Can Spot Motor-Manual Brushing Treatments Be Effective for Controlling Aspen and Increasing Spruce Growth in Regenerating Mixedwood Stands?

Abstract

Broadcast motor-manual or manual brushing treatments applied to control aspen (Populus tremuloides Michx.) in young spruce (Picea glauca (Moench) Voss) plantations often result in increases in the number of aspen stems and the amount of aspen competition. In this study data collected at three locations is used to examine the potential effectiveness of spot brushing treatments, applied over radii ranging between 1.0 and 2.5 m around individual spruce, to aspen that are two, four or six years old. Results indicate that spot manual or motor-manual treatments result in reductions in the number and size of aspen stump sprouts compared to the untreated control. However, when aspen size and vigour are reduced due to site or other factors, as observed for blocks at one location, post-treatment aspen densities may still be high (e.g., above 5000 stems ha⁻¹). Spot treatments applied when aspen regeneration was two years old were observed to be less effective than treatments applied at ages 4 or 6. At two study locations where control was effective, spot brushing treatments significantly increased spruce diameters after treatment compared to untreated. Broadcast or spot treatment using basal bark application of triclopyr ester (Release^®) herbicide at one of the study sites resulted in similar increases in spruce diameter to those observed for motor-manual treatment.

1. Introduction

Mixedwood sites are among the most productive and most common forest types in northern Alberta. White spruce (Picea glauca (Moench) Voss) (hereafter termed spruce) dominated mixedwoods and pure spruce stands occur on a small portion of these sites and commonly develop through succession from stands dominated by aspen. Abundant regeneration from root suckers and rapid growth of trembling aspen (Populus tremuloides Michx.) (hereafter termed aspen) makes it difficult to recreate spruce dominated mixedwood stands immediately following harvesting.

While the option exists to convert mixedwood stands to monocultures of aspen or spruce, this may not be desirable since it ignores the potential benefits to stand yield from growing mixtures (including the nursing effects that aspen provides for young white spruce) [1,2] and would result in a forest that differs from current mixed conditions. In a spruce plantation aspen cover can reduce radiation frost damage [3,4], winter injury [5], suppress growth of other competitors (such as marsh reedgrass (Calamagrostis canadensis (Michx.) Beauv)) [6] and reduce damage to spruce due to white pine weevil (Pissodes strobi Peck) [7]. Other benefits of growing mixtures include potential increases in carbon storage and increased biodiversity. However, dense aspen cover can also seriously reduce survival and growth of white spruce when competitive effects exceed beneficial effects.

A common approach being used is to create a mixedwood stand by thinning the aspen at a young age [8,9,10], however, this approach is costly and low densities of aspen may lead to development of substantial grass cover, which may compete with the white spruce and can be difficult to control in this situation. While effective, the use of broadcast herbicide treatments for competition control is intended to create spruce dominated stands. Spot herbicide treatments, involving treating a small area around selected planted trees and leaving an untreated matrix of broadleaf trees between treated spots were examined by Pitt et al. (2015) [11] with treatments applied in a 2 m radius of planted white spruce. These treatments were found to effectively improve early growth of spruce compared to untreated. Manual and motor-manual cutting/brushing treatments are a potential alternative to herbicides [12] and may be more readily accepted by first nations and the general public [13]. However, cutting treatments applied as a broadcast treatment have been observed to result in substantial regrowth of aspen through stump sprouting and root suckering [14,15,16,17]. As an alternative to broadcast motor-manual clearing, retaining a component of residual trees by treating only small spots and retaining untreated aspen in the surrounding matrix may reduce aspen resprouting and reduce the number of retreatments required. While it would be expected that effectiveness of spot cutting treatments will increase with age of the aspen, there is no information available for western Canada to support this. Spot treatments using herbicide or cutting tools have the benefit of selectivity compared to broadcast herbicide treatments and provide a potentially effective approach for retaining mixedwood stand characteristics [18].

In this paper I present results from studies conducted at three locations in Alberta where effects of radius and age of cutting treatments on growth of aspen sprouts and suckers and growth of planted white spruce were examined. Hypotheses being tested are: (1) that spot brushing results in reduced regrowth of aspen compared to broadcast brushing treatment; (2) that aspen resprouting following spot brushing increases with age of aspen at treatment; (3) that aspen regrowth increases with radius of treated spots; and (4) that spot brushing is similar in effectiveness to spot herbicide treatment.

2. Materials and Methods

Three experiments were established at different times at three locations in Alberta: Mistahae, Judy Creek and LacLaBiche (Figure 1).

2.1. Mistahae

The Mistahae study site was a four-year-old white spruce plantation with approximately 9000 young aspen per hectare located northeast of Slave Lake, Alberta (55.7308° N 114.1369° W). The study site is located in the Central Mixedwood Natural Subregion [19] on a mesic site with fine textured (silty clay loam) Luvisol [20] developed on a fine textured glacial till parent material. According to ClimateNA [21] (30 year (1981–2010) Normal) Mean Annual Temperature at this location is 1.3 °C, Mean Warmest Month Temperature is 15.8 °C, Mean Coldest Month Temperature is −16.0 °C, Mean Annual Precipitation is 475 mm with 336 mm falling between 1 May and 30 September, there are 154 Frost Free Days and the Frost-Free Period is 109 days. Following harvesting of a mature white spruce-trembling aspen mixedwood stand in 1996 the site was shear bladed and then planted.

The following seven treatments were applied:

• Untreated (control) (U)
• Complete removal—triclopyr ester (Release) (TA)
• Complete removal motor-manual (BA)
• 1 m radius treatment with triclopyr ester (Release) applied as a low volume basal treatment to all aspen around 300 selected spruce/ha (6 m × 6 m spacing, approximately). (T1)
• 2 m radius treatment with triclopyr ester (Release) applied as a low volume basal treatment to all aspen around 300 selected spruce/ha (6 m × 6 m spacing, approximately). (T2)
• 1 m radius motor-manual applied to all aspen around 300 selected spruce/ha (6 m × 6 m spacing, approximately), (B1)
• 2 m radius motor-manual applied to all aspen around 300 selected spruce/ha (6 m × 6 m spacing, approximately). (B2)

This study used a completely randomized design, with each treatment randomly assigned to four plots. Treatment plots were 50 m × 50 m (0.25 ha).

Spot radial treatments (T1, T2, B1 and B2) were applied to only 300 white spruce per hectare (75 spruce per plot) in order to avoid creating long strip treatments when treatment radius exceeds 1.5 m and in order to retain a matrix of surrounding aspen in each plot. Triclopyr ester (Release^®) herbicide was applied as a low volume basal treatment in October 2000. Release herbicide was mixed with isoparaffinic mineral oil at 20% concentration (v/v) and applied as a streamline treatment to each aspen stem using low pressure backpack sprayers fitted with narrow angle flat fan (1502) nozzles. Motor-manual treatments were completed in mid-June 2001 using brushsaws. Cut height was between 10 and 30 cm, and all slash was laid flat after treatment.

Nine (9) treated spruce located closest to plot center (or similar spruce located approximately 6 m apart in untreated plots or in broadcast treatment plots) were selected for measurement in each treatment plot. In addition, four (4) dominant aspen located approximately 6 m NW, NE, SW, and SE of plot center were selected in each plot for measurement. Numbered aluminum tags were attached to each sample tree using coated copper wire, loosely tied around each stem.

Basal diameter (15 cm above root collar) and height of spruce was measured in May of 2001 and 2007. Resprouting of broadleaf trees was assessed in a 1 m radius assessment plot at each of the 9 sampled spruce in treatments B1, B2 and BA in May of 2005. The number of live stems of each species (aspen, balsam poplar (Populus balsamifera L.) or paper birch (Betula papyrifera Marsh.)) and the top height of each species was recorded.

2.2. Judy Creek

In 2004 a study was initiated adjacent to the Judy Creek Mixedwood Study site near Whitecourt, Alberta (54.3816° N 115.5878° W) [11] to examine effects of stand age (age 2, 4, and 6 (2004, 2006 and 2008)) and radius of brushing (1.5 and 2 m radius) on growth of aspen sprouts and suckers and on growth of released white spruce. The study site is located on the border of the Lower Foothills and Central Mixedwood Natural Subregions [19]. Soils were assessed as mesic, fine-textured luvisols [20], with parent material consisting of a fluvial veneer overlying ablation till and a rounded cobble layer at about 25–40 cm, resulting from glacio-fluvial action. Texture ranged between silty loam and clay, to a sandy-loam veneer over silty clay-loam. According to ClimateNA [21] (30 year (1981–2010) Normal) Mean Annual Temperature at this location is 2.8 °C, Mean Warmest Month Temperature is 14.6 °C, Mean Coldest Month Temperature is −8.7 °C, Mean Annual Precipitation is 526 mm with 366 mm falling between 1 May and 30 September, there are 153 Frost Free Days and the Frost-Free Period is 106 days. For this study, the aspen dominated block was harvested in the winter of 2001–2002 and white spruce were planted in the summer of 2002 at approximately 2.5 m spacing. The experiment involved a total of 7 treatments: untreated, plus 3 treatment ages × 2 treatment radii, with 12 spruce randomly selected for each treatment. Treated spruce were located at least 7.5 m apart to avoid effects of adjacent treatments. Treatments used handclippers and shears.

Selected trees were tagged and height and stem diameter (5 cm above root collar) was measured in May of 2004 and remeasured in May of 2013 and 2017. Aspen sprouts were measured in the central 1 m radius plot in early May of 2013 (age 10).

2.3. Lac LaBiche

The Lac LaBiche study was established in three blocks, two blocks which were harvested in 2004 and planted with white spruce (Blocks 17711 [55.0489° N 111.7578° W] and 13731 [55.0683° N 111.7633° W]) and one block harvested and site prepared using a Terrateck Mounder in early 2004, and planted with white spruce (Block 33961 [55.0292° N 111.7703° W]). The study site is located in the Central Mixedwood Natural Subregion [19] on a mesic site with a fine textured (silty clay loam) Luvisolic soil [20] developed on a fine textured glacial till parent material. According to ClimateNA [21] (30 year (1981–2010) Normal) Mean Annual Temperature at this location is 1.5 °C, Mean Warmest Month Temperature is 16.4 °C, Mean Coldest Month Temperature is −15.3 °C, Mean Annual Precipitation is 451 mm with 317 mm falling between 1 May and 30 September, there are 155 Frost Free Days and the Frost-Free Period is 113 days. Spruce (container grown 412A stock) were planted at approximately 3 m spacing in blocks used for the study.

This study was established in 2006 to test the effects of: (a) age of aspen: 2, 4, and 6 years; and (b) radius of treatment: 1, 1.5, 2, 2.5 m. In addition to these 12 treatments there was an untreated control. The age 2 treatments were applied in May 2006 using handclippers, age 4 treatments were completed in 2008 and age 6 treatments were completed in 2010. Individual planted white spruce are the treatment unit used for this study, with each treatment randomly assigned to 10 spruce in blocks 13731 and 33961 and to 5 spruce in block 17711 (for a total of 325 trees). Selected trees were located at least 5 m apart in each block to minimize influences of adjacent treatments.

Diameter (15 cm above root collar) and height of white spruce were measured in May of 2013 and September 2017. Number and height of sprouts and suckers of aspen and other broadleaf species within 1 m of the treated tree was measured in May 2013.

2.4. Statistical Analysis

Statistical analyses were completed using proc GLM in SAS version 9.4 (SAS Institute, Cary, NC, USA). Analysis of aspen height and diameter for the Mistahae study treated plot as the experimental unit and subplots as subsamples within plot. Analysis of covariance was used for analysis of spruce responses in the Mistahae study with initial (2001) spruce height or basal diameter used as a covariate for analysis of treatment effects on spruce height and diameter (respectively), with plot treated as the experimental unit, and trees nested within plot as subsamples. Judy Creek data were analyzed as a completely randomized factorial experiment, with individual trees treated as the experimental unit. Aspen and spruce data from the three Lac LaBiche sites were analyzed as a randomized complete block design treating block as a random effect, using block × treatment as the error term for hypothesis testing and tree as the experimental unit. In all cases, analysis of treatment effects on spruce size included only live trees.

3. Results

3.1. Mistahae

Brushing a 1 or 2 m radius significantly reduced the height and density of regrowing aspen over that observed in the broadcast brushed (BA) plots (

Table 1. Treatment effects on aspen height, number of aspen sprouts in 2005 and spruce height and diameter in 2007 for the Mistahae study. Analysis of treatment effects on spruce size is based on analysis of covariance, using height or diameter in 2001 as a covariate. Letters in each column indicate significant differences detected using Scheffe’s Test at α = 0.05. SD shown in brackets. Significant p values (α = 0.05) are shown in bold.

Treatment	Aspen Height 2005 (cm)	Number of Aspen Sprouts and Suckers 2005 (#/ha)	Spruce Height 2007 (cm)	Spruce Diameter 2007 (mm)
B1—1 m radius cutting	48.3 b (14.3)	3714 b (3188.4)	183.1 (59.4)	30.9 ab (9.0)
B2—2 m radius cutting	61.7 b (55.5)	7109 b (7760.1)	184.9 (51.2)	30.9 ab (9.5)
BA—complete removal cutting	197.6 a (43.7)	22,282 a (14,899.5)	204.9 (62.9)	37.4 a (12.8)
T1—1 m radius triclopyr	.	.	189.8 (61.2)	26.9 ab (7.9)
T2—2 m radius triclopyr	.	.	186.2 (51.3)	29.1 ab (7.3)
TA—complete removal triclopyr	.	.	175.8 (51.9)	33.3 a (11.1)
U—untreated	.	.	152.1 (36.4)	21.8 b (6.9)
covariate	.	.	H01	D01
p	<0.01	<0.01	0.26	<0.01
Contrast(p values)
1. U vs. (T1, T2, B1, B2)			0.02	<0.01
2. (B1, B2, T1, T2) vs. (BA, TA)			0.75	<0.01
3. U vs. (B1, B2)			0.06	<0.01
4. (B1, B2) vs. (T1, T2)			0.64	0.98
5. (B1, T1) vs. (B2, T2)			0.76	0.70

). In 2005, aspen regrowth in the 1 m and 2 m radius brushing treatments were on average less than 30% of the height of aspen regrowth in the BA treatment. Aspen overtopped spruce in the BA in 2005 (data not shown) but not in the B1 and B2 treatments. Compared to broadcast brushing, radial brushing reduced the number of aspen stems and their height over the 5 years following brushing.

Analysis of covariance was used to account for influences of initial size of individual spruce on their subsequent growth (in 2001 spruce height ranged from 12 to 82 cm and basal diameter ranged from 2.2 to 15.8 mm), in all cases initial size was significant (p < 0.05) in the models. In May 2007, ANOVA indicated that spruce height was not significantly affected by treatment but spruce diameter was significantly larger in the BA and TA treatments compared to the untreated while spruce diameter in the radial treatments (B1, B2, T1 and T2) was intermediate between and did not differ significantly from the broadcast (BA, BT) or untreated. Five linear contrasts shown in Table 1 indicate that: (1) radial treatments (B1, B2, T1 and T2), lead to a significant increase in spruce diameter (p < 0.01) and height (p = 0.02) compared to untreated (U), (2) that broadcast treatments (BA, BT) resulted in a significant increase in spruce diameter (p < 0.01) (but not height (p = 0.75)) compared to radial treatments, (3) that radial cutting (B1,B2) lead to a significant (p < 0.01) increase in spruce diameter, while differences in height were marginally non-significant (p = 0.06), (4) that spruce diameter (p = 0.98) or height (p = 0.64) did not differ significantly between radial cutting or radial herbicide treatments, and (5) that radius of treatment (1 m vs. 2 m) did not significantly affect spruce diameter (p = 0.70) or height (p = 0.76) measured six years after treatment.

3.2. Judy Creek

With the exception of the 2 m radius treatment in year 2 (2004) (treatment 5, 2004-2 m), which had an average of 2652 aspen per hectare, radial brushing treatments had fewer than 290 aspen sprouts per hectare within the 1 m radius plots centered on the treated spruce (

Table 2. Treatment effects on number and height of aspen sprouts within a 1 m radius plot centered on the crop spruce seedling in 2013 for the Judy Creek study. Letters in each column indicate significant differences detected using Scheffe’s Test at α = 0.05. SD shown in brackets. (despite ANOVA detecting a significant effect of treatments on aspen sprout height, Scheffe test did not indicate differences). Significant p values (α = 0.05) are shown in bold.

Treatment	Year Treated	Treatment Radius (m)	Number of Aspen Sprouts Per Ha	Aspen Sprout Height (cm)
1	-	untreated	0 b (.)	0 (.)
2	2004	1.5	0 b (.)	0 (.)
3	2006	1.5	289 b (960)	28 (93.5)
4	2008	1.5	265 b (919)	30 (103.9)
5	2004	2	2652 a (3278)	79 (104.8)
6	2006	2	0 b (0)	0 (0.0)
7	2008	2	0 b (0)	0 (0.0)
p			<0.01	0.03

). Height of aspen sprouts was also low for all treatments except treatment 5 (2004-2 m), but average height of aspen sprouts was still shorter than average spruce height (

Table 3. Summary of treatment effects on spruce diameter and height at Judy Creek in 2013 and 2017. Letters in each column (where shown) indicate significant differences detected using Scheffe’s Test at α = 0.05. SD shown in brackets. Significant p values (α = 0.05) are shown in bold.

			2013 Results			2017 Results
Treatment	Year Treated	Radius	n	Spruce Diameter (mm)	Spruce Height (cm)	n	Spruce Diameter (mm)	Spruce Height (cm)
1	-	untreated	10	13.5 b (4.83)	61 b (23.2)	5	15.2 (4.97)	98 (38.5)
2	2004	1.5	11	19.5 ab (4.04)	111 ab (56.3)	9	22.8 (7.91)	165 (104.8)
3	2006	1.5	10	25.5 a (7.94)	151 a (70.7)	9	30.8 (13.33)	217 (133.0)
4	2008	1.5	11	20.7 ab (5.15)	118 ab (41.1)	10	22.9 (6.66)	153 (55.9)
5	2004	2	12	26.0 a (6.11)	157 a (74.2)	8	29.1 (11.34)	184 (112.4)
6	2006	2	9	23.8 a (5.45)	126 ab (55.8)	7	27.6 (12.19)	177 (101.0)
7	2008	2	12	20.0 ab (4.25)	98 ab (37.4)	9	24.7 (6.76)	159 (42.4)
p				<0.01	<0.01		0.09	0.44

) for treatment 5 in 2013.

Spruce diameter in 2013 was significantly larger in the 2004-2 m, 2006-1.5 m, and 2006-2 m radius treatments than in the untreated. Spruce height in 2013 was significantly lower in the untreated than in treatments 3 (2006-1.5) and 5 (2004-2). Hare damage was substantial starting in 2009 and has influenced both height, particularly for shorter trees, and has probably also influenced diameter growth of untreated (treatment 1). Hare damage and substantial mortality of smaller trees, particularly in the untreated, has led to the lack of differences in spruce height and diameter in 2017.

Aspen density in the untreated in 2010 equaled 25,761 stems/ha (8.8 stems per 3.416 m² plot). Aspen height in the untreated was 1.9 m (sd = 1.41) in 2010 and 4.4 m (sd = 0.67) in May 2013. In 2013 untreated surrounding aspen average 4.7 m in height (sd = 0.55). Aspen densities on untreated portions of the study site were still above 20,000 stems/ha and heights above 6.0 m in 2017.

Significant interactions between treatment radius and year of treatment precludes use of two-way ANOVA to examine effects of these two factors on number and size of aspen sprouts and on spruce diameter and height in 2013, consequently interpretation of results for Judy Creek should be based on results shown in Table 2 and Table 3.

3.3. Lac LaBiche

Data from the three blocks at Lac LaBiche were analyzed as a randomized complete block design. Treatments were less effective in the Lac LaBiche blocks than for the other two locations and there was much more aspen sprouting and aspen growth after treatment (

Table 4. Treatment effects on number and height of aspen sprouts within a 1 m radius plot centered on the crop spruce seeding for the 3 Lac LaBiche blocks in 2013. Despite ANOVA detecting a significant effect of treatments on aspen sprout number and height, Scheffe test did not indicate differences. Significant p values (α = 0.05) are shown in bold.

Treatment	Year Treated	Treatment Radius (m)	Number of Aspen Sprouts Per Ha (2013)	Average Height of Aspen Sprouts (2013) (cm)
1	-	untreated	-	-
2	2006	1.0	6179 (6109)	138 (58.4)
3	2006	1.5	6869 (6733)	133 (42.6)
4	2006	2.0	9726 (6027)	125 (63.0)
5	2006	2.5	7036 (6148)	104 (46.8)
6	2008	1.0	3853 (4188)	110 (68.6)
7	2008	1.5	5026 (5013)	102 (73.7)
8	2008	2	6534 (6666)	128 (88.5)
9	2008	2.5	7115 (5459)	96 (74.4)
10	2010	1.0	6553 (6313)	98 (58.8)
11	2010	1.5	5128 (4650)	76 (42.7)
12	2010	2.0	10,486 (8845)	106 (81.5)
13	2010	2.5	10,111 (8134)	98 (43.0)
p			0.04	0.03

).

Results shown in

Table 5. Summary of treatment effects on spruce diameter (D15) and height for the 3 blocks at Lac La Biche in 2013 and 2017.

			2013 Results			2017 Results
Treatment	Year Treated	Radius	n	Spruce Diameter (mm)	Spruce Height (cm)	n	Spruce Diameter (mm)	Spruce Height (cm)
1	-	untreated	18	29.2 (7.07)	171 (50.9)	18	60.2 (20.88)	309 (98.7)
2	2006	1.0	18	32.5 (11.51)	168 (66.7)	18	64.6 (26.48)	312 (133.1)
3	2006	1.5	19	34.9 (11.33)	182 (61.8)	19	73.6 (26.32)	341 (130.5)
4	2006	2.0	18	34.9 (10.88)	190 (67.5)	18	72.2 (25.74)	354 (122.8)
5	2006	2.5	19	34.3 (13.58)	186 (73.6)	19	75.2 (33.26)	351 (146.6)
6	2008	1.0	19	31.1 (8.13)	169 (52.4)	18	68.1 (22.35)	328 (100.4)
7	2008	1.5	18	32.4 (10.74)	175 (60.4)	19	67.6 (25.19)	336 (121.7)
8	2008	2	19	34.7 (11.96)	168 (56.8)	18	72.2 (28.45)	336 (134.0)
9	2008	2.5	17	33.7 (6.87)	180 (49.5)	16	70.2 (20.92)	332 (101.8)
10	2010	1.0	17	33.4 (9.15)	178 (58.4)	17	68.9 (23.39)	325 (109.7)
11	2010	1.5	18	32.1 (10.24)	163 (51.5)	17	66.1 (24.72)	310 (110.4)
12	2010	2.0	17	34.6 (9.60)	182 (65.6)	16	75.1 (25.14)	348 (135.8)
13	2010	2.5	17	32.9 (12.38)	168 (66.4)	18	66.2 (26.62)	306 (131.6)
p				0.66	0.27		0.33	0.33

indicate a lack of significant treatment effects on spruce diameter or height for the Lac LaBiche blocks in 2013 or 2017.

Analysis of this dataset as a factorial experiment (

Table 6. Results from analysis of Lac LaBiche blocks as a factorial experiment showing means and Scheffe Groupings for each factor. Significant p values (α = 0.05) are shown in bold. Letters in each column indicate significant differences detected using Scheffe’s Test at α = 0.05.

		Aspen		Spruce
		Number Sprouts Per Ha (2013)	Sprout Height 2013 (cm)	Diameter 2013 (mm)	Height 2013 (cm)	Diameter 2017 (mm)	Height 2017 (cm)
Year	2006	7546 ab	124 a	34.2	182	71.5	340
	2008	5592 b	109 ab	33.0	173	69.5	333
	2010	8027 a	91b	33.2	173	68.9	322
	Untreated	0		29.2	172	60.2	309
	p	0.04	<0.01	0.60	0.25	0.40	0.11
Radius	2.5 m	8048 ab	94	33.7	178	70.6	330
	2.0 m	8842 a	120	34.8	180	73.1	346
	1.5 m	5684 b	104	33.2	174	69.2	330
	1.0 m	5465 b	114	32.3	171	67.2	322
	0.0 (untreated)	0		29.2	171	60.2	309
	p	<0.01	0.06	0.49	0.75	0.39	0.53
Year × Radius	p	0.63	0.63	0.82	0.36	0.30	0.39

) indicates that treatment radius significantly (p < 0.01) affected the number of sprouts, with number of sprouts increasing with treatment radius and with 2 m radius having significantly larger numbers of sprouts than 1.5 and 1.0 m radius (and with 2.5 m radius being intermediate). However, treatment year did not have significant (p = 0.06) effects on number of sprouts in 2013. In contrast, sprout height was affected by year (p < 0.01) but not by treatment radius (p = 0.06), with sprouts being taller in the 2006 treatment (124 cm) than in the 2010 treatment (91 cm). Treatment radius or treatment year did not have significant (p > 0.05) effects on spruce diameter or height in 2013 or 2017.

In 2013 aspen height in untreated areas averaged 223 cm in block 13731, 353 cm in block 17711, and 379 cm in block 33961, with an overall average of 308 cm. A detailed survey in block 13731 in 2017 indicated an average aspen density of 12,156 sph, with aspen height averaging 211 cm and a top height (height of 100 largest aspen per hectare) of 332 cm.

4. Discussion

Does spot brushing result in fewer and shorter suckers than broadcast treatments?

An interesting result from the Mistahae study is that motor-manual cutting of a 1 or 2 m radius around individual white spruce resulted in substantial suppression of the regrowth of aspen compared to complete removal treatments. At Judy Creek aspen numbers in the 1 or 2 m radius treatment spots were also lower and sprouts were much shorter than is typically observed following brushing of larger (e.g., 10 m × 10 m) areas. For the Lac LaBiche blocks the 1.0 and 1.5 m radius treatments had significantly fewer aspen sprouts than the 2.0 m radius, although numbers of sprouts were much higher (exceeding 5400 sph) than at the other sites. Overall results are consistent with findings from other studies that indicate that retaining a component of residual trees may reduce hardwood resprouting and reduce the number of retreatments which are required [17]. In the Lac LaBiche blocks, substantial stump sprouting was observed following radial treatments and is thought to result from poor aspen vigour in the surrounding untreated areas leading to poor apical dominance and/or a lack of shading. Our results indicate that for the Mistahae and Judy Creek sites surrounding stems are exerting sufficient apical dominance and/or cast sufficient shade to inhibit growth of aspen sprouts and suckers while this does not appear to be the case for the Lac LaBiche blocks.

At both the Judy Creek and Lac La Biche sites, aspen height declined going from early to late treatments, although differences were only significant for the Lac La Biche blocks. This trend is believed to be a reflection of the number of years for aspen regrowth following treatment, but may also be resulting from increases in apical dominance and shade associated with increasing height of surrounding aspen at the time of treatment.

As observed by Pitt et al. (1999) [16] effects of aspen control treatments may be influenced by site conditions affecting the vigour of aspen. This was evident for the Lac La Biche blocks, where the 1.0 or 1.5 m radius treatments, although having significantly fewer sprouts still had large numbers of aspen. However, these numbers were still substantially lower than the 23,000 [16], 50,000 [15], or 91,000 [17] reported for broadcast cutting treatments in young aspen stands. In the Lac LaBiche blocks aspen densities and heights were much lower than is expected for these sites and possibly reflect impacts of soil compaction during harvesting or other site issues. Poor vigour of aspen, and weak competitive effects on these sites were the likely reason for a lack of treatment effects on spruce.

Are spruce growth responses similar to herbicide treatments?

The lack of difference between spot herbicide and spot brushing treatments at Mistahae indicates the potential of the spot brushing treatments to improve spruce growth. Cyr and Thiffault (2009) [22] report that a release treatment involving removal of hardwood stems within 1 m of each crop black spruce at age 14 resulted in increased growth over the ensuing 8 years. Pitt et al. (2004) [12] found that brushing a 1 m radius around black spruce seedlings resulted in increased diameter and height at 10 years which were better than basal bark treatment with triclopyr, but inferior to repeated annual control of competing vegetation. Kabzems et al. (2022) [10] observed that white spruce diameter and height growth were similar at age 21 for manual or chemical (triclopyr) spot treatments applied in a 1.25 m radius.

While the 2 m radius cutting treatment resulted in the best growth of white spruce over the first six years after treatment at the Mistahae site, and it was significantly improved over that in the untreated, the difference between this treatment and the 1 m radius treatment was not significant. At this age either a 1 m or 2 m radius appears to be equally effective. However, since the difference between aspen and spruce height is expected to increase through to age 30 or 40, a larger treatment radius (or stepwise increases in treatment radius over time) or followup thinning treatments [8,9,10] may be required to provide light levels conducive to good spruce growth. Basal bark applications of triclopyr ester in a 1 or 2 m radius and over an entire plot resulted in significant increases in spruce diameter compared to untreated control six years after treatment. Application of triclopyr to entire plots resulted in a significant increase in marsh reedgrass cover (data not shown), but still provided significant improvements in spruce diameter growth over the 6 year period since treatment.

Judy Creek results indicate the potential to reduce competition from aspen and improve growth of white spruce by using spot cutting in a 1.5 or 2 m radius two or more years after aspen regeneration. The 2 m radius treatment in year 2 did have more sprouts than the 1.5 m radius treatment, suggesting that unless treatments are delayed until age 4, use of a smaller treatment radius may be desirable. Maintaining a matrix of surrounding aspen around each treated tree is important to controlling sucker growth and to mitigating spring and summer frost and winter injury [11]. Compared to the radial herbicide treatments examined in the adjacent portion of this study site [11] these spot cutting treatments are providing similar growth response, consistent with reports from other studies [10,12,22].

Although spot treatments can result in improvements in spruce growth over untreated, results from the Mistahae site and Pitt et al. 2015 [11] indicate that broadcast treatments are likely to result in better spruce growth than spot treatments. Consequently, there is a cost of using spot treatments in terms of reduced spruce growth but this must be balanced against the benefits of maintaining mixedwood stands.

5. Conclusions

Spot brushing within a 1 to 1.5 m radius of planted white spruce when aspen are two to six years old provided effective reductions in aspen sprouting and resulted in improvements in growth of spruce at two of the three locations studied. Results suggest that treating a 2.0 m radius should be delayed until aspen are at least four years old. However, effectiveness was reduced on sites with poor aspen vigour.

At the Mistahae site, spruce diameter growth responses to spot brushing were similar to those resulting from spot herbicide treatments but were less than observed for broadcast treatments. Growth of white spruce was significantly improved following motor-manual brushing of a 1 or 2 m radius, or motor manual brushing or triclopyr application to entire plots in this four year old spruce plantation dominated by trembling aspen.

Additional studies are required and should involve applying treatments to entire treatment plots (as done at the Mistahae site for this study) to provide additional information on effects of treatment radius and number of treated spruce on growth of both aspen and spruce, and such studies should include comparison with broadcast brushing treatments. Further studies relating to influence of environmental factors and aspen vigour on effectiveness of spot brushing treatments would also support improvements in the targeting of these treatments to sites where they are most effective. Long-term studies exploring the growth and yield implications of these treatments are needed to assist in decision making.