Influence of gaps on some selected tree characteristics of edge trees in Norway spruce plantations

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Forest Ecology and Management 255 (2008) 2643–2649 www.elsevier.com/locate/foreco

Influence of gaps on some selected tree characteristics of edge trees in Norway spruce plantations Oriana Pfister a,*, Urban Nilsson a, Pelle Gemmel b a

Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, Box 49, SE-230 53 Alnarp, Sweden b SCA Forest Products AB, SE-851 88 Sundsvall, Sweden Received 11 July 2007; received in revised form 9 January 2008; accepted 10 January 2008

Abstract The present study examined branch and stem characteristics of trees growing around gaps in Norway spruce plantations. Trees located at the edge of gaps with a radius of either 5 m or 7 m either unplanted or with supplementary planting were compared to trees in the original closed plantation. The experiment was carried out in two locations in the south of Sweden and the design included four blocks at each location and one replicate of each treatment in each block. The measurements were carried out on standing and felled trees in 2005 at the time of the first thinning. The results indicate a significant effect of gaps and supplementary planting on the diameter of the largest branch, the number of living branches close to breast height, branch basal area, height to the first living branch and taper. The effect of supplementary planting on branch and stem characteristics was greater in large gaps than in small ones. This study indicated that supplementary planting may be useful in affecting stem-form and branch parameters, although other studies have shown that the trees that result from supplementary planting contribute little to total production. # 2008 Elsevier B.V. All rights reserved. Keywords: Picea abies; Competition; Replacement planting; Supplementary planting; Wood quality

1. Introduction In Sweden, during the years 2002–2004, about 20 million seedlings of Norway spruce were planted each year to replace dead seedlings (Anon., 2007). The Swedish Forestry Act requires supplementary planting when the number of seedlings is insufficient to guarantee a uniform and satisfactory stand density (Anon., 2001). According to the Forestry Act, supplementary planting should be carried out within 1–5 years of the time that the plantation was originally established, in order to meet the main goal of sufficient volume production. However, the replacement of dead seedlings seems to contribute very little to the volume of the stand (Wakely, 1968; Gemmel and Nilsson, 1990). Gemmel and Nilsson (1990) studied early competition in the same gap experiment examined in this paper, and showed that seedlings from supplementary planting were not as tall as those planted originally. Some years later the same authors, Nilsson and Gemmel (2007), examining the same experiment, observed that the volume growth

* Corresponding author. Tel.: +46 40 41 51 17; fax: +46 40 46 23 25. E-mail address: [email protected] (O. Pfister). 0378-1127/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.01.024

(m3 ha1) of supplementary seedlings was lower than that of the original seedlings. Moreover, they noticed that trees located close to the gaps that had not been replanted had a higher volume growth than trees in the closed stands and this additional volume almost compensated for the loss of stem volume production as a result of gaps. Trees at the edge of gaps grow faster than trees in closed stands (Gemmel and Nilsson, 1990; Nilsson and Gemmel, 2007). Gemmel (1988) showed that 6 years after planting, the height and diameter of seedlings at the edge of gaps was larger than that of trees in the closed stand; trees at the edge of gaps still had a larger diameter and higher volume than trees in closed stands at the time of the first thinning (Nilsson and Gemmel, 2007). Growth rate determines the quality of wood and, in general, fast growing trees have lower wood quality than slow growing ones (Johansson, 1992; Perstorper et al., 1995; Pape, 1999a). Several characteristics are influenced by an acceleration in growth, irrespective of whether it is caused by a variation in stand density or by an artificial increase in the availability of water and nutrients. Wood density, which is one of the most studied and discussed properties, decreases with accelerating tree growth, since it is negatively correlated to annual ring width (Olesen,

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1976; Lindstro¨m, 1996; Pape, 1999b; Ma¨kinen et al., 2002). A decrease in wood density causes lower yields of pulp and paper and reduces strength of boards (Lundgren, 2003). Low wood density is also found in juvenile wood (Olesen, 1977; Yang, 1987). Juvenile wood is produced in the first rings nearest to the pith, and it is inferior in quality compared to mature wood (Yang, 1987; Danborg, 1994). Thus, fast growth in the early years will increase juvenile wood content and decrease the quality of the trees. It is well established that branch diameter influences both the appearance and strength of wood and is positively correlated with the diameter of the tree at breast height (Johansson, 1992; Vestøl et al., 1999; Pfister et al., 2007); in addition, large trees have thicker branches than small ones (Nylinder, 1959). Several studies on initial spacing of trees have demonstrated that fast growing trees develop thicker branches than slow growing trees (e.g. Johansson, 1992; Ma¨kinen and Hein, 2006). Moreover, the height to the first living branch tends to decrease with decreasing competition (Ma¨kinen and Hein, 2006). Living branches continue to grow as the stem grows and their presence in the lower part of the stem decreases its quality. However, some end-use products require living knots and a slow increase in crown height can be valuable. With regard to the form of the bole, contradictory results have been reported often because of different methods of measurement or classification. Agestam et al. (1998) found small differences in stem taper between planted and naturally regenerated stands whereas Nylinder (1959), Johansson (1992) and Deans and Milne (1999) reported increased taper with increasing spacing. Moreover, contradictory results have been found with respect to fertilization regime. It has been shown that an increase in nitrogen supply increases taper (Mead and Tamm, 1988). Wiklund et al. (1995) found that although taper decreased with irrigation, fertilization had no effect. According to Johansson (1992), crookedness is not influenced by initial spacing. In contrast, Nylinder (1959), Prescher and Sta˚hl (1986) and Pfister et al. (2007) found that trees tend to be somewhat straighter in densely spaced stands than in more open ones. Competition has a strong impact on the development of stands (Mithen et al., 1984; Pettersson, 1992; Nilsson, 1993)

since it determines growth (Hamilton, 1969; Ku¨ppers, 1985; Gemmel and Nilsson, 1990; Siipilheto, 2006) and quality characteristics of trees (Johansson, 1992; Brazier and Mobbs, 1993; Nilsson and Gemmel, 1993; Agestam et al., 1998; Watt et al., 2005). Mithen et al. (1984) demonstrated that plant growth seems to be influenced by the ‘‘available area’’ during the first phase of growth. For this reason the replacement of dead seedlings at an early age can guarantee the competition level necessary to improve the quality of the stand. Thus, because of its positive effect on quality, replacement planting may be an economically sound silvicultural treatment even if the effect on volume production is limited. The purpose of the present study was to investigate the effects of gaps on the quality characteristics of edge trees. Gaps of different sizes and gaps with supplementary planting were examined. Two hypotheses were tested: (1) gaps cause increased branch size and taper in edge trees; (2) supplementary planting significantly reduce branch size and taper in edge trees. 2. Materials and methods 2.1. Experimental design The data for this study were collected from a replacement planting experiment established in 1978 at two sites in southern Sweden. Originally the experiment was designed to study the effects of competition on yield and growth in stands of Norway spruce where supplementary planting had been employed (Gemmel, 1984) (Fig. 1). The Ullasjo¨ site (568460 N, 138060 E) was originally planted in the spring of 1972 with 3-year-old Norway spruce seedlings (2/1), from Czechoslovakia (Gemmel, 1988). Seedlings of the same species, provenance and age, were planted in the spring of 1976 at Kna¨red (568310 N, 138150 E). The experiment was established in spring 1978 by clearing circular plots of a radius of either 7 m or 5 m, then either replanting with 3-year-old (2/1) Norway spruce seedlings or leaving the gap unplanted. At that time, the density of the stand was 2500 seedlings ha1 at Ullasjo¨ and 2700 seedling ha1 at Kna¨red (Gemmel, 1988). More details about the experiment can be found in Gemmel (1984) and Nilsson and Gemmel (2007).

Fig. 1. Experimental design from Nilsson and Gemmel (2007) modified. Small gap of 5 m radius; large gap of 7 m radius; control plot of 7 m radius. Shaded area around small and large gaps indicates the area from which felled sample trees were selected.

O. Pfister et al. / Forest Ecology and Management 255 (2008) 2643–2649

This study included five treatments (Fig. 1): gaps of a radius of 5 m and 7 m either left unplanted (in this study referred to as small and large gaps, respectively) or supplementary planted with Norway spruce (in this study referred to as small supplementary planted and large supplementary planted gaps, respectively), and plots with a radius of 7 m radii established in the original closed plantation to act as a reference (in this study referred as control plots). On each site, the experiment was arranged in four blocks and each treatment had one replicate per block. Thus, data were collected from eight circular plots for each treatment.

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where m is the overall mean, CK is the treatment effect and ai and bj are the random effects for site and block. Differences between treatment least-square means were evaluated using Tukey–Kramer significant mean separation test. In addition, regression functions using the diameter of the largest branch as the dependent variable and DBH and indicator variables for treatments were developed. Three regression functions were developed. Model 1 had indicator variables for intercept, model 2 had indicator variables for slope and model 3 had indicator variables for both slope and intercept. The regression models were:

2.2. Measurements and calculations BDmax ¼ a þ bDBH þ cI LG þ dI SG þ eI LGSP Data were gathered 1 year after thinning, in the winter of 2005, from sample trees located at the edges of the artificially created gaps. Standing sample trees (264) and felled sample trees (80) were selected and marked before thinning. The selection of sample trees was based on the diameter distribution before thinning of the trees standing within an area 2 m from the edge of the gaps (Fig. 1). In order to represent the entire diameter distribution, sample trees were randomly selected within each quartile of this distribution. For standing trees the following were recorded: diameter at breast height (DBH), diameter of the thickest dead and living branch in each of the three whorls nearest to breast height and the number of branches in the same three whorls divided into 5 mm diameter classes. Dead and living branches were recorded separately. Sixteen sample trees from each treatment were selected for destructive measurements (felled sample trees). DBH, diameter every second meter from 4 m above the ground, the height of the trees and the height to the first living branch were recorded. The number of branches in each whorl between 1.3 m and 8 m up the stem were measured and divided into diameter classes of 5 mm. For both standing and felled sample trees, the diameter of branches was measured parallel to the longitudinal axis of the trees. The basal area of branches was calculated by assigning each branch the mean value of the diameter class. Subsequently, the trunks were cross cut into two logs, each 4 m in length, and the crookedness of each log was recorded by measuring the maximum distance that the centre of the log deviated, horizontally, from a straight line joining the pith at the bottom to the pith at the top, here called deviation. The crookedness index was calculated as the ratio, as a percentage, of this deviation to the length of the log. Taper was calculated as the difference between the stem diameter at 1.3 m and at 4 m divided by the distance on the stem between these two measurements. 2.3. Statistical analyses Statistical significance of the difference between the treatments was analysed using mixed effects model (MIXED procedure of SAS, SAS Institute Inc., Cary, NC, USA) including random site and block effects. The model used to test the treatment effects was: Y i jK ¼ m þ ai þ b j þ CK þ ei jK

þ fI SGSP

ðmodel 1Þ

BDmax ¼ a þ bDBH þ cI LG DBH þ dI SG DBH þ eI LGSP DBH þ fI SGSP DBH

ðmodel 2Þ

BDmax ¼ a þ bDBH þ cI LG þ dI SG þ eI LGSP þ fI SGSP þ gI LG DBH þ hI SG DBH þ iI LGSP DBH þ jI SGSP DBH

ðmodel 3Þ

where BDmax was diameter of the largest branch (mm); DBH was diameter at breast height (cm); ILG, ISG, ILGSP, ISGSP were indicator variables (0, 1) for the treatments large gap (LG), small gap (SG), large supplementary planted gap (LGSP) and small supplementary planted gap (SGSP). Finally, a, b, c, d, e, f, g, h, i and j were regression coefficients. The regression analysis showed that the coefficients for both intercept and slope were significantly different from zero when analysed separately. However, if slope and intercept were considered together, their coefficients were not different from zero. In order to determine, for the intercept, the magnitude of difference that would be significant, a sequential test of differences between regression lines was conducted for increasing values of DBH. 3. Results Trees at the edge of large gaps had a significantly larger DBH than trees in all other treatments; there was no significant difference between DBH in small supplementary planted gaps and control plots (Table 1). The average diameter of the thickest branch at breast height was significantly larger for trees at the edge of large gaps than for trees in all other treatments; there was no significant difference between small supplementary planted gaps and control plots (Table 1). For all treatments, the size of the trees was positively correlated with the size of the thickest branch in the three whorls nearest to breast height (Fig. 2). A regression analysis with diameter of the thickest branch as dependent variable and DBH and indicator variables for treatments as independent variables showed that trees at the edge of large gaps had a significantly larger branch diameter than trees in the control plots, when DBH exceeded 9 cm

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Table 1 Characteristics of standing sample trees

DBH (cm) Thickest branch (mm) at breast height No. of branches at breast height No. of living branches at breast height

Large gap

Small gap

Large SP gap

Small SP gap

Control

P-value

18.0a 24.1a 21.5a 3.0a

15.6b 20.1b 20.9a 2.1a

15.0b 18.9b 22.1a 0.04b

14.6bc 17.9bc 20.4a 0.2b

12.4c 14.9c 19.3a 0b

0.0001 0.0001 0.1373 0.0001

SP = supplementary planted. Least-square means on the same line with the same letters are not significantly different at the 0.05 probability level.

(Fig. 2; Table 2). Compared to trees in control plots, trees at the edge of small gaps had a significantly larger branch diameter when their DBH exceeded 12 cm. Also, compared to the control plots, trees at the edge of large supplementary planted gaps had a significantly larger branch diameter when their DBH was more than 14 cm. The diameter of the largest branch of trees at the edge of small supplementary planted gaps and control plots were not statistically significantly different (Fig. 2; Table 2). The total number of branches in the three whorls nearest to breast height was similar among treatments but the number of living branches varied significantly (Table 1). No living branches at breast height were found in the control plots; however, the number of branches was significantly different from zero for trees at the edge of large and small gaps (Table 1). The number of branches per meter between 1.3 m and 8 m was similar between treatments (Table 3). However, the frequency distribution of branch size varied between treatments (Fig. 3). In the control plots the frequency distribution was concentrated on the smallest size-classes whereas trees at the edge of large gaps had, relative to other treatments, high frequencies in the large size-classes (Fig. 3). The basal area of all the branches between 1.3 m and 8 m up the stem was largest in trees at the edge of large gaps; this was the case for both the first and the second logs. These values were significantly different from the other treatments (Fig. 4).

Fig. 2. Relationship between diameter at breast height and thickest branch in the three whorls nearest breast height. Each line was calculated using a regression function with DBH and indicator variables for slope and intercept as independent variables (cf. Table 2, model 3). Lines for large SP gap and small gap are overlapping. SP = supplementary planted.

The average height to the first living branch was significantly greater in trees in the control plots than in the other treatments (Table 3). Trees standing at the edge of gaps exhibited a greater taper than trees in the control plots, while there was no significant difference in taper between supplementary planted gaps and control plots (Table 3). Crookedness was not significantly different between treatments (Table 3). 4. Discussion A number of indicators of future wood quality (diameter of the largest branch, branch basal area, height to first living branch, number of living branches near breast height and taper) were significantly affected by gaps and supplementary planting. Thus, this study indicates that the expected wood quality in terms of branch size and taper may be lower for trees close to gaps than for trees within the original closed plantation and that these parameters are improved by supplementary planting. The two hypotheses that branch size and taper will be increased by gaps and decreased by supplementary planting could not be rejected. The positive correlation between DBH and the thickest branch in the three whorls around breast height was consistent with several previous studies (e.g. Johansson, 1992; Salminen and Varmola, 1993; Klang, 2000; Pfister et al., 2007). Since tree size was affected by the presence of gaps, part of the larger branch diameter for trees at the edge of gaps can be explained by the correlation between tree size and branch diameter. However, there was an additional effect of the treatments on the size of the thickest branch that was not explained by tree size. The diameter of the thickest branch of trees growing at the edge of gaps was significantly greater than that of trees of the same size in the control plots. Several studies of tree spacing have also shown a similar treatment effect, but this effect is often of minor importance (Johansson, 1992; Vestøl et al., 1999; Pfister et al., 2007). The difference between spacing experiments and this study may be the spatial arrangement of trees: here trees at the edge of unplanted gaps had almost unrestricted growing space in one direction. Supplementary planting had a greater effect on branch size in large gaps than in small gaps. This was firstly because branch size was lower for trees next to small gaps than for trees near large gaps so the potential for reducing the size of branches by supplementary planting was less. Another possible cause could be differences in the size of the seedlings used for supplementary planting. Nilsson and Gemmel (2007) demonstrated that the growth of seedlings used in supplementary

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Table 2 Parameter estimates and summary statistics for diameter of the largest branch regressions with and without indicator variables for intercept and slope as independent variables Model 1. With indicator variables for intercept

Model 2. With indicator variables for slope

Model 3. With indicator variables for slope and intercept

Coeffa

Coeff

Coeff

S.E.b

P-value

4.406 0.855

0.906 0.059

<0.0001 <0.0001

Indicator variable of intercept: Large gap 4.42 Small gap 1.975 Large SP gap 2.622 Small SP gap 0.952

0.814 0.791 0.854 0.946

<0.0001 0.00132 0.0021 0.315

Intercept DBH

6.586 0.682

Indicator variable of slope: Large gap Small gap Large SP gap Small SP gap MSE CV (function) R2 n

0.297 0.156 0.195 0.104

17.43 21.37 0.587 263

S.E.

P-value

0.917 0.081

<0.0001 <0.0001

0.054 0.057 0.059 0.066

<0.0001 0.0067 0.0012 0.1169

17.39 21.34 0.587 263

S.E.

P-value

5.871 0.735

1.778 0.138

0.0011 <0.0001

1.179 1.407 1.997 2.551

2.627 2.832 2.604 3.383

0.6539 0.6196 0.4441 0.4516

0.219 0.048 0.067 0.259

0.173 0.201 0.181 0.234

0.2069 0.7714 0.7122 0.2706

17.52 21.43 0.584 263

SP = supplementary planted. a Coefficient. b Standard error. Table 3 Characteristics of felled sample trees

No. branches per meter Height to the first living branch (m) Taper (mm/m) Crookedness index log 1 (%) Crookedness index log 2 (%)

Large gap

Small gap

Large SP gap

Small SP gap

Control

P-value

12.6a 1.95c 9.4a 0.42a 0.32a

12.1a 2.46bc 9.6a 0.73a 0.20a

11.8a 3.66b 7.9ab 0.51a 0.19a

12.3a 3.95b 8.1ab 0.76a 0.44a

12.6a 5.47a 6.0b 0.76a 0.27a

0.8789 0.0001 0.0042 0.0628 0.0878

SP = supplementary planted. Least-square means on the same line with the same letters are not significantly different at the 0.05 probability level.

planting was lower in small gaps than in large ones; the effect of the smaller supplementary planted seedlings on the branch size of the surrounding originally planted trees may have been lower. The height to the first living branch was lower for trees at the edge of gaps than for trees in the other treatments. Since living branches will continue to increase in size, the present differences in branch size between trees at the edge of gaps and other treatments will probably continue to increase. In addition, it will be difficult to remove trees with poor branch quality and large taper at the edge of gaps in future thinning operations, since cutting these trees will increase the size of the gap. Therefore, it is likely that trees with large branch size and large taper at the edge of gaps will remain until the time of final felling, while trees with similar branch and stem characteristics in the closed stand are likely to be removed during thinning operations. Because of this, the difference in average branch and stem-form characteristics at the time of final felling between trees at the edge of gaps and trees in a closed stand may be even greater than was found in this study. Taper was significantly greater in trees at the edge of gaps compared to trees in control plots. This can be explained by a

Fig. 3. Total number of branches on the tree divided into branch diameter class. One standard error of the mean is indicated by vertical bars. SP = supplementary planted.

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by the stabilizing effect of close surrounding trees (Høibø, 1991; Kairiu¯ksˇtis and Malinauskas, 2001), although Johansson (1992) found no effect of increased spacing on crookedness in Norway spruce. Our thought was, therefore, that crookedness should be greater for trees next to gaps than for trees in the closed stand. The crookedness index, however, was not significantly different between treatments. We have no good explanation for this unexpected result but, nevertheless, it indicates that crookedness may not be an important factor to consider with respect to trees at the edge of gaps. In conclusion, this study showed that it is possible to affect branch and stem-form characteristics by supplementary planting and that the effect is greater in larger gaps. Therefore, supplementary planting may be driven by the desire for decreased branch size and taper even though earlier studies (e.g. Nilsson and Gemmel, 2007) have shown that the contribution of supplementary planting to stand volume production is low. However, supplementary planting in small gaps is not recommended for reasons of either production or improvement of branch and stem-form characteristics. References

Fig. 4. Mean branch basal area per treatment in the first and second logs. For the calculation, the mean value of the diameter class was assigned to all branches in each class. Columns with the same letters have means that are not significantly different from each other.

difference in growth allocation caused by treatment-induced differences in resource availability and environmental conditions within the stand. Taper is affected, amongst other things, by the air movement within a stand (Larson, 1963; Valinger, 1992). Trees exposed to more windy conditions exhibit more taper than those that are more protected and trees at the edge of the gaps are likely to have been exposed to windier conditions than trees in control plots. Furthermore, taper is greater in the living crown than below the first living branch (Edgren and Nylinder, 1949). Since the height of the first living branch was lower for trees at the edge of gaps than for trees in control plots, increased taper within the living crown may partly explain the observed difference. A higher crookedness index has been found in more widely spaced stands compared to denser ones; this has been explained

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