Effects of Thinning on Dynamics and Drought Resistance of Aspen-White Spruce Mixtures: Results From Two Study Sites in Saskatchewan

Abstract

Drought stress associated with warm temperatures is causing increased mortality and reduced growth of trees in drier portions of the boreal forests of western Canada with both warming and drought expected to increase over the coming decades. While thinning is often shown to reduce drought stress, there is little information on its effects in stands comprised of mixtures of trembling aspen and white spruce that are common in the forests of this region. This study examined effects of pre-commercial thinning on aspen and spruce growth, response to drought stress, and stand dynamics for two study sites located in Saskatchewan, Canada. In unthinned plots aspen densities declined from initial densities of 40,000 to 200,000 trees ha−1 to 2,639 trees ha−1 at age 26. Twenty-one years after thinning (i.e., at age 26) diameter of aspen and spruce had increased, with largest trees being found at the lowest aspen densities (200 aspen ha−1 for aspen and 0 aspen ha−1 for spruce). Aspen density affected average height but not height of aspen top height trees. Spruce height decreased significantly with increasing aspen density. Crown width and live crown ratio of both aspen and spruce declined with increasing aspen density. Data from cores collected from aspen and spruce indicate significant positive effects of tree size (basal area at beginning of the year), and CMI (Climate Moisture Index) on basal area increment of both aspen and spruce while increasing aspen basal area (m2ha−1) had negative effects on aspen and spruce growth. Increasing tree size (basal area) and aspen competition (basal area ha−1) both had negative effects on drought resistance and resilience for both aspen and spruce. Yield projections provided by the Mixedwood Growth Model (MGM) suggest that the mixture of 1,500 aspen ha−1 and 1,000 spruce ha−1 has the potential to provide a 23% increase in total mean annual increment (MAI) with a single harvest at age 100, compared to unthinned aspen stands, but with a 44% reduction in spruce MAI compared to pure spruce stands. Thinning of aspen to densities below 4,000 trees ha−1 at age 5 resulted in reduced aspen yields but increased spruce yields.

Precommercial Thinning Increases Spruce Yields in Boreal Mixedwoods in Alberta, Canada

“…Aspen survival rates in the thinned plots, representing the combined effects of mortality and ingress, ranged between 90 and 100% over the first eight years following thinning, with no significant differences among the three thinning levels. This is in agreement with findings of other studies of young natural aspen stands that starting with low densities these stands tend to maintain initial density, or increase density over short periods [13,15]. Consistent with our findings of no difference in aspen survival rates between thinning intensities, Kabzems et al [14] observed that mortality of aspen over a 20-year period ranged from 2.5% to 10.4% with no significant impact of aspen density, among four aspen densities created by thinning, on aspen mortality.…”

“…To compensate for this effect, analysis was completed using only the dominant subset of aspen in each plot (200 stems ha −1 top DBH aspen). Similar studies in this region have used 256 aspen ha −1 [30] or 100 aspen ha −1 [13][14][15] for analysis of aspen growth response to silvicultural treatments.…”

“…Forests 2021, 12, 412 2 of 20 However, aspen overtops spruce during the first 50 to 100 years of stand development and reduces its growth [12]. Reducing aspen densities through precommercial thinning in young boreal mixed stands generally increases spruce growth [13][14][15]. Reducing aspen density by thinning of boreal aspen or mixed stands can also increase growth of residual aspen [13][14][15][16][17][18], accelerate achievement of merchantable diameters, and improve resistance and resilience of aspen and spruce to drought [15].…”

Evaluating the impact of stand composition and competition on white spruce and aspen wood attributes in the boreal mixedwood

“…Much less is known on the effect of tree mixtures on individual tree wood attribute development, especially in the boreal mixedwood. Mixedwood stands provide opportunities to apply variation in silviculture techniques such as planting densities and thinning treatments to manage for desired combinations of wood properties and yield (Comeau 2021). Wood attributes (Table 1.1) within both soft and hardwoods have impacts on tree survivability and the grade of economic viability.…”

“…This suggests that maintaining aspen at lower densities in intimate mixture can increase spruce wood quality and maintain the ecological benefits of aspen and their potential positive impacts of forest resilience (Macdonald et al 2010). Changes in measures of productivity for the spruce and aspen components of a mixture will vary with management choices and desired outcomes (Comeau 2021). Increased competition from aspen may have a negative impact on spruce volume growth (Cortini et al 2012), even though intimate mixture stands often exhibit similar total volume production (spruce and aspen volume growth combined) or potentially increased total yield (Kweon and Comeau 2019).…”