Survival, growth, and growth allocation of planted Scots pine trees after different levels of biomass removal in clear-felling

Forest Ecology and Management 177 (2003) 65±74

Survival, growth, and growth allocation of planted Scots pine trees after different levels of biomass removal in clear-felling Gustaf Egnell*, Erik Valinger Department of Silviculture, Swedish University of Agricultural Sciences, SE-901 83 UmeaÊ, Sweden Received 23 November 2001; received in revised form 25 April 2002; accepted 19 May 2002

Abstract A great concern in forestry today is whether whole-tree harvesting in¯uences site productivity and whether it is consistent with the principle of sustainable use of forest resources. To evaluate this a randomised ®eld experiment established 24 years ago in Scots pine (Pinus sylvestris L.) in southern Sweden was used. Treatments were conventional stem harvest (CH), whole-tree harvest (WTH), and branch and stem harvest (BSH). Seedling survival was unaffected by treatments. The total basal area over bark at breast height (1.3 m, m2 ha 1) was signi®cantly reduced following WTH from the 15th year after planting. Sample trees on CH plots produced 20% more wood biomass than WTH and BSH plots, while biomass produced within the crown was unaffected by treatment. Height growth was greater for sample trees on CH plots during the last year of measurement, while basal area and volume under bark were larger from the 12th year onwards when compared with the WTH treatment. BSH showed a decreased basal area growth under bark during the two 4-year-periods, 13±16 and 17±20 years after planting, and a decreased volume growth under bark from year 9 onwards in comparison with CH. Radial growth was increased for CH up to 3 m of the stems during the 9±12-year period and at 3 m during the 13±16-year period in comparison with the other two treatments. The implications of the results are discussed in terms of long-term site productivity and overall silvicultural outcome. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Production; Pinus sylvestris; Whole-tree harvest; Slash removal

1. Introduction As two-thirds of Sweden is covered with forests, wood fuel is a renewable energy source with a high potential. Today, residues from the forest industry are almost completely exploited for energy purposes. Thus, the potential increase is to be gained by removing logging residues from cutting operations, especially whole-tree harvesting (WTH), which is a *

Corresponding author. Tel.: ‡46-90-7865874; fax: ‡46-90-7867669. E-mail address: [email protected] (G. Egnell).

common practice in ®nal cuttings in southern and central Sweden. In WTH, more biomass is gained at the expense of a relatively larger removal of mineral nutrients from the site compared with other harvesting intensities (Boyle et al., 1973; MaÈlkoÈnen, 1976; Kimmins, 1977; Carey, 1980; Mann et al., 1988). A primary concern is therefore the in¯uence of WTH on site productivity, i.e. whether WTH is consistent with the principle of sustainable forestry. Budget analyses and models have been used to forecast such effects (Weetman and Webber, 1972; Freedman et al., 1981; Weetman and Algar, 1983; Freedman et al., 1986; Olsson et al.,

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 3 2 - 8

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1993). The budget and model studies are based on a mixture of empirical and assumed input and output levels of nutrients. This makes the results uncertain (Mann et al., 1988) and demonstrates the need for long-term ®eld experiments that provide processbased and empirical evidence of the effects of WTH on future site productivity. Following the energy crisis in the seventies a number of ®eld experiments were established in Sweden to study the effects of whole-tree harvesting on site productivity. In a previous paper Egnell and Leijon (1999), showed signi®cant negative effects of wholetree harvesting on height-growth of subsequently planted Norway spruce (Picea abies (L. Karst.) seedlings, whereas no such effect could be detected in Scots pine. However, on the more fertile of the two Scots pine (Pinus sylvestris L.) sites, basal area was reduced following WTH (Egnell and Leijon, 1999), indicating a treatment effect on stem taper and a negative effect of WTH on total above ground biomass production of single trees. Those results were achieved 15 years after planting. As the experiment had grown for another 9 years and it was time to thin the stand an opportunity to evaluate the longer-term effects was presented. Sample trees could be taken and used for stem analyses to estimate treatment effects on growth and total above-stump biomass production. In so doing, it would be possible to determine whether the effect on biomass production was still present (cf. Egnell and Leijon, 1999) or if the effects presented were transient.

The objective of the study was thus to establish the ®eld performance of planted Scots pine seedlings during the ®rst 24-year period of stand establishment following different levels of biomass removal after clear-cut harvesting. Seedling survival and basal area growth at stand level, and above stump biomass production, and biomass allocation at tree level, have been analysed. The results are discussed in terms of site and stand productivity, overall silvicultural outcome, and permanent versus temporary effects. 2. Material and methods 2.1. Experimental design The ®eld experiment was established in autumn 1975 when a naturally regenerated mixed forest of Scots pine and Norway spruce (305 m3 ha 1) was clear-cut near Kosta in the province of SmaÊland (568520 N, 158500 E, 240 m a.s.l.). The site was a mesic dwarf-shrub type of medium fertility, with an average precipitation of 600 mm per year and the soil was an orthic podzol. The experiment was arranged in a randomised block design, with three treatments (harvest intensities) and four blocks (Fig. 1). Assessment plots (20 m  20 m) were separated by 10 m wide buffer zones. The three levels of harvest intensities were: (1) conventional stem harvest (CH), removal of commercial stems to top diameter of 5 cm; (2) whole-tree

Fig. 1. Map showing the location and design of the experiment with three treatments replicated in four blocks (I±IV): conventional stem harvest (CH), in which only commercial stems with top diameter 5 cm were removed; whole-tree harvest (WTH), i.e. all above-stump biomass removed; and branch and stem harvest (BSH), i.e. needles left on site by removing the branches 1 year after harvest, when the needles had fallen.

G. Egnell, E. Valinger / Forest Ecology and Management 177 (2003) 65±74 Table 1 Estimated biomass (Mg ha 1) and nutrient (kg ha 1) removal for each treatment: conventional stem (CH) harvest; whole-tree harvest (WTH), all above-stump biomass removed; branch and stem harvest (BSH), needles left on site (From BjoÈrkroth and RoseÂn, 1977) Treatment

CH WTH BSH

Biomass

155 197 188

Nutrients N

P

K

Ca

Mg

106 352 211

10 35 22

50 158 93

141 278 227

25 49 38

harvest (WTH) removing all above-stump biomass; and (3) branch and stem harvest (BSH) removing the stems and branches of commercial trees when needles had fallen off, 1 year after harvest. Biomass left behind in treatments 1 and 3 was evenly distributed on the ground. Biomass and nutrient removal were estimated through stand speci®c allometric functions based on 8±10 sample trees per stand, on which biomass and nutrient content in stem bark and wood, living and dead branches, and needles were measured and registered (Table 1, BjoÈrkroth and RoseÂn, 1977). To avoid excessive soil disturbance while harvesting, logging machines operated in the buffer zones between the experimental plots during the winter. Scots pine seedlings of local provenance were planted in spring 1977 at the beginning of the second growing season following the harvest. The seedlings were planted in exposed mineral soil in the centre of manually scari®ed patches (40 cm  40 cm) at 1.7 m spacing (144 seedlings per assessment plot, i.e. 3 600 seedlings ha 1). To avoid damage by pine weevils (Hylobius abietis L.), the seedlings were pre-treated with the insecticide DDT. 2.2. Measurements In autumn 2000, 24 growing seasons (i.e. 24 years) after planting, all trees were callipered at breast height (DBH, 1.3 m). In total, an average of 98, 106, and 104 trees were measured in each block of CH, WTH, and BSH treatments, respectively. Damaged and dead trees were recorded. Based on calliper data, the diameter for the mean basal area per tree (db) was calculated for each plot

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using the formula: db ˆ Sb3 =Sb2 , where b is basal area at breast height for each tree. Three undamaged sample trees with a diameter equal or close to the diameter of the mean basal area per tree were randomly selected on each plot giving 36 stems that were felled for destructive measurements in 2000. Total tree height (0.01 m) was measured on every tree felled. Relative stem biomass was estimated by sampling of stem discs, 2 cm thick, at stump height (1% of tree height), breast height (1.3 m), and at every meter along the bole. The stem discs were weighed in the ®eld to the nearest 0.1 g using a mechanical ®eld scale. Crown biomass was estimated by sampling live and dead branches on the felled trees. From every whorl of branches one living branch was sampled and all branches were counted. To ensure that all directions of the crown were equally represented, the stem was divided into quadrants. The ®rst living branch encountered in a quadrant was sampled and weighed in the ®eld to the nearest g, using a mechanical ®eld scale. The quadrants were rotated clockwise through the crown. Remaining living branches were grouped and weighed in the ®eld (g). All dead branches in the crown were sampled and weighed in the ®eld to the nearest g. From the weighed dead branches a sub-sample of approximately 1 kg was collected. All collected sample material was stored in jute sacks at 5 8C for about 2 months, until further laboratory work was carried out. The diameter over and under bark was measured with a ruler in two perpendicular directions, rounded to the nearest millimetre. To obtain dry weight, the discs were dried at 85 8C for 48 h and weighed to the nearest 0.1 g. When weighed, the discs were smoothed with sandpaper to make annual ring width measurements possible. Thereafter, they were soaked in water for 48 h to swell and obtain original size (Eklund, 1951). The annual ring widths were measured along a transect (rounded to the nearest 0.001 mm) using the WinDENDRO software (Guay et al., 1992). The arithmetic mean of the two corresponding annual-ring widths was used in the further calculations to obtain radial growth and diameters under bark for the successive years 1977±2000. Branch material was dried at 70 8C for 24 h. After drying, the needles and shoots were separated. To obtain dry weight, the branch material was re-dried for a further 24 h at 70 8C. Dead branches were dried for 48 h at 70 8C. All branch material was weighed to the nearest 0.1 g. Thereafter,

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total dry weight of crown fractions was calculated according to Albrektson (1980), where the relation between dry and fresh weight of sample from each fraction was multiplied with total fresh weight of fraction. As fresh weight of the whole stem was not obtained in the ®eld, relative dry weight of wood and bark was obtained by adding dry weight of sample discs per tree. Volume under bark (Vc) for each tree was calculated through sectional measurements after Loetsch et al. (1973) n 1X Vc ˆ Li  …gb;i ‡ gt;i ‡ sqrt…gb;i  gt;i †† 3 iˆ1 where n is the number of sections, Li the length of section i, gb,i the basal area at the bottom of section i, and gt,i is the basal area at the top of section i. To reconstruct the volume for the years 1977±2000 for the material, the annual rings and height increment values were used. As no measurement of height to every whorl of branches was done, tree heights for the successive years were set to half the distance between the upper disc from the actual year and the next disc.

2.3. Statistical calculations Treatment effects on survival, and basal area growth over bark ha 1 after 24 years were analysed by using Tukey's studentized test on all main effect means (GLM, Anon., 1999). Dependent variables in the models were: survival rate (arcsin-transformed to meet the assumption of a normal distribution, p0 ˆ arcsin (p0.5)) and basal area over bark (m2 ha 1) at 1.3 m. Multiple pair wise comparisons between treatments on single trees to establish the effects of treatment on the dependent variables: dry weight of wood and bark, needles, shoot axes, and dead branches; and in radial, height, basal area, and volume under bark increments were made using Tukey's studentized test on all main effect means. The model used in all statistical analyses (GLM, SAS Institute Inc., 1989; Anon., 1999) was in the form Yijk ˆ m ‡ B ‡ Tij ‡ eijk where Yijk is the response in the dependent variable, m the overall mean effect, B the block effect, Tij the effect of treatment, and eijk are assumed independent, with random distribution. If P < 0:05, the result of the statistical analysis was regarded as signi®cant.

Fig. 2. Survival of planted Scots pine seedlings during the ®rst 24 years following different harvest intensities: conventional stem harvest (CH, open square), whole-tree harvest (WTH, ®lled square), and branch and stem harvest (BSH, ®lled up triangle).

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Fig. 3. Basal area development over bark (m2 ha 1) of planted Scots pine seedlings during the ®rst 24 years following different harvest intensities. For notations see Fig. 2. Treatments indicated with different letter differ signi®cantly (P < 0:05).

Fig. 4. Height growth (m) of sample trees per 4-year period. CH (solid line), WTH (dashed line), BSH (dotted line). Treatments indicated with different letter differ signi®cantly (P < 0:05).

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3. Results 3.1. Stand survival and basal area growth over bark

Table 2 Current dry weight of different fractions above stump after 24 years relative to control (%) Fraction

Relative weight of treatment (%)

Twenty-four years after planting, seedling survival was unaffected by treatment (P > 0:05, Fig. 2). Total basal area over bark (m2 ha 1) was signi®cantly reduced following WTH 15 (14.5%), 20 (16.3%), and 24 years (16.7%) after planting (P < 0:05, Fig. 3), compared with CH.

Wood, bark Needles Shoot axes Dead branches Total crown

3.2. Sample tree growth

Data in the same row followed by the same letters are not signi®cantly different (P > 0:05). See Table 1 for treatment de®nitions.

During the 24-year period, the CH-trees produced 20% more wood and bark biomass above stump (P < 0:05, Table 2) than the WTH- and BSH-trees. Although there was a consistent trend of approximately

10% less biomass in all evaluated crown fractions in WTH- and BSH-trees, no treatment effects could be detected.

CH 100 100 100 100 100

a a a a a

WTH

BSH

79 87 87 86 87

80 91 92 88 91

b a a a a

b a a a a

Fig. 5. Radial growth (mm) of sample trees per 4-year period at different heights within the bole. CH (solid line), WTH (dashed line), BSH (dotted line). Treatments indicated with different letter differ signi®cantly (P < 0:05).

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Fig. 6. Basal area growth under bark (m2) at DBH (1.3 m) of sample trees per 4-year period. For notations see Fig. 4. Treatments indicated with different letter differ signi®cantly (P < 0:05).

Fig. 7. Volume growth under bark (m3) of sample trees per 4-year period. For notations see Fig. 4. Treatments indicated with different letter differ signi®cantly (P < 0:05).

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Height of sample trees was unaffected by treatment until the last measurement, i.e. after 24 years, where CH-trees were 0.6 m higher than WTH-trees (P < 0:05, Fig. 4). Within the 24-year assessment of radial growth a decrease after WTH (P < 0:05, Fig. 5) was found up to 3 m height during the period 9±12 years after planting and at 3 m during the 13±16 year period. The growth pattern of radial growth indicated a shift upwards of maximum growth in the bole over each 4-year period. Basal area under bark at breast height (1.3 m) was signi®cantly reduced (P < 0:05) after BSH from the period 9±12 until 21±24, and for WTH from the period 13±16 to the very last period evaluated (Fig. 6). Volume under bark was lower for both WTH and BSH from the 9±12 year period and for the remaining study period (P < 0:05, Fig. 7). The stem form development was not signi®cantly affected by treatment (P > 0:05). 4. Discussion The present study was performed on a nitrogenlimited site in Sweden (cf. Tamm, 1991), where it is likely that the growth effects, to some extent, re¯ect harvest intensity in terms of harvested amount of nitrogen (cf. Table 1), rather than harvested amount of other nutrients. However, Olsson et al. (1996b) did not register any differences in total nitrogen or carbon store in the forest ¯oor and top mineral soil (0±20 cm) 15 years after planting at Kosta (coarse woody debris was not included in the study). They concluded that the pronounced effects of clear felling on soil nitrogen and carbon storage overshadowed the effects related to the harvest of branches and needles. Total pools of nitrogen give limited information on site and stand productivity, these being more closely related to available nitrogen pools and particularly to turnover rates of nitrogen from different pools (Davidson et al., 1992; Stark and Hart, 1997). Treatment effects after 24 years re¯ected the effects already observed after 15 years (Egnell and Leijon, 1999) with growth reductions in the subsequent tree crop following more intense harvest regimes. This indicates a negative effect on nitrogen pools available for the subsequent tree crop on this nitrogen-limited site (cf. Tamm, 1991). The higher survival rate following WTH and BSH, established after 15 years (Egnell and Leijon, 1999),

was not signi®cant after 24 years. This was due to a higher self-thinning rate between year 15 and 20 in WTH and BSH (cf. Fig. 2), particularly among the smallest trees. The action of self-thinning could have been avoided with an earlier thinning or a wider spacing of the stand, which is usually the case in practical Swedish forestry. A higher survival rate on WTH and BSH plots could then have counteracted some of the production losses following these treatments. After 15 years, basal area over bark was almost the same following BSH and CH, suggesting that leaving the needles on site would counteract the negative effects of extracting more of the woody biomass at ®nal-cut. There has been a drop, although not signi®cant, in basal area growth after year 15 in BSH compared with CH (cf. Fig. 3). Sample tree data further support this drop on BSH plots (cf. Fig. 7). This late treatment effect could be due to a delayed net-mineralisation of nitrogen from the coarse woody logging residues as suggested by HyvoÈnen et al. (2000). For the last study period (21±24 years), the CHtrees had increased their height in comparison to WTH. A possible reason for this late height-growth reaction is the late thinning as indicated by the uplifted green crown of the pines. Translocated nitrogen from the lower crown could have been used for height growth. This effect would be more pronounced in the stand with higher basal area (CH). At present, there is an approximately 20% decrease in wood and bark biomass growth per tree after WTH and BSH. The measurements showed a decrease in site index at the actual site according to HaÈgglund and Lundmark (1982) by 2 m. The initial site index 24 m (dominant height at age 100) reported by Egnell and Leijon (1999) for the experimental site is still valid for CH. For one rotation this reduction corresponds to a production loss of 0.8 m3 ha 1 per year in a fully stocked stand. As the lowest legal ®nal stand age at Kosta is 85 years (Anon., 1994), a permanent decrease in site productivity (site index) would correspond to a total loss of 68 m3 for the entire rotation period. In the worst scenario, this production loss means that the trees on sites where whole-tree harvesting has been performed must grow approximately 13 years longer before the same total production is achieved. If, on the other hand, all the negative effects on growth have

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been reached at this stage due to nitrogen de®ciency at seedling establishment and no more loss of volume growth can be expected, a loss of only 3 years volume growth will be the result (cf. Fig. 7). From the sample tree analyses, it could be seen that the effect on basal area and volume growth was mainly a result of higher radial growth during two of the studied periods, years 9±12 and 13±16. This indicates that the effect on site productivity has levelled out whereas stand productivity is still affected by differences in basal area (e.g. Barclay and Brix, 1985). Despite the increasing differences in height growth, basal area and volume growth under bark between CH and whole-tree harvesting, a loss of only 3 years volume growth is perhaps the most likely. Previously presented results from fertilisation experiments with nitrogen in thinning shows that (e.g. Brix, 1983; Barclay and Brix, 1985; Valinger et al., 2000) the duration of a fertilisation response is short-lasting. When the available nitrogen is utilized the ®rst response in the trees is a decreased needle growth followed by a reduced stem growth some years later Ê gren, 1991; Valinger et al., 2000). In (Ingestad and A the case of a nitrogen pool in branches and needles left after cutting, this response period could be somewhat prolonged due to delayed utilization of nutrients released from the coarse litter fraction (HyvoÈnen et al., 2000). HyvoÈnen et al. (2000) estimate that the response period can be longer than 15 years after clear-cutting, because the rate of nitrogen and phosphorus release is slower from woody logging residues than from needles. Another cause for a prolonged response period could be that a greater amount of nutrients is bound in the root systems of the cut trees due to their ability to compete initially with seedlings for available nutrients from decomposed needles and slash (Albrektson et al., 2000). Present data does not indicate whether the treatment effects on growth are temporary or permanent. It is therefore important to continue to follow this experiment further. The growth effects recorded during this period are most likely related to the amounts of nitrogen harvested on this nitrogen-limited site (cf. Tamm, 1991). In the longer term, with repeated whole-tree harvests, other nutrients may become limiting. After 15 years, Olsson et al. (1996a) showed a 16% lower base saturation in the humus layer following WTH at Kosta.

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This could be counteracted if the wood-ashes are brought back to the site, whereas nitrogen losses have to be replaced by adding a nitrogen fertiliser. When interpreting the results from this experiment for practical implications there are some important aspects to consider. In this experimental situation, almost 100% of available biomass above stump was harvested on WTH plots. Eriksson (1993) showed that only 70% of potential biomass was harvested during whole-tree harvesting of three spruce stands in southern Sweden. The biomass left on site was dominated by ®ne fractions with higher nutrient concentrations, thus, less than 70% of potential nutrients were harvested. Furthermore, the Swedish National Board of Forestry recommends that as much of the needles as possible are left on site when whole-tree harvesting is practiced. This implies that practical whole-tree harvesting is somewhere between the WTH (almost 100% harvested) and BSH of this study. 5. Conclusions WTH caused a production loss during the ®rst 24 years. Needles left in BSH helped maintain production during the ®rst 15 years. After that, there was also a tendency for productivity at BSH to drop compared with CH. The reason for this delayed response was probably a delayed net mineralisation of nitrogen from coarse woody logging residues. Data did not clearly indicate that the effect of logging residues on growth of the subsequent tree crop was about to level out after 24 years. It was, therefore, too early to conclude if the negative effect of removing logging residues was over. Most likely the reported loss in growth after wholetree harvest is less in a practical situation where some of the logging residues are left on site. A better management of the higher survival rate following the treatment, together with the possibility to replant earlier when logging residues are removed could further counteract production losses. Acknowledgements The Swedish National Energy Administration funded the study. Many people at the Department of Silviculture, Swedish University of Agricultural

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Sciences, have given valuable contribution to the study. We are particularly indebted to GoÈran BjoÈrkroth, who initiated this study, Rudolf Kollenmark who conducted the ®eldwork, Lena Walfridsson and Rune Johansson for the laborious work in the biomass laboratory. References Albrektson, A., 1980. Tallens biomassa; storlek-utveckling-uppskattningsmetoder. In: Biomass of Scots pine (Pinus sylvestris L.) Amount-Devlopment-Methods of Mensuration. Department of Silviculture, Swedish University of Agricultural Sciences, ISBN 91-576-0338-3. Albrektson, A., Valinger, E., Leijon, B., SjoÈgren, H., Sonesson, J., 2000. Fine root production and nitrogen content in roots of Pinus sylvestris L. after clear-felling. Scand. J. For. Res. 15, 188±193. Anon., 1994. SkogsvaÊrdslagenÐHandbook. Skogsstyrelsens foÈrlag, JoÈnkoÈping, ISBN 91-88462-11-0. Anon., 1999. SPSS for Windows, Rel. 10.0.5. 1999. SPSS Inc., Chicago. Barclay, H.J., Brix, H., 1985. Fertilization and thinning effects on a Douglas-®r ecosystem at Shawningan Lake: 12-year growth response. Can. For. Serv. Pac. For. Res. Cent. Inf. Rep. BC-X271, 34 pp. Boyle, J.R., Phillips, J.J., Ek, A.R., 1973. Whole-tree harvesting: nutrient budget evaluation. J. For. 71, 760±762. Brix, H., 1983. Effects of thinning and nitrogen fertilization on growth of Douglas-®r: relative contribution of foliage quantity and ef®ciency. Can. J. For. Res. 13, 167±175. Carey, M.L., 1980. Whole-tree harvesting in Sitka spruce. possibilities and implications. Irish For. 37, 48±63. Davidson, E.A., Hart, S.C., Firestone, M.K., 1992. Internal cycling of nitrate in soils of a mature coniferous forest. Ecology 73, 1148±1156. Egnell, G., Leijon, B., 1999. Survival and growth of planted seedlings of Pinus sylvestris and Picea abies after different levels of biomass removal in clear-felling. Scand. J. For. Res. 14, 303±311. Eklund, B., 1951. UndersoÈkningar oÈver krympnings-och svaÈllningsfoÈraÈndringar hos borrspaÊn av tall och gran (Investigations of shrinkage and swelling changes in increment cores of pine and spruce). Meddelanden fraÊn Statens SkogsfoÈrsoÈksanstalt 39, 7. Eriksson, L.-G., 1993. MaÈngd traÈdrester efter traÈdbraÈnsleskoÈrd. Rapport fraÊn Vattenfall Utveckling AB. Projekt Bioenergi 28, 0±27. Freedman, B., Duinker, P.N., Morash, R., 1986. Biomass and nutrients in Nova Scotian forests, and implications of intensive harvesting for future site productivity, and implications of intensive harvesting for future site productivity. For. Ecol. Manage. 15, 103±127.

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