Pre-commercial thinning effects on growth, yield and mortality in even-aged paper birch stands in British Columbia

Forest Ecology and Management 190 (2004) 163–178

Pre-commercial thinning effects on growth, yield and mortality in even-aged paper birch stands in British Columbia Suzanne W. Simarda,*, Trevor Blenner-Hassettb,1, Ian R. Cameronc,2 a

Forest Science Department, University of British Columbia, 2424 Main Mall, Vancouver, BC, Canada V6T 1Z4 b Forestec Forestry Consultants, 1289 Monashee Crescent, Kamloops, BC, Canada V2C 6B4 c J.S. Thrower and Associates Ltd., 103-1383 McGill Road, Kamloops, BC, Canada V2C 6K7 Received 7 June 2003; received in revised form 28 August 2003; accepted 24 September 2003

Abstract The aim of this study was to quantify 5-year growth, yield and mortality responses of 9- to 13-year-old naturally regenerated, even-aged paper birch (Betula papyrifera Marsh.) stands to pre-commercial thinning in interior British Columbia. The study included four residual densities (9902–21,807 stems ha
1 (unthinned control), 3000, 1000 and 400 stems ha
1) and four sites with 3-fold within-site replication in a randomised block design. The largest, straightest, undamaged trees were selected to leave during thinning. Thinning reduced stand basal area from 5.90 m2 ha
1 in the control to 2.50, 1.53 and 0.85 m2 ha
1 in the three thinning treatments, representing 42, 26 and 15% of control basal area, respectively. After 5 years, total stand volume per plot remained lower in the three thinning treatments than the control (50.20, 30.07, 18.99 and 11.86 m3 in the control, 3000, 1000 and 400 stems ha
1 treatments), whereas mean stand diameter, diameter increment, height, and height increment were increased by thinning, and top height (tallest 100 trees ha
1) was unaffected. When a select group of crop trees (largest 250 trees ha
1) in the thinning treatments was compared with the equivalent group in the control, there was a significant increase in mean diameter, diameter increment, basal area, basal area increment, and volume increment. Mean height, height increment, top height, and total volume were unaffected by thinning. Crop tree diameter increment was the greatest following thinning to 400 stems ha
1 for all diameter classes. Thinning to 1000 stems ha
1 resulted in lower diameter increment than thinning to 400 stems ha
1 but tended to have higher volume increment. Dominant trees responded similarly to subdominant trees at 400 stems ha
1, but showed the greatest response at 3000 stems ha
1. Results suggest that pre-commercial thinning of 9–13-year-old stands to 1000 stems ha
1 would improve growth of individual trees without seriously under-utilising site resources. # 2003 Elsevier B.V. All rights reserved. Keywords: Pre-commercial thinning; Density; Growth and yield; Paper birch; Site quality

1. Introduction *

Corresponding author. Tel.: þ1-604-822-1955; fax: þ1-604-822-9102. E-mail addresses: [email protected] (S.W. Simard), [email protected] (T. Blenner-Hassett), [email protected] (I.R. Cameron). 1 Tel.: þ1-250-851-3808. 2 Tel.: þ1-250-314-0875.

Paper birch (Betula papyrifera Marsh.) is an important early seral species in the interior of British Columbia, representing a potential harvestable volume in pure and mixed stands of 20  106 m3 over 0:25  106 ha (Massie, 1996). Average juvenile height growth of paper birch is among the highest of all tree

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2003.09.010

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species on moist upland sites and riparian areas in the young forests of the Interior Cedar Hemlock (ICH) zone and wetter regions of the Interior Douglas-fir (IDF) zone (Lloyd et al., 1990). Paper birch has traditionally been considered a weed in these forests because of its competitive effects on commercial conifer species (Simard et al., 2001), the underdeveloped market for birch wood (Manning, 1975), and the poor quality and advanced age of the existing inventory (Comeau and Thomas, 1996). However, there has been a recent increase in the use and market value for high quality stems in veneer, lumber, flooring, furniture, and speciality products (Peterson et al., 1997a), raising the potential for paper birch to be an important timber species in British Columbia. Similar to silver birch (Betula pendula Roth.) in Scandinavia, paper birch has the potential for intensive management using thinning and pruning to produce clean, straight stems in short rotations for veneer and pulp products (Cameron et al., 1995). To meet the increasing demand for high quality birch logs in British Columbia as quickly as possible, there is a need to develop suitable sawlog regimes for paper birch that include a prescription for thinning (Graham, 1998; Comeau et al., 1999). In British Columbia, relatively pure even-aged, high density paper birch stands grow quickly and appear to self-thin rapidly. On a chronosequence of mesic-subhygric ICH sites, Simard and Vyse (1992) found that paper birch density reduced on average from 20,000 stems ha
1 at 10 years, to about 2500 stems ha
1 at 40 years and 500 stems ha
1 by 70 years. As the stands self-thinned, many of the residual trees grew faster and the stand structure advanced to a more mature stage, characterised by larger diameter trees, fewer trees per hectare, and the growth of shade tolerant conifers in the understorey (Simard and Vyse, 1992; Cameron, 1996b). During the rapid self-thinning years, managers have an opportunity to control paper birch stand quality and product development by thinning to an appropriate residual stand density and structure. Thinning in mixed hardwood stands has been shown to shorten sawlog rotations, increase harvested merchantable volume, and improve stem quality (Solomon and Leak, 1969; Hibbs et al., 1989; Nowak, 1996; Oliver and Larson, 1996; Miller, 1997).

However, little is known about the effects of thinning to a range of residual basal areas on growth and volume production of young paper birch stands in western Canada. Most paper birch density management studies apply to mixed broadleaf stands in eastern North America or employ intermediate (>35 years old) rather than early thinnings (e.g., Solomon and Leak, 1969; Miller, 1997; Graham, 1998; Comeau et al., 1999). Intermediate thinning may be commercially valuable and can minimise volume losses if suppressed trees are removed shortly before they die (Perry, 1985), whereas early thinning can result in greater growth responses provided residual trees are vigorous (Oliver and Larson, 1996). Paper birch may benefit from earlier thinning on ICH sites in British Columbia because mean annual increment of relatively pure, unthinned birch stands is estimated to culminate by approximately age 50 years (Simard and Vyse, 1992). For sawlog production, Cameron (1996a) recommends early thinning of silver birch to 3000 stems ha
1 when stems are 3– 6 m tall, and to 1500 stems ha
1 at 8–10 m. Solomon and Leak (1969) recommends a similar regime for paper birch in eastern Canada, in which stands are thinned to 1000 stems ha
1 when they reach a mean diameter at breast height of 13 cm. When these recommendations are applied to paper birch growing in southern British Columbia, we estimate that the first thinning to between 1000 and 3000 stems ha
1 be applied between the ages of 8–12 years, but it is important to test these guidelines experimentally before they are recommended for commercial sawlog production. A long-term field experiment was established in 1991 to study the effects of differing levels of residual density (400, 1000, 3000 and untreated control) that followed pre-commercial thinning on the mortality, growth and yield in 9–13-year-old naturally regenerated, even-aged paper birch stands. Ancillary studies were established to measure: (1) photosynthesis, water use efficiency and nitrogen use efficiency, and (2) crown and taper responses of paper birch. These studies were previously reported by Wang et al. (1995) and Utunen (1997), respectively. In this paper, we discuss the effects of differing densities of paper birch on the mortality, growth, and yield of crop trees, stands, and size classes of trees during the 5 years following treatment.

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2. Methods 2.1. Study sites This study was conducted in four 9–13-year-old paper birch-dominated stands in the ICHmw subzone of the southern interior of British Columbia, Canada. The ICHmw is characterised by a continental climate with cool, snowy winters and warm, moist summers (Lloyd et al., 1990). Mean temperature during the growing season is 14–16 8C, mean minimum temperature in January is
7 to
12 8C, and annual precipitation is 430–670 mm, of which 25–30% falls as snow. The experimental sites were located at Barnes Creek, Burton Creek, Eagle Bay and Lee Creek, between 650 and 850 m in elevation, and within 200 km north and east of Kamloops, BC. Each site was uniform in slope and aspect, and soils were deep, well drained, of loamy sand to sandy loam texture, and derived from glacial moraine parent material. The soils were classified either as Humo-Ferric Podzols or Dystric Brunisols (Soil Classification Working Group, 1998). The four sites varied in site quality as measured by 5-year top height increment of paper birch (Table 1). Stand and site characteristics are summarised in Table 1. Following clearcutting of the original stands, all sites had been mechanically site prepared and the slash piles burned, except at Burton Creek, where no site preparation was performed. History records and

165

forest floor characteristics suggested, of the mechanically prepared sites, that mineral soil exposure was extensive at Barnes Creek and Lee Creek and confined to small areas at Eagle Bay. All sites had regenerated to a mixture of species, but paper birch was consistently dominant, comprising at least 24% of the stands and exceeding 5000 stems ha
1. According to Gregory and Haack (1965) and Safford (1983), all sites were considered fully stocked with paper birch, which were of both seed- and sprout-origin. Total density tended to decrease with increasing site quality, from 21,807 stems ha
1 at Barnes Creek to 9902 stems ha
1 at Burton Creek. The lower site quality but higher density at Barnes Creek and Lee Creek appeared to have resulted from the mechanical site preparation treatments that removed the nutrient-rich forest floor and increased the receptivity of the seedbed for germinants. 2.2. Experimental design and treatments The study included four thinning levels applied on four sites with 3-fold within-site replication (n ¼ 3) in a randomised block design (N ¼ 48). The four levels of thinning were unthinned control, 3000, 1000 and 400 stems ha
1 (hereafter referred to as control, T3000, T1000, and T400, respectively). The four thinning treatments were randomly assigned to 12 treatment plots on each site. Treatment plots were 75 m  75 m (0.5625 ha), in which 35 m  35 m (0.1225 ha) measurement plots were centred.

Table 1 Stand and site characteristics of the four experimental sites Site

Latitude/ Longitude

Elevation Slope (m) (%)

Aspect Total density prior to thinninga Stand age Five-year Number of Basal Birch proportion at thinning top height stems area of density in stems (y) increment (m)b (stems ha
1) (m2 ha
1) per hectare before thinning (%)

Barnes creek Lee creek Eagle Bay Burton creek P-value a

508310 N/1188520 W 508570 N/1198330 W 508550 N/1198120 W 518300 N/1198290 W

700 850 650 750

5–10 10–20 10–30 10–30

S.E. S.E. N NW

13 12 9 13

2.8 3.0 3.5 3.1

21807 19438 13827 9902

ac a b b

0.0001

4.82 6.78 2.78 4.41

bc a c b

24 56 41 87

0.0001

Measurements completed in April–May 1991 prior to treatment application. Height increment of tallest 100 trees ha
1 measured pre-treatment was used as a measure of site quality. c Site means followed by different letters were significantly different from each other (P  0:05). Stem frequency and basal area did not vary significantly among treatments prior to thinning (P > 0:05). b

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Trees were selected and marked for thinning in May, 1991, and all sites were thinned in June and July, 1991. Paper birch were primarily selected as leave trees, but cottonwood (Populus balsamifera L. ssp. trichocarpa T. & G.), trembling aspen (Populus tremuloides Michx.), or conifers were occasionally selected in gaps where no birch individuals occurred. Thinning was applied according to the principle of thinning from below, whereby trees were removed mainly from the lower crown classes (Smith et al., 1997). Selection criteria for paper birch leave trees were: (1) dominant or co-dominant crown class; (2) uniform spacing relative to other leave trees; (3) seedorigin; (4) single, straight, healthy stem. Conifer species that were sometimes selected as crop trees included Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), western larch (Larix occidentalis Nutt.), lodgepole pine (Pinus contorta Dougl.), western white pine (Pinus monticola Dougl.), hybrid spruce (Picea engelmannii var. glauca Perry ex Engelm.), western redcedar (Thuja plicata D. Don.), or western hemlock (Tsuga heterophylla (Raf.) Sarg.). Trees were cut near the root collar using circular brush saws. All leave trees located inside the measurement plots were identified with numbered tags. Sprouts were cleaned from thinned stands in 1998 using circular brush saws. Further cleanings were not necessary because of the decline in sprout vigour with increased crown closure. All thinning treatments shifted the average diameter distribution from an inverse-J shape prior to thinning, to a more normal shape with a higher frequency of larger diameter trees after thinning (Fig. 1). Pre-treatment density appeared to be higher in the control and T3000 than in T1000 and T400 (Fig. 1), but because of the variability between and within sites, differences among treatments were not significant (P > 0:05, Table 1). Mean residual densities were close to the targeted densities at Eagle Bay and Lee Creek, but at Barnes Creek and Burton Creek they were lower in T3000 and T1000 and higher in T400. At Eagle Bay, mean densities after thinning in T3000, T1000, and T400 were 2855, 933, and 427 stems ha
1, and at Lee Creek they were 3018, 1102, and 414 stems ha
1, respectively. Corresponding mean densities at Barnes Creek were 2144, 732, and 468 stems ha
1 and at Burton Creek they were 2207, 860, and 509 stems ha
1.

2.3. Measurements Prior to thinning in early spring, 1991, the diameter (diameter at breast height) of all trees inside the measurement plots was measured. Post-thinning measurements of crop trees, conducted in the fall of 1991, were diameter and total height. In controls, where the number of measurement trees was very large, height was measured on a subset of trees, and the remaining heights estimated using Weibull height–diameter functions for paper birch, black cottonwood and trembling aspen, and linear functions for all conifer species. In the three thinning treatments, height of all leave trees was measured. The trees were re-measured again in the fall of 1996. Height increment and diameter increment was calculated as the difference between 1996 and 1991 height and diameter, respectively. 2.4. Statistical analysis The analysis examined the effects of thinning treatment on the growth, yield, and mortality of paper birch. Trees were grouped into three classes for data analysis: stand, crop tree, and initial diameter size classes. Stand values were determined from all trees, regardless of their size. Crop tree values were determined from the 250 largest birch stems ha
1 for each measurement period so that equivalent groups of trees could be compared among treatments. Crop tree-level values were analysed to differentiate the true response to thinning from the technical effect of thinning (i.e., the chainsaw effect) that is represented by the standlevel responses. Trees were also split into initial (1991) 2 cm diameter classes (0.00–2.00, 2.01–4.00, 4.01–6.00, 6.01–8.00, and 8.01–10.00 cm) to investigate crop tree variation in diameter increment with initial tree size. Mean stand and crop tree variables calculated were height, top height, quadratic diameter, total volume per hectare, and total basal area per hectare. Mean top height was the average height of the 100 largest birch stems per hectare. Quadratic mean diameter was the diameter of a tree with average basal area at breast height. Basal area and volume were calculated for each tree, and total basal area per hectare and total volume per hectare were determined by summing values for all trees in the plot. Tree volumes were calculated for the whole stem inside

3000

3000

2000

2000

Frequency

Frequency

S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178

1000

1000

0

0 0

2

4

6

8

10

Diameter Class

12

0

2

3000

3000

2000

2000

1000

4

6

8

10

12

8

10

12

Diameter Class

(b)

Frequency

Frequency

(a)

1000

0

0 0

(c)

167

2

4

6

Diameter Class

8

10

12

0

(d)

2

4

6

Diameter Class

Fig. 1. Frequency of trees by diameter class averaged over the four study sites at Barnes Creek, Lee Creek, Eagle Bay, and Burton Creek. Grey bars indicate frequency before thinning (spring, 1991) and black bars indicate frequency after thinning (fall, 1991), which was done in June 1991. Diameter distributions are shown for (a) control, (b) T3000, (c) T1000, and (d) T400.

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bark using numerical integration of the taper function by Kozak (1988) and British Columbia Ministry of Forest coefficients based on Forest Inventory Zones. Stand, crop tree and diameter size class variables were compared among thinning treatments for each year after thinning using a mixed model analysis of variance (ANOVA) with 3-fold replication in a randomised block design (Kirk, 1982; McKone and Lively, 1993). Thinning treatment was a fixed effect and site was a random effect. Homogeneity of variances was checked for all variables and years before ANOVA was performed. The partitioning of the variation in the ANOVA model was as follows:

3. Results 3.1. Mortality Most mortality (95.4%) between 1991 and 1996 occurred in the unthinned control, 3.8% in T3000, and 0.7% in T1000 and T400 combined (Fig. 2). In the control, all mortality occurred in the smaller diameter classes (<5 cm), which represented suppressed and intermediate trees, whereas the larger, selected crop trees remained vigorous (Fig. 2). In T3000, most trees that died were <3 cm in diameter, whereas the few that died in T1000 and T400 were of various sizes (data not shown).

Source of variation

Factor type

Degrees of freedom

Expected F-test (d.f.)

Thinning treatment (T) Site (S) Interaction (TS) Error Total

Fixed Random Random

t
1¼3 s
1¼3 ðt
1Þðs
1Þ ¼ 9 tsðn
1Þ ¼ 32 tsn
1 ¼ 47

MST/MSTS (3, 9) MSS/MSE (3, 32) MSTS/MsE (9, 32)

Where significant differences occurred (a ¼ 0:05), multiple comparisons were made among treatment means using the Waller–Duncan–Bayes LSD procedure (Duncan, 1975).

3.2. Stand-level growth and yield responses In the spring of 1991, prior to thinning, stand basal area did not vary significantly among treatments and

Table 2 Thinning effects at the stand level on mean tree size, total basal area, and total volume in 1991 and 1996 Year

Treatment

1991

Control T3000 T1000 T400

7.678 8.208 7.966 7.850

Standard error P-value

0.278 0.7992

1996

Control T3000 T1000 T400 Standard error P-value

a

Top height (m)

11.675 11.300 11.133 10.825

aa a a a

a a a a

0.391 0.8626

Mean height (m)

Quadratic mean diameter (cm)

3.308 4.792 5.842 6.083

2.125 3.383 4.442 4.583

c b a a

c b a a

0.182 0.0001

0.145 0.0001

4.667 7.142 8.433 8.783

3.367 5.800 7.633 8.442

c b a a

0.262 0.0001

d c b a

0.223 0.0001

Values with the same letters within a year are not significantly different (a ¼ 0:05).

Total basal area (m2 ha
1) 5.900 2.500 1.525 0.850

a b c d

0.239 0.0001 13.383 7.308 4.417 2.792

a b c d

0.485 0.0001

Total volume (m3 ha
1) 16.100 7.525 4.933 2.817

a b c d

0.800 0.0001 50.200 30.067 18.992 11.858

a b c d

2.738 0.0001

S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178 500

400

400

300

300

Frequency

Frequency

500

200

200

100

100

0

0 0

2

(a)

4

6

8

10

12

0

2

(b)

Diameter Class

500

400

400

300

300

6

8

10

12

8

10

12

Frequency

Frequency

4

Diameter Class

500

200

200

100

100

0

0 0

(c)

169

2

4

6

Diameter Class

8

10

12

0

(d)

2

4

6

Diameter Class

Fig. 2. Cumulative mortality (1991–1996) by diameter class in the controls at (a) Barnes Creek, (b) Lee Creek, (c) Eagle Bay, and (d) Burton Creek.

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Table 3 Thinning effects at the stand level on height, diameter, total volume, and total basal area increment (increments were calculated for the period between 1996 and 1991) Treatment

Top height incrementa (m)

Control T3000 T1000 T400

3.542 3.092 3.167 2.975

Standard error P-value

0.196 0.2184

ab a a a

Mean height increment (m)

Quadratic mean diameter increment (cm)

Volume increment (m3 ha
1)

Basal area increment (m2 ha
1)

1.358 2.350 2.592 2.700

1.242 2.417 3.192 3.858

34.100 22.542 14.058 9.042

7.483 4.808 2.892 1.942

c b ab a

0.112 0.0001

d c b a

0.114 0.0001

a b c c

2.033 0.0001

a b c d

0.289 0.0001

a Top height increment was the same at the crop tree level as at the stand level because in both cases it was determined from the tallest 100 stems ha
1 only. b Values with the same letters are not significantly different (a ¼ 0:05).

averaged 4.6 m2 ha
1 over the four sites (P > 0:05, Table 1). Stand basal area was reduced by 39–58% with each increase in thinning intensity, and similar treatment differences were maintained through 1996 (P < 0:0001, Table 2). Stand basal area increased by 2–3 between 1991 and 1996 due to growth of the leave trees. Basal area increment between 1991 and 1996 followed the same pattern as basal area, and was the greatest in the control and smallest in T400 (P < 0:0001, Table 3). Similar to stand basal area, stand volume immediately decreased with thinning and differences among thinning treatments remained significant through 1996 (P < 0:0001, Table 2). Mean volume increment from 1991 to 1996 followed the same pattern among treatments, except the difference between T1000 and T400 was not significant (P < 0:0001, Table 3). Quadratic mean diameter and mean stand height were significantly affected by thinning in all three measurement years (P < 0:0001, Table 2), but there was no effect on top height either in 1991 (P ¼ 0:7992) or 1996 (P ¼ 0:4923, Table 2). The increases in mean diameter and height were expected because the largest trees were selected to leave during thinning. The lack of top height response occurred because thinning selected for the tallest trees, and more than 100 trees plot
1 remained following all treatments, except T400, where approximately 49 trees plot
1 remained. Differences in quadratic mean diameter between the thinning levels increased with time because diameter increment increased with each increase in thinning intensity (P < 0:0001, Table 2). Thinning in 1991 resulted in an increase in mean

height from 3.3 m in the control, to 4.8 m in T3000, and 5.9 m in T1000 and T400, but there was no difference between the two lowest density treatments (P < 0:0001, Table 2). The same treatment pattern in mean height and mean height increment occurred in 1996 (P < 0:0001, Tables 2 and 3). 3.3. Crop tree growth and yield responses There were no significant differences in crop tree top height, mean height, arithmetic mean diameter, quadratic mean diameter, total basal area, and total volume among treatments in the fall following thinning in 1991 (P > 0:05, Table 4). Treatment effects on crop tree size and increment became significant in 1996, 5 years following thinning. In 1996, mean diameter and total basal area of crop trees were significantly larger in the three thinning treatments (means were 10.2 cm and 2.1 m2 ha
1) than in the control (9.0 cm and 1.6 m2 ha
1) (P < 0:05, Table 4). There were no differences among the three thinning intensities for any of these parameters, and T400 did not differ from the control (P > 0:05, Table 4). Top height, mean height, and total volume were not significantly affected by thinning (P > 0:05, Table 4). Mean diameter increment, total volume increment, and total basal area increment of crop trees between 1991 and 1996 increased significantly with thinning (P < 0:05, Fig. 3). Mean diameter increment increased with increasing thinning intensity, but differences between T1000 and T400 were not significant (P < 0:05, Fig. 3c). Basal area increment and volume increment of crop trees also improved with thinning,

S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178

171

Table 4 Thinning effects at the crop tree level on mean tree size, total basal area, and total volume between 1991 and 1996 (top height is not shown because values were the same at the crop tree level as at the stand level since in both cases it was determined from the tallest 100 stems ha
1 only) Year

Treatment

1991

Control T3000 T1000 T400

7.405 7.617 7.541 7.092

Standard error P-value

0.238 0.4287

1996

Control T3000 T1000 T400 Standard error P-value

a

Mean height (m)

10.592 10.667 10.600 10.050

aa a a a

a a a a

0.306 0.4622

Quadratic mean diameter (cm) 6.258 6.250 6.267 5.608

a a a a

0.244 0.1711 9.000 10.442 10.133 10.042

b a a ab

0.336 0.0269

Total basal area (m2 ha
1) 0.808 0.800 0.817 0.650

a a a a

0.063 0.1050 1.633 2.092 2.192 2.033

b a a ab

0.137 0.0371

Total volume (m3 ha
1) 2.975 2.850 2.900 2.225

a a a a

0.332 0.3708 7.750 9.882 10.275 8.992

a a a a

0.870 0.1957

Values with the same letters within a year are not significantly different (a ¼ 0:05).

but thinning to densities lower than T3000 had no additional beneficial effect. In contrast to the bole increment measures, neither mean height increment nor top height increment of crop trees was affected by thinning through the entire measurement period (P > 0:05, Fig. 3d). 3.4. Diameter growth responses by initial diameter class Thinning had a highly significant effect on the growth of all trees, regardless of initial diameter class (P < 0:001, Fig. 4). Trees of all sizes responded to all three levels of thinning, but the greatest diameter increment occurred in T400 (P < 0:001, Fig. 4). In the 5 years after thinning, trees grew 1.6–1.9 cm faster in T400 than the control, and there was no difference in the magnitude of response among initial diameter classes. The relative responses in T400 were considerably greater among the smallest trees (0–2 cm initial diameter), however, whose diameter increment increased 4.5 times over control values, compared with only a 1.5 times increase among trees that were initially 6–12 cm in diameter. Thinning to a higher density had a somewhat different effect, where diameter class at the time of thinning affected the absolute diameter growth response of individual trees. In T3000, gains in diameter increment over the

control were greater among co-dominant and dominant trees (8–12 cm initial diameter) trees than among subdominant (0–2 cm) trees (1.1 versus 0.7 cm gain). The relative gains followed a similar, albeit more muted pattern as in T400, where growth of subdominant trees thinned to 3000 stems ha
1 was 2.5 times that of equivalent control trees, compared with only a 1.4-fold gain among co-dominant and dominant trees. 3.5. Effectiveness of blocking by site Because sites were not replicated, we could not test the effect of site on paper birch growth and yield responses to thinning. However, the expected F-test for site (FS) provided an approximate test of the effectiveness of blocking by site (Peterson, 1985). We found that FS exceeded Fcritical for mean height, total volume, total basal area and quadratic mean diameter increment, indicating that blocking by site was effective at reducing experimental error (Table 5). Mean crop tree size and growth increments by site are presented in Table 5 but without any statement of significance or standard error because there is no estimate of error of the means (Peterson, 1985). The ranking of sites by mean diameter increment during the 5 years post-thinning matched site patterns in 5-year top height increment measured prior to

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S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178 9

1.6 a

a ab

7 6

b

5 4 3 2

Basal area increment (m2 ha-1)

Volume increment (m3 ha-1)

8

Control

(a)

T3000

T1000

1.0

T400

b

0.8 0.6 0.4

(b)

Thinning treatment

5.0

Control

T400

3.5

b

4.0 3.5 c

2.5 2.0 1.5 1.0

T3000

Thinning treatment

4.0

a ab

4.5

Mean height increment (m)

Quadratic mean diameter increment (cm)

T1000

1.2

0.0

0

a

a

a

a

T3000

T1000

T400

3.0 2.5 2.0 1.5 1.0 0.5

0.5 0.0

0.0 Control

(c)

a

0.2

1

3.0

a a

1.4

T3000

T1000

T400

Thinning treatment

Control

(d)

Thinning treatment

Fig. 3. Treatment comparison of (a) volume increment, (b) basal area increment, (c) quadratic mean diameter increment, and (d) mean height increment between 1991 and 1996. Values with the same letters are not significantly different (a ¼ 0:05).

treatment in 1991 (Table 1). Mean diameter increment tended to be the greatest at the highest sites, Eagle Bay and Burton Creek, and the lowest at the poorest site, Barnes Creek.

4. Discussion Paper birch crop trees responded to all intensities of thinning, but the greatest mean diameter increment response occurred following thinning to 1000 or 400 stems ha
1. When crop trees were separated into initial size classes, mean diameter increment response

was consistently the greatest in the 400 stems ha
1 treatment. Our results agree with other thinning studies, where trees generally grew the largest in the most heavily thinned treatments (Johnstone, 1981; Leak and Solomon, 1997; Smith et al., 1997; Medhurst et al., 2001). Crop trees also responded to thinning with increased diameter, basal area, basal area increment, and volume increment. Thinning resulted in crop trees that were 13% (1.2 cm) larger in mean diameter and that had 29% (0.5 m2 ha
1) more basal area than unthinned crop trees after 5 years. Basal area increment and volume increment responded more dramatically, and were 48 and 64% greater, respectively, in the three

S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178

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Fig. 4. Mean diameter increment of crop trees from 1991 to 1996 by thinning treatment and initial diameter class. Values with the same letters in each diameter class are not significantly different (P < 0:0001 for all diameter classes).

Our response was comparable to a thinning response in a paper birch-dominated stand of mixed eastern hardwoods, however, where stand age (25 years) and density (10,000 stems ha
1) at the time of thinning were more comparable to our study (Marquis, 1969). Marquis (1969) found that paper birch grew 0.20 cm per year faster in a heavy thinning treatment than the control during the first 10 years after treatment, but this advantage slowed to 0.16 cm per year over the subsequent 21 years (Leak and Solomon, 1997). It was also comparable to 14–20-year-old red alder (Alnus rubra Bong.) diameter response to thinning on highly productive coastal Oregon sites, which grew 0.78– 0.85 cm per year in thinned plots compared with 0.55 cm per year in control plots (Hibbs et al., 1995). Our diameter increment results, along with

thinning treatments than the control. Some studies suggest that such rapid growth increments may eventually result in a convergence in stand basal area in thinned treatments with controls, provided there are not subsequent, frequent thinnings (Oliver and Murray, 1983; Hasenauer et al., 1997). The magnitude of response in our study was greater than that in several other thinning studies on paper birch (Solomon and Leak, 1969; LaBonte and Leso, 1990; Graham, 1998) and other broadleaf tree species (Strong and Erdemann, 2000), probably because our stands were younger and initial stand densities much higher before thinning. Mean diameter increment of our crop trees averaged 56% (0.28 cm per year) greater in T400 and T1000 than in the control, compared with a 41% (0.20 per year) increase in T3000.

Table 5 Site means for crop tree height, diameter, total volume, and total basal area in 1991 and 1996 (increments were calculated for the period between 1996 and 1991) Treatment

Barnes creek Lee creek Eagle bay Burton creek FSa a

Mean height (m)

Quadratic mean diameter (cm)

Total volume (m3 ha
1)

Total basal area (m2 ha
1)

Quadratic mean diameter increment (cm)

Volume increment (m3 ha
1)

Basal area increment (m2 ha
1)

1991

1996

1991

1996

1991

1996

1991

1996

7.667 7.542 6.233 8.217

10.342 10.517 9.692 11.358

6.608 6.000 5.058 6.717

10.092 9.575 9.367 10.583

3.042 2.733 1.558 3.617

9.225 8.417 7.708 11.550

0.892 0.733 0.525 0.925

2.058 1.842 1.767 2.283

3.483 3.575 4.308 3.867

6.183 5.683 6.150 7.933

1.167 1.108 1.242 1.358

12.461

5.039

9.724

2.646

6.814

3.680

8.474

2.972

4.681

2.280

1.394

FS provides an approximate test of the effectiveness of blocking by sites at reducing experimental error.

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the fact that crowns remained open in T1000 and T400 but were closed in T3000 after 5 years, suggest that the diameter response in T1000 or T400 should continue to increase over the control for a greater number of years than that in T3000. However, the longevity of the increase, and whether the growth rate in T400 eventually exceeds the other treatments, can only be determined with long-term measurements. The absolute diameter growth response to thinning to 400 stems ha
1 did not vary with initial size of crop trees, but in the 3000 stems ha
1 treatment it was the greatest among trees that were dominant at the time of thinning. The relative growth response to thinning was the largest in the smallest diameter class regardless of thinning intensity, but the overall contribution of these small stems to stand basal area growth was small. Other studies have also found differential thinning responses among different tree size classes (Barclay and Brix, 1985; Pukkala et al., 1998). For example, subdominant Eucalyptus nitens trees either did not respond or responded to a lesser degree compared with dominant trees (Medhurst et al., 2001). Others have suggested that dominant trees may not respond to thinning unless neighbouring dominant trees are removed (Miller, 1997), but that this may be avoided if thinning is done early in the rotation before height stratification becomes pronounced (Medhurst et al., 2001). The results of our study suggest thinning in young, uniform paper birch stands should aim to retain dominant trees because of their individual growth response potential and for their contribution toward the stand basal area response. The increased diameter growth resulting from thinning was associated with increased net photosynthetic rate among thinned trees measured 1 and 2 years after treatment (Wang et al., 1995). Wang et al. (1995) found in the same stands that net photosynthetic rates were the highest in T1000 and T400 (20.7 mmol CO2 m
2 s
1) and the lowest in the control (2.3 mmol CO2 m
2 s
1) 1 year after thinning. Specific leaf area decreased in a similar pattern among thinning treatments, and water use efficiency and nitrogen use efficiency increased (Wang et al., 1995). Thinning increased photosynthetically active radiation under the canopy from 4% of full light in the control to 89% in T400 and T1000 in 1992 (Wang et al., 1995). Although light undoubtedly accounted for much of the growth responses to thinning (e.g.,

Roberge, 1988; Peterson et al., 1997b), increased water and nitrogen availability also likely played important roles in the physiological responses measured by Wang et al. (1995). Utunen (1997) also found on our sites that leaf mass of individual trees increased with crown size in response to thinning (Utunen, 1997), which was likely correlated with sapwood cross-sectional growth (Waring and Schlesinger, 1985). Peterson et al. (1997b), in work with loblolly pine (Pinus taeda L.), similarly attributed increased diameter growth of leave trees following thinning to increased leaf mass, increased crown volume, and enhanced physiological activity. Increased foliar efficiency, along with increased foliar biomass, has contributed to Douglas-fir growth responses to thinning (Mitchell et al., 1996). In contrast to mean diameter increment, mean volume increment tended to be higher in T1000 and T3000 than T400, and T400 did not differ significantly from the control. The rapid volume growth increases post-thinning in T1000 and T3000 suggest that the residual trees refilled the growing space rapidly in the intermediate treatments. The lack of volume increment response in T400, however, may have resulted from a multiplication of height and diameter error effects in volume calculations, or, alternatively, from thinning shock, allocation of carbon away from stem towards root growth, or inadequate utilisation of ‘growing space’ or resources. Thinning shock was unlikely in T400 because (a) we found a significant diameter increment response and (b) Wang et al. (1995) found no reduction in net photosynthetic rate during the first year following thinning, possibly because the dominant and codominant leave trees were able to adjust rapidly to the increased light conditions. Alternatively, thinning to 400 stems ha
1 may have opened the stands to such a degree that crowns were no longer closed and the site was no longer fully occupied, resulting in incomplete use of growing space for the first 5 years following thinning. In spite of higher net photosynthetic rates and mean diameter increment, mean diameter of crop trees in T400 did not differ significantly from the control 5 years after thinning, which may have resulted from greater allocation of carbon towards root than diameter growth in order to fully utilise available soil resources and re-build crown cover. Paper birch crowns still had not closed in T400 5 years after

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thinning. In comparison, paper birch planted to 300 stems ha
1 on a medium site in New Hampshire did not close crowns for at least 20 years (Hoyle, 1984). In young stands of silver birch, trembling aspen or oak, heavy pre-commercial thinning below 1000 stems ha
1 has resulted in volume and value losses (Niemisto¨ , 1991; Hocker, 1982; Dwyer and Lowell, 1988). Thinning had no effect on height of paper birch crop trees during the 5-year measurement period, agreeing with the results of several other broadleaf thinning and weeding studies (Marquis, 1969; Hoyle, 1984; Graham, 1998; Simard and Hannam, 2000; Medhurst et al., 2001). Stand density has a well-known effect on diameter but not height of trees, unless density is extremely low or high, particularly on poor sites (Niemisto¨ , 1991). In our study, it appears that the lowest and the highest densities were within the range where paper birch height is unaffected. Our results contrast with Niemisto¨ (1995a), who found that silver birch grown at 400 stems ha
1 had reduced height growth in comparison to denser stands. They also differ from studies in young red alder, where thinning frequently results in height growth losses for short periods of time (Hibbs et al., 1995). Nevertheless, thinning does not usually affect height growth of residual trees because allocation of carbon toward height growth is a higher priority than toward diameter growth (Lanner, 1985). In addition, seasonal carbon allocation patterns suggest that height growth occurs in the spring, when soil resources are not limiting, and diameter growth occurs later in the summer, when summer drought tends to limit photosynthesis on these sites (Wang et al., 1995). Increasing summer water and nutrient availability by reducing stand density should therefore primarily affect diameter growth of the residual trees. Mean diameter increment response to thinning, averaged over all thinning treatments, tended to increase with site quality in our study (this trend could not be tested statistically because we lacked replication of sites). Mean diameter increment averaged 12% greater on the two higher quality sites, Burton Creek and Eagle Bay, than the two lower quality sites, Barnes Creek and Lee Creek. Growth responses to thinning are well known to increase with site index, site fertility and nitrogen fertilisation (Safford and Czapowskyj, 1986; Harrington and Wierman, 1990; Mitchell et al.,

175

1996; Hasenauer et al., 1997), presumably because of increased potential nutrient availability. Site quality differences are important because thinning prescriptions that are appropriate for sawlog production on high quality sites may be inappropriate on sites of low quality, where cutting too heavily may under-utilise growing space for a prolonged period of time (Strong and Erdemann, 2000; Smith et al., 1997). Following thinning, the controls were considered overstocked, T3000 was adequately stocked, and T1000 and T400 were appropriately stocked for minimal future competition-related mortality according to published stocking guidelines for paper birch (Safford, 1983; Solomon and Leak, 1969; Gregory and Haack, 1965). Predictably, almost all the mortality between 1991 and 1996 occurred in the controls (95.4%), not in the thinned plots, and appeared primarily due to intraspecific competition. A similar mortality pattern in control versus thinned plots has been documented in studies of paper birch (Marquis, 1969) and other broadleaf and conifer species (e.g., Johnstone, 1981; Strong and Erdemann, 2000). On average, 5.5% of control trees died during the 5-year measurement period in our study. This was considerably less than the 30% mortality observed over a 5-year period by Marquis (1969), which may have been related to differences in shade tolerance of the composite species in the eastern hardwood mixture. Removal of suppressed trees in our study accounts for some of the lower mortality in the thinned treatments versus the control, but thinning probably also reduced risk of mortality to smaller leave trees. Mortality tended to increase with site quality, where only 2.3% of control trees died at Lee Creek compared with 11.1% at Burton Creek, in spite of higher initial densities at Lee Creek (19,438 stems ha
1, 6.78 m2 ha
1) than at Burton Creek (9902 stems ha
1, 4.41 m2 ha
1). Rate of self-thinning has been shown in other studies to increase with site productivity because of faster growth rates and increased inter-tree competition (Simard, 1990; Radtke and Burkhart, 1999). Mortality has also been shown to increase with fertilisation, possibly by increasing inter-tree competition (Mitchell et al., 1996) or virulence of root pathogens (Safford and Czapowskyj, 1986). Thinning stands to low densities has the potential to negatively affect stem quality through increased branch size and stem taper, particularly when stands

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are young. Two years after thinning was applied in our experiment, Utunen (1997) found that paper birch trees in the heavily thinned stands (T1000 and T400) tended to have larger crowns (with larger and more abundant branches) and greater stem taper than those in the control or lightly thinned stands (T3000). These results concur with Niemisto¨ (1995b), who found that silver birch stands established at densities greater than 1600 stems ha
1 had shorter crowns with fewer branches and stems with better taper than stands established at lower densities. On our study sites, Utunen (1997) also found that the greatest crown responses tended to occur among trees in subordinate crown positions (75% of the dominant canopy height), possibly because they received proportionately more of the light freed by thinning than trees that had already occupied codominant or dominant positions before thinning (Miller, 1997). This crowndifferentiated response was supported by the diameter size class analysis in this paper, where relative response of subdominant trees in T400 compared with equivalent-sized trees in the control was proportionally greater than that of dominant trees. The tendency for larger crowns to occur in T400 and T1000 on our sites resulted from free-growth prior to crown reclosure with nearby residual trees, which still had not occurred in those treatments when the crown study was conducted. Most of the increased branch volume among subordinates in T1000 and T400 was distributed to the lower portion of the crown (Utunen, 1997). This distribution response may have facilitated taper increases following thinning because taper was strongly correlated with branch basal area and increased substantially at the base of the crown. Crown and taper responses in the lowest thinning intensity, T3000, were very similar to the control, probably because crowns closed quickly in that treatment following thinning. We found little epicormic branching or sprouting in response to any of the thinning treatments in our study, which reportedly is a primary reason to avoid heavy thinnings in birch and other hardwood species (e.g., Strong and Erdemann, 2000). We observed that thinning resulted in considerable top damage in our stands due to snow bending and moose browsing. Snow damage and breakage has been common in other thinned birch stands (LaBonte and Leso, 1990; Leak and Solomon, 1997), but it appears to decrease with greater age at the time of thinning

because of increased stem stability (Graham, 1998). Moose browsing of birch has also increased with density reduction in other studies because of greater animal access in the stands (Simard and Heineman, 1996; Karlsson and Albrektson, 2001).

5. Conclusions This study provides information on growth, yield and mortality responses of paper birch to pre-commercial thinning, representing a unique study in young even-aged paper birch stands in western North America. This new information will be useful for density management of young paper birch stands for rapid diameter growth and high wood quality. Our results generally agree with earlier studies in eastern North America paper birch stands (Cameron, 1996a; Safford, 1983; Solomon and Leak, 1969; Gregory and Haack, 1965), suggesting that eastern pre-commercial thinning guidelines are generally adequate for young paper birch stands in British Columbia. We found that pre-commercial thinning to 1000 stems ha
1 was the most successful treatment at balancing minimal competition-related mortality with maximum growth potential and, according to the earlier study of Utunen (1997), minimal effects on branching and taper. Thinning to 400 stems ha
1 resulted in the largest and the most sustained diameter response, but at this low density, site resources appeared to be under-utilised. Thinning to 1000 stems ha
1 resulted in comparatively lower diameter increments but tended to have higher stand basal area and volume increment because of fuller site occupancy. Thinning to 3000 stems ha
1 at this age would result in the smallest branches and the lowest taper for the highest veneer-quality sawlogs according to Utunen (1997), but we found that crowns refilled quickly at this density, likely requiring subsequent, frequent thinnings to maintain rapid diameter growth rates. The precise residual density for various product objectives is likely to vary among sites. Management plans for the vast untapped paper birch stands in western North America should consider stand structure, stand density, stand age, site quality, and tree vigour (O’Hara, 1990), and how they differ from the study conditions reported in this paper.

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Acknowledgements Funding for this research project was provided by the Hardwood Management Subprogram of the Canada-British Columbia Partnership Agreement on Forest Resource Development (1991–1995) and Forest Renewal BC (1996–2001). Funding assistance by Forest Renewal BC does not imply endorsement of any statements or information contained herein. We are particularly appreciative to Alan Vyse, who originally conceived of the idea and has provided ongoing support, and Barbara Zimonick, who assisted with administration of the project. We thank Jacob Boateng, Phil Comeau, Wayne Johnstone, Steven Omule, Frank Sheran, Jim Wright, and Alan Vyse for reviewing the working plan, and Mike Carlson, Rob Enfield, Dennis Lloyd, and Jim Wright for suggestions on site locations. We appreciate the advice provided by Valerie LeMay on statistical design and analysis. We are grateful to Cora Gordon for assistance with layout of the experiments, and Vicky Berger, Lynette Ryrie, Mary Fraser, and Jonas Fraser for completing the pre-treatment measurements and thinning treatments. We thank the many people who assisted with tree measurements over the years. Assistance with data entry was provided by Hilary Parsons. We thank anonymous reviewers for valuable comments on earlier drafts of the manuscript. References Barclay, H.J., Brix, H., 1985. Fertilization and thinning effects on a Douglas-fir ecosystem at Shawnigan Lake: 12-year growth response. Inf. Rep. BC-X-271. Can. For. Serv., Pac. For. Centre, Victoria, BC. Cameron, A.D., 1996a. Managing birch woodlands for the production of quality timber. Forestry 69, 357–371. Cameron, I.R., 1996b. Vertical stratification in some 50-year-old mixed-species stands in the Interior Cedar-Hemlock zone. In: Comeau, P.G., Thomas, K.D. (Eds.), Proceedings of the Workshop on Silviculture of Temperate and Boreal Broadleaf-conifer Mixtures, Land Manage. Handbook No. 36, Richmond, BC, February 28–March 1, 1995. BC Ministry of Forests, Victoria, BC, pp. 122–125. Cameron, A.D., Dunham, R.A., Petty, J.A., 1995. The effects of heavy thinning on stem quality and timber properties of silver birch (Betula pendula Roth). Forestry 68, 275–286. Comeau, P.G., Thomas, K.D. (Eds.), 1996. In: Proceedings of the Workshop on Silviculture of Temperate and Boreal Broadleafconifer Mixtures, Land Manage. Handbook No. 36, Richmond,

177

BC, February 28–March 1, 1995. BC Ministry of Forests, Victoria, BC. Comeau, P.G., Wang, J.R., Letchford, T., Coopersmith, D., 1999. Effects of spacing paper birch-mixedwood stands in central British Columbia. FRBC Project HQ96423-RE (MOF EP 1193). Extension Note 29. BC Ministry of Forests, Victoria, BC, 7 pp. Duncan, D.B., 1975. t-Tests and intervals for comparisons suggested by the data. Biometrics 31, 339–359. Dwyer, J.P., Lowell, K.E., 1988. Long-term effects of thinning and pruning on the quality, quantity, and value of oak lumber. N. J. Appl. For. 5, 258–260. Graham, J.S., 1998. Thinning increases diameter growth of paper birch in the Susitna Valley, Alaska: 20 year results. N. J. Appl. For. 15, 113–115. Gregory, R.A., Haack, P.M., 1965. Growth and yield of wellstocked aspen and birch stands in Alaska. US For. Serv. Res. Pap. NOR-2, 12 pp. Harrington, C.A., Wierman, C.A., 1990. Growth and foliar nutrient response to fertilization and precommercial thinning in a coastal western red cedar stand. Can. J. For. Res. 20, 764–773. Hasenauer, H., Burkhart, H.E., Amateis, R.L., 1997. Basal area development in thinned and unthinned loblolly pine plantations. Can. J. For. Res. 27, 265–271. Hibbs, D.E., Emmingham, W.H., Bondi, M.C., 1989. Thinning red alder: effects of method and spacing. For. Sci. 35, 16–29. Hibbs, D.E., Emmingham, W.H., Bondi, M.C., 1995. Responses of red alder to thinning. West. J. Appl. For. 10, 17–23. Hocker, H.W., 1982. Effects of thinning on biomass growth of Populus tremuloides plots. Can. J. For. Res. 12, 731–737. Hoyle, M.C., 1984. Plantation birch: what works. what doesn’t. J. For. 82, 46–49. Johnstone, W.D., 1981. Precommercial thinning speeds growth and development of lodgepole pine: 25 year results. Inf. Rep. NORX-237. Can. For. Serv., North. For. Res. Centre, Edmonton, Alta. Karlsson, A., Albrektson, A., 2001. Height development of Betula and Salix species following pre-commercial thinning through breaking the tops of secondary stems: 3-year results. Forestry 74, 41–51. Kirk, R.E., 1982. Experimental Design. Wadsworth, Belmont, CA. Kozak, A., 1988. A variable exponent taper equation. Can. J. For. Res. 18, 1363–1368. LaBonte, G.A., Leso, R.J., 1990. Cleaning paper birch in a birchaspen stand in Maine: a 34-year case history. N. J. Appl. For. 7, 22–23. Lanner, R.L., 1985. On the sensitivity of height growth to spacing. For. Ecol. Manage. 13, 143–148. Leak, W.B., Solomon, D.S., 1997. Long-term growth of crop trees after release in northern hardwoods. N. J. Appl. For. 14, 147–151. Lloyd, D., Angove, K., Hope, G., Thompson, C., 1990. A Guide to Site Identification and Interpretation for the Kamloops Forest Region. Land Manage. Handbook No. 23. BC Ministry of Forests, Victoria, BC, pp. 127–223. Manning, G.H., 1975. British Columbia’s neglected hardwood resource. Report BC-X-118. Can. For. Serv., Pac. For. Centre, Victoria, BC, 18 pp.

178

S.W. Simard et al. / Forest Ecology and Management 190 (2004) 163–178

Marquis, D.A., 1969. Thinning in young northern hardwoods: 5-year results. USDA For. Serv. Res. Pap. NE-139, 22 pp. Massie, M.R.C., 1996. Hardwood utilization and supply in British Columbia. In: Comeau, P.G., Harper, G.J., Blache, M.P., Boateng, J.L., Thomas, K.D. (Eds.), Proceedings of the Workshop on Ecology and Management of British Columbia Hardwoods, FRDA Rep. 255, Richmond, BC, December 1–2, 1993. BC Ministry of Forests, pp. 1–7. McKone, M.J., Lively, C.M., 1993. Statistical analysis of experiments conducted at multiple sites. Oikos 67, 184–186. Medhurst, J.L., Beadle, C.L., Nielson, W.A., 2001. Early-age and later-age thinning affects growth, dominance, and intraspecific competition in Eucalyptus nitens plantations. Can. J. For. Res. 31, 187–197. Miller, G.W., 1997. Stand dynamics in 60-year-old Allegheny hardwoods after thinning. Can. J. For. Res. 27, 1645–1657. Mitchell, A.K., Barclay, H.J., Brix, H., Pollard, D.F.W., Benton, R., deJong, R., 1996. Biomass and nutrient element dynamics in Douglas-fir: effects of thinning and nitrogen fertilization over 18 years. Can. J. For. Res. 26, 376–388. Niemisto¨ , P., 1991. Growing density and thinning models for Betula pubescens stands on peatlands in northern Finland. Folia For. 782, 36. Niemisto¨ , P., 1995a. Influence of initial spacing and row-to-row distance on growth and yield of silver birch (Betula pendula). Scand. J. For. Res. 10, 245–255. Niemisto¨ , P., 1995b. Influence of initial spacing and row-to-row distance on the crown and branch properties and taper of silver birch (Betula pendula). Scand. J. For. Res. 10, 235–244. Nowak, C.A., 1996. Wood volume increment in thinned, 50- to 55year-old, mixed species Allegheny hardwoods. Can. J. For. Res. 26, 819–835. O’Hara, K.L., 1990. Twenty-eight years of thinning at several intensities in a high-site Douglas-fir stand in western Washington. West. J. Appl. For. 5, 37–40. Oliver, C.D., Larson, B.C., 1996. Forest Stand Dynamics. Updated Edition. Wiley, New York. Oliver, C.D., Murray, M.D., 1983. Stand structure, thinning prescriptions, and density indices in a Douglas-fir thinning study, western Washington, USA. Can. J. For. Res. 13, 126– 136. Perry, D.A., 1985. The competition process in forest stands. In: Cannell, M.G.R., Jackson, J.E. (Eds.), Attributes of Trees as Crop Plants. Institute of Terrestrial Ecology, Abbots Ripton, Hunts, England, pp. 481–506. Peterson, R.G., 1985. Design and Analysis of Experiments. Marcel Dekker, New York. Peterson, E.B., Peterson, N.M., Simard, S.W., Wang, J.R., 1997a. Paper Birch Managers’ Handbook for British Columbia. For. Can. and BC Ministry of Forests, Victoria, BC, 133 pp. Peterson, J.A., Seiler, J.R., Nowak, J., Ginn, S.E., Kreh, R.E., 1997b. Growth and physiological responses of young loblolly pine stands to thinning. For. Sci. 43, 529–534.

Pukkala, T., Miina, J., Kellomki, S., 1998. Response to different thinning intensities in young Pinus sylvestris. Scand. J. For. Res. 13, 141–150. Radtke, P.J., Burkhart, H.E., 1999. Basal area growth and crown closure in a loblolly pine spacing trail. Forest Sci. 45, 35–44. Roberge, M.R., 1988. Effects of thinning, patch clearcutting, site preparation, and planting on development of yellow birch in Quebec. N. J. Appl. For. 5, 248–251. Safford, L.O., 1983. Silvicultural guide for paper birch in the northeast (revised) (Betula papyrifera). USDA For. Serv., NE US Northeast For. Exp. Stn., Res. Pap. 535, 29 pp. Safford, L.O., Czapowskyj, M.M., 1986. Fertiliser stimulates growth and mortality in a young Populus–Betula stand: 10year results. Can. J. For. Res. 16, 807–813. Simard, S.W., 1990. A retrospective study of competition between paper birch and planted Douglas-fir. For. Can. and BC Ministry of Forests FRDA Report No. 147. Simard, S.W., Hannam, K.D., 2000. Effects of thinning overstory paper birch on survival and growth of interior spruce in British Columbia: implications for reforestation policy and biodiversity. For. Ecol. Manage. 129, 237–251. Simard, S.W., Heineman, J.L., 1996. Nine year response of Douglas-fir and the mixed hardwood–shrub complex to chemical and manual release treatments on an ICHmw2 site near Salmon Arm. FRDA Rep. 257. Simard, S.W., Vyse, A., 1992. Ecology and management of paper birch and black cottonwood in southern British Columbia. Land Manage. Rep. No. 75. BC Ministry of Forests, Victoria, BC. Simard, S.W., Heineman, J.L., Mather, W.J., Sachs, D.L., Vyse, A., 2001. Effects of Operational Brushing on Conifers and Plant Communities in the Southern Interior of British Columbia: Results from PROBE 1991–2000. Land Manage. Handbook No. 48. BC Ministry of Forests, Victoria, BC. Smith, D.M., Larson, B.C., Kelty, M.J., Ashton, P.M.S., 1997. The Practice of Silviculture: Applied Forest Ecology, 9th ed. Wiley, New York, NY, 537 pp. Soil Classification Working Group, 1998. The Canadian System of Soil Classification. Agric. and Agri-food Canada Publ. 1646 (Revised). Solomon, D.S., Leak, W.B., 1969. Stocking, growth and yield of birch stands. In: Birch Symposium Proceedings. N.E. For. Exp. Sta., US Dept. Agric., pp. 106–118. Strong, T.F., Erdemann, G.G., 2000. Effects of residual density on growth and volume production in even-aged red maple stands. Can. J. For. Res. 30, 372–378. Utunen, M., 1997. Growth response of paper birch crown to thinning in moist warm Interior Cedar-Hemlock-subzone of British Columbia. M.Sc. Thesis. University of Helsinki, Finland. Wang, J.R., Simard, S.W., Kimmins, J.P., 1995. Physiological responses of paper birch to thinning in British Columbia. For. Ecol. Manage. 73, 177–184. Waring, R.H., Schlesinger, W.H., 1985. Forest Ecosystems: Concepts and Management. Academic Press, Orlando, FL.