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Density manipulation of overstory aspen provides increased light levels to promote growth of understory spruce, while it may also impact the air and soil temperature. We monitored temperature for three growing seasons in young mixedwood stands of variable density at two boreal locations in Alberta, Canada. Results indicate that partial aspen cover provides frost protection that may be location specific. Mean air temperature was similar or increased slightly across aspen densities during the growing season, while mean soil temperature significantly increased with decreasing overstory cover. The amount of accumulated air and soil heat over the growing season was inversely related to aspen density. The differential impact of density on temperature contributes to variation in growth relationships and may explain regional differences in competition effects. [ABSTRACT FROM AUTHOR]

The 19-20-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 12-16 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. [ABSTRACT FROM AUTHOR]

Populus balsamifera L., Betula papyrifera Marsh., Populus tremuloides Michx. and Picea glauca (Moench) Voss seedlings were grown in specialized pots that maintained a constant water-table height and allowed monitoring of water use by the tree/pot under high water-table conditions and different soil temperatures. The trees were grown at imperfectly and poorly drained water table conditions and at 5, 10 or 20°C soil temperature. In P. balsamifera , net assimilation and transpiration remained high under wet soil conditions and increased with higher soil temperatures. Populus balsamifera growth and leaf area development were severely restricted at soil temperatures of 5°C. Both B. papyrifera and P. tremuloides had low transpiration and pot level water-use rates at both water-table conditions and these did not significantly increase with increasing soil temperature. Picea glauca was negatively affected by high water tables but showed minimal response to soil temperature changes. The study suggests that P. balsamifera would be a good hydrological nurse crop to lower the water table when soils are warm, while B. papyrifera is likely to be a good nurse species in cool and imperfectly drained sites. [ABSTRACT FROM AUTHOR]