Methods

Cutblocks near Fawcett Lake, Alberta were selected for pre-commercial thinning trials. These cutblocks were harvested in the winter of 2004/2005 and were planted with white spruce tree seedlings. As of 2022, the cutblocks consisted primarily of white spruce, trembling aspen, paper birch and balsam poplar trees. Figure 1. shows a portion of one of the cutblocks.

Figure 1. One of the study cutblocks during summer 2022, before thinning operations. This cutblock later became treatment units 5a and 5b. Photo taken from the south-east facing north-west.

Areas of each cutblock were chosen for pre-commercial thinning operations. These areas were chosen to be representative of the growing conditions in adjacent areas of the cutblock. Each thinned area was given treatment unit numbers ending in “a” (1a, 2a, 3a, 4a, 5a). For each “a” treatment unit an adjacent unthinned area of similar size within the same regenerating cutblock was chosen. These areas were given treatment unit numbers ending in “b” (1b, 2b, 3b, 4b, 5b). A map of cutblocks and thinned areas is provided in figure 2.

Figure 2. Map of study cutblocks and treatment units.

Thinning operations were done to the following specifications:

1. Select 2500 trees per hectare to remain.

2. No selected trees can be within 1.2m of another selected tree

3. Select white spruce first followed by pine (jack or lodgepole), tamarack, black spruce, trembling aspen, and balsam poplar.

4. Remove all birch, alder, willow, or other bushes within 1.2m of a selected tree

5. Quality specifications (selected tree health and form, downing of cut vegetation, damage to selected trees).

Figure 3 shows parts of treatment units 5a and 5b after thinning operations.

Figure 3. Treatment units 5a and 5b after thinning. Photo taken from the east facing west. Arrows show the boundary between treatment units.

The resulting thinned stands have a tree/shrub distribution biased towards conifers and a maximum, but not a minimum, tree/shrub density. The average density values are shown in figure 4.

Figure 4. Species and density distribution of thinned and unthinned treatment units.

Note: Sw=white spruce, Sb=black spruce, Pj=jack pine, Lt=tamarack, Bw=white birch, Pb=balsam poplar, Aw=trembling aspen

A total of 80 plots to be used in this study were been distributed randomly across each “a” and “b” treatment unit subject to conditions (e.g. must be 20 meters from boundaries and other plots). Figure 5. shows how plots were distributed in treatment units 5a and 5b.

Figure 5. Map of treatment units 5a and 5b. Note that additional plots were mapped to allow plots in unsuitable locations to be dropped. Only 16 of these plot centers were used.

Data was collected during the first growing season after thinning operations (summer 2023) by installing TOMST TMS-4 dataloggers at each plot center. Dataloggers recorded the following:

Given that thinning treatment removes trees to decrease competition, it would be tempting to quantify the remaining trees in a plot (basal area) and use it as a metric of competition. However, analyzing temperature and moisture in thinned and unthinned plots better reflects the realities of operational thinning treatments. Each category (thinned vs. unthinned) captures the resulting variation in basal area within the treatment.