Managed Mixtures of Aspen and White Spruce 21 to 25 Years after Establishment

Abstract

Intimate mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are a key feature of western Canadian boreal forests. These mixtures have the potential to produce high yields of merchantable fibre and provide numerous ecological services. Achievement of this potential has been difficult, and often expensive, to realize as a regeneration goal in managed forests. We report 21 to 25 year results of managed mixtures on two study sites where the white spruce was planted, and the density of aspen natural regeneration manipulated within five years of the stand initiation disturbance. On both sites, white spruce mortality did not increase with increasing aspen density. While height and diameter growth of white spruce declined with increasing aspen density, the effect was not entirely consistent across the two sites. Abrasion from aspen branches was the most common source of damage to spruce crowns. Mixed stands had greater merchantable volume production than pure spruce stands based on model projections. Application of aspen harvest at year 60, while protecting the spruce component for a second harvest entry at year 90, was projected to optimize combined yield for the mixedwood stands.

Effects of Aspen and Spruce Density on Size and Number of Lower Branches 20 Years after Thinning of Two Boreal Mixedwood Stands

“…ns = model not significant (α = 0.05). Pre-commercial thinning of aspen in mixed stands of aspen and white spruce has been shown to increase the diameter, crown width, and live crown ratio and reduce slenderness of the spruce [16,17], as also found in this study. Other studies show similar effects of stand density on crown size of spruce, lodgepole pine, and subalpine fir in western British Columbia [25], black spruce in Ontario [8], Norway spruce in France [21], and Scots pine in Finland [26].…”

“…Effects of competition on height to crown base, crown length, and crown ratio also depend on species shade tolerance, with tolerant species having longer crowns and lower height to crown base than intolerant species [15]. Consequently, it may be desirable to retain moderate to high densities of intolerant deciduous competitors in young stands to control knot size in moderately shade-tolerant spruce, even though the fastest growth of white spruce is achieved at very low aspen densities [16,17].…”

Managing tree species diversity and ecosystem functions through coexistence mechanisms

“…Similar difficulties arise in other European forests involving codominance by shadetolerant and shade-intolerant species such as Pinus sylvestris-Fagus sylvatica and Abies alba-Picea abies stands (del Río et al 2017;Vallet and Pérot 2011). In western Canadian boreal forests, overyielding mixed stands of Populus tremuloides-Picea glauca (Man and Lieffers 1999) also follow a succession to Picea glauca dominance in the absence of major disturbance (Kabzems et al 2016). Such observations call into question the feasibility of species diversity-based management if mixtures favourable for ecosystem functions are naturally transient and require substantial intervention to maintain species coexistence.…”

Cohort aggregation modelling for complex forest stands: Spruce–aspen mixtures in British Columbia

“…In multi-layered mixtures it is often observed that the height growth of the dominated species is reduced in relation to that expected in a monoculture (e. g., Kabzems et al, 2016). More generally, a similar suppression of shorter by taller cohorts can occur in uneven-aged stands, or among trees in individual-based models.…”