indicate that up to 45% of the variation in spruce growth and up to 37% of the variation in pine growth over this 20-year period can be explained by selected climatic variables. Monthly climate variables showed a stronger relationship to conifer growth than seasonal and annual variables. Climate variables related to the preceding year accounted for more than half of the variables in the final equations, indicating a lagged response in conifer growth. Future projections indicated that height growth of young lodgepole pine plantations in the sub-boreal zone could benefit (in the short term) from longer growing seasons by up to 12% on untreated stands. Untreated young white spruce plantations in the boreal zone may suffer height growth decreases of up to 10% due to increased drought stress. Vegetation control and mechanical site preparation treatments appear to mitigate effects of climate change to some extent.