Height growth of planted conifer seedlings in relation to solar radiation and position in Scots pine shelterwood

Forest Ecology and Management 224 (2006) 258–265 www.elsevier.com/locate/foreco

Height growth of planted conifer seedlings in relation to solar radiation and position in Scots pine shelterwood Martin Strand a,*, Mikaell Ottosson Lo¨fvenius b, Urban Bergsten c, Tomas Lundmark d, Ola Rosvall e a

Umea˚ Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, SE-901 83 Umea˚, Sweden b Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea˚, Sweden c Department of Silviculture, Swedish University of Agricultural Sciences, SE-901 83 Umea˚, Sweden d Vindeln Experimental Forests, Swedish University of Agricultural Sciences, Svartberget Field Station, SE-922 91 Vindeln, Sweden e Forestry Research Institute of Sweden, Box 3, SE-918 21 Sa¨var, Sweden

Abstract Seedlings of different provenances of Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl., var. latifolia Engelm.) and Norway spruce (Picea abies (L.) Karst.) were planted in three Scots pine shelterwoods (125, 65 and 43 stems ha
1) and a clear-cut, all in northern Sweden. The sites were mounded and planting took place during 2 consecutive years (1988 and 1989). The solar radiation experienced by the individual seedlings was determined using a simulation model. Height development of the seedlings was examined during their first 6 years after planting. During the final 3 years of the study, height growth of Norway spruce was relatively poor, both in the shelterwoods and the clear-cut area. Height growth of lodgepole pine was significantly greater than that of Scots pine, both in the shelterwoods and the clear-cut. In contrast to Norway spruce, Scots pine and lodgepole pine displayed significantly greater height growth in the clear-cut than in the shelterwoods. For all three species in the shelterwoods, regression analyses showed that height growth was more strongly correlated with the distance to the nearest tree than with the amount of radiation reaching the ground, i.e. growth was reduced in the vicinity of shelter trees. Therefore, we conclude that the significant reduction in height growth of seedlings of Scots pine and lodgepole pine in Scots pine shelterwoods was partially caused by factors associated with the distance to the nearest shelter tree. Because the substrate was a nitrogen-poor sandy soil, we suggest that root competition for mineral nutrients, especially nitrogen, accounts for the reduction in height growth. # 2006 Elsevier B.V. All rights reserved. Keywords: Shelterwood; Solar radiation; Height growth; Scots pine; Lodgepole pine; Norway spruce

1. Introduction Management of boreal forests involves various activities, including thinning and selective cutting. This creates a wide variation in light levels reaching the ground, depending on the density of the retained trees and their size and spatial distribution. Light is of fundamental importance for plants and several studies have characterized the spatial and temporal variation in solar radiation beneath the canopy by means of measurements and/or modelling (Reifsnyder et al., 1971; Pukkala et al., 1991, 1993; Cescatti, 1997a,b; Martens et al., 2000; Gendron et al., 2001). In a comprehensive review, Lieffers et al. (1999) discussed state-of-the-art models with

* Corresponding author. Tel.: +46 90 7868265; fax: +46 90 7868165. E-mail address: [email protected] (M. Strand). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.12.038

respect to the dynamics of solar radiation in northern boreal forests. The growth responses of naturally regenerated and planted conifer seedlings to light availability have also been extensively investigated (e.g., Klinka et al., 1992; Chen et al., 1996; Chen, 1997; Wright et al., 1998; Coates and Burton, 1999; Williams et al., 1999). Generally in these studies, height growth was found to increase with increasing light availability during the growing season. Shade-intolerant species tend to have higher growth rates at high light levels, but a weaker response to an increase in light at low light levels compared to shade-tolerant species (e.g., Wright et al., 1998; Coates and Burton, 1999; de Chantal et al., 2003). However, the growth of tree seedlings in silvicultural systems that retain partial forest canopy cover can be simultaneously limited by a number of resources in addition to light, including soil moisture and mineral nutrients (e.g., Canham et al., 1996; Palik et al., 1997; Lajzerowicz et al., 2004).

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One of the objectives of this study was to examine whether the height growth of planted seedlings of three conifer species was related to the amount of radiation experienced by the seedlings. To this end, the global radiation near the ground in three shelterwoods with different stem densities was determined by measurements and simulations. Differences between the species in their response to light availability were expected because Scots pine (Pinus sylvestris L.) and lodgepole pine (Pinus contorta Dougl., var. latifolia Engelm.) are shadeintolerant species whereas Norway spruce (Picea abies (L.) Karst.) is a shade-tolerant species. In this study, the shelterwood trees were growing on nitrogen-poor sandy soils. Therefore, the growth responses to light were likely to be affected by the availability of mineral nutrients, especially nitrogen (e.g., Drever and Lertzman, 2001). If competition for mineral nutrients and water is symmetrically distributed around the trees, we would expect height growth of understorey seedlings to be correlated with the distance to the nearest tree. Indeed, a number of studies have found that height growth of understorey seedlings is reduced in the vicinity of large trees in Scots pine forests (Hagner, 1962; Kuuluvainen et al., 1993; Niemisto¨ et al., 1993; Skoklefald, 1995; Valkonen et al., 2002). Therefore, we also examined whether height growth was related to the distance to the nearest tree. Modifying forest harvest practices could provide a way to enhance profits from land uses other than traditional forestry, e.g., reindeer production, with relatively low timber opportunity costs (cf. Bostedt et al., 2003). The optimum conditions for the required tree species with respect to light, for example, could possibly be met by identifying appropriate shelterwood densities and gap sizes. 2. Material and methods 2.1. Experimental site and plant material The field study examined three pine shelterwoods, each 1 ha in area, and a clear-cut area of 1.8 ha, all located on a wide plateau at an altitude of approximately 173 m within the Svartberget Experimental Forest (648140 N, 198490 E), Vindeln, Sweden. The surrounding hills reach altitudes of about 300 m, and glades and clear-felled areas of the pine heath vegetation are exposed to summer frosts. The soil is podzolised sandy silt, classified by Giesler et al. (2000) as a Haplic Arenosol (FAO, 1988). The organic layer is thin (2–4 cm) and the ground water level is estimated to be several metres below the surface. The soil conditions and the ground vegetation in the three shelterwoods and the clear-cut area were very similar during the period of this study. The shelterwood site was established in 1982 in a 150-year-old (1987) stand of Scots pine trees with a height of ca 20 m. During the winter of 1983–1984, three plots of about 1 ha each were evenly thinned to densities of 138, 91 and 48 stems/ha. In 1985, the stem densities were reduced by windfelling to 125, 65 and 43 stems/ha. Shelterwood tree height, stem diameter at 1.5 m and geometric positions were determined using a theodolite equipped for electronic distance measurements (Wild Co.). The shelterwood plots are located next to each other in a row (Fig. 1),

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bordered on three sides by the undisturbed forest with a stem density of about 380 stems/ha (the expected density of a mature pine stand at this site). The northern side is adjacent to an area of forest regeneration with seed trees. The middle shelterwood is separated from the other two shelterwoods by 10–20 m strips of undisturbed forest. Undisturbed forest surrounds the clear-cut area on all sides. Seedlings were planted in early summer 1988 and 1989 after mounding in autumn 1987. Three provenances of Scots pine, two of lodgepole pine, and two of Norway spruce were used in this study (Table 1). Each provenance was assigned to four rows (two of which were planted in the first year and two in the second; Fig. 1), each containing eight plants, in each shelterwood and eight rows (four of which were planted in the first year and four in the second) in the clear-cut area. The position of the rows of each provenance was randomised among the 36 possible positions in each shelterwood, and the same randomisation procedure was applied to the rows of each provenance in the clear-cut area. The total height of the seedlings was measured annually from 1990 to 1994, and again in 1996, after the end of the growing season. Only seedlings with no recorded visible damage in the yearly inventories from 1988 to 1996 were included in this study. 2.2. Simulation of global radiation A computer model was developed to simulate the global radiation in the shelterwoods. Measurements of global radiation near the ground during two growing seasons (1985 and 1986) were used to evaluate the validity of the model. In total, 12 solarimeters (type CM-5, Kipp and Zonen, Delft, the Netherlands, waveband 0.31–2.8 mm), four in each of the three shelterwoods, were located one metre above the ground. The daily irradiance at each sensor position was derived from 10 min mean values (with 6-s sampling intervals). To compute the global radiation reaching the surface of the earth, the model calculates the extraterrestrial solar radiation, takes the atmospheric attenuation into account, and determines the diffuse sky radiation. The sun altitude alone was used to calculate the clearsky diffuse component of the global radiation. Based on date, time and latitude, the model calculated the clear-sky direct and diffuse radiation above the shelterwood canopy. A subroutine simulated the tree shading and estimated the daily irradiance at any point on the shelterwood floor. The short-wave radiation at the upper limit of the atmosphere varies somewhat as a result of sun spot activity and variations in the Earth–Sun distance. Considering the annual variation in Earth–Sun distance, several methods for calculating the extraterrestrial radiation are available in the literature (Perttu et al., 1980; Mohammad and Pradeep, 1987). In this study, the model estimates the solar constant (I0) on a given day using the formula: I0 ¼ 1353 þ 45:326 cos ðdn cÞ þ 0:88018 cos ðdn 2cÞ
0:00461 cos ðdn 3cÞ þ 1:8037 sin ðdn cÞ þ 0:09746 sin ðdn 2cÞ þ 0:19412 sin ðdn 3cÞ

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Fig. 1. Above: shelter trees and seedling positions in the three shelterwood plots, each approximately 1 ha. The shelter trees are marked with grey dots and the plant positions are marked by their identification numbers (1, 2 and 4, Scots pine; 5 and 6, lodgepole pine; 9 and 10, Norway spruce). Below: simulated clear-sky irradiance reaching the forest floor of the shelterwood plots on 1 July 1996.

where dn = Julian day + 0.5 and c = 2p/366 for the leap year 1996. The computation of the sun’s altitude and azimuth, at a given local apparent solar time, follows the spherical trigonometry presented by Oke (1987). Global radiation (G) reaching the earth’s surface in clear sky conditions was determined by the expression: G ¼ ðI0 e
b=sinðhÞ Þ sin ðhÞ þ D where e is the base of natural logarithms; b, a constant that represents the total atmospheric extinction (the value 0.2 was

Table 1 Species and provenances ID No.

Species

Provenance Latitude N

1 2 4 5 6 9 10

Scots pine Scots pine Scots pine Lodgepole pine Lodgepole pine Norway spruce Norway spruce

0

68810 658000 678060 638280 628080 658000 638130

Altitude (m) 300 300 200 610 570 475 245

used here); h, the sun’s altitude; and D, the diffuse radiation based on the formula: D¼k

pffiffiffiffiffiffiffiffiffiffiffiffiffi sin ðhÞ

where k is a constant near the value 185. This simple method for estimating the global radiation was in agreement with observed data from the reference climate station at the Svartberget Experimental Forest and generated appropriate background data for the shelterwood simulation. The simulation of the shadows formed by the shelter trees was based on the forest shading model presented by Granberg (1988). The shadow of each shelter tree was projected onto the ground surface using the sun’s position to determine its length and direction. The simulated pine trees were modelled as a ‘cone-on-a-stick’, where the cone consisted of 25 whorls with four branches each and additional outer twigs, thus producing shade akin to partly transparent tree crowns. In accordance with observations made at the experimental site, the cone base projection to the ground was set to a diameter of 6 m, tree height to 20 m and crown base at 10 m. All simulated shelter trees were allocated the same properties. Given the graphic resolution of the VGA computer monitor used, a square of 400  400 pixels was suitable for representing 1 ha of shelterwood, one pixel corresponding to a square with sides 0.25 m. Since all lines drawn on the screen were one pixel wide,

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all parts of the simulated trees, stems, branches and twigs had the same diameter of 0.25 m. The simulation model used the shelter tree positions to plot the shade cast, with black pixels inside the predefined white square representing the shelterwood area on the screen. The resulting plot of the tree shadows on screen was analysed by determining the status of each pixel within the plot area. The value of a white pixel was set to the calculated clear-sky global radiation, and the black pixels were set to the value of the clearsky diffuse radiation. The surrounding forest was treated by the simulation model as a 20 m high opaque wall. A time resolution of 10 min was used and the simulated global radiation for a specific point was based on the mean value of nine pixels; the central pixel and its adjacent eight pixels. The ratio between the daily irradiance at each seedling position and the above-canopy clear-sky irradiance was defined as the global radiation transmission (GRT). The daily global radiation transmission from May to October 1996 was derived for each seedling and used for all years in the analysis. To estimate the actual irradiance, the simulated daily clear-sky irradiance was replaced by the daily irradiance measured at the Svartberget reference climate station. 2.3. Statistical analyses Data from the shelterwoods and the clear-cut were analysed separately using the SPSS statistical package (SPSS 12.0.1, SPSS Inc., IL, U.S.A.). Multiple linear regression by provenance and planting year, using the stepwise method, was performed for the data from the shelterwoods. Height or annual height growth were used as dependent variables and distance to the nearest tree, competition index and irradiance as independent variables. The competition index (Stoll et al., 1994) was calculated for each seedling as the sum of the basal area of each tree within 10 m, weighted by the inverse of its distance to the seedling. Analysis of variance by species was used to evaluate the effects of provenance and planting year (fixed factors) on height or annual height growth. Because the distance to the nearest tree was the only factor that explained a significant portion of the variation in height or annual height growth in most regression analyses, this factor was used as a covariate in the models for the shelterwoods. Analysis of variance was also used to evaluate differences in height growth between species. Differences in height or annual height growth between the shelterwoods and the clear-cut were evaluated for each provenance with an unpaired t-test. 3. Results 3.1. Microsite irradiance The simulated irradiance at the locations of the seedlings below the shelterwood canopy was between 565 and 2061 MJ/ m2 for the period May to October 1996 (GRT-values were between 34 and 89%). Positions with the lowest GRT-values were located near the forest edge in the southwest part of all shelterwoods; the highest GRT-values were found in the centre

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Fig. 2. Measured and simulated global radiation in the centre of the pine shelterwood with 43 stems/ha on a clear day (26 June 1986).

of the most open shelterwood plot (Fig. 1). The spatial variation of GRT reflects the summer solar conditions at high latitudes where the shade cast by shelter trees is in the east-west direction twice a day since the hours of daylight is close to 21. Later in the growing season, during October, the spatial variation was considerably weaker due to the sun being at a lower elevation, thus increasing shading by the shelter trees and the surrounding intact forest. On clear days, the simulated global radiation was close to the 10-min mean value of measured global radiation at the same location (Fig. 2). The difference between the simulated and measured clear-sky daily irradiance was typically within 15%. 3.2. Height growth After three growing seasons, seedlings of lodgepole pine and one provenance of Scots pine were significantly taller in the clear-cut than in the shelterwoods (Table 2). Furthermore, the annual height growth during the next three growing seasons was significantly higher in the clear-cut area than in the

Table 2 Height 3 years after planting and annual height growth 4–6 years after plantinga Variable and provenance

Shelterwoods

Clear-cut

Height year 3 (cm) Scots pine (No. 1) Scots pine (No. 2) Scots pine (No. 4) Lodgepole pine (No. 5) Lodgepole pine (No. 6) Norway spruce (No. 9) Norway spruce (No. 10)

31.9  1.6 36.5  1.8 36.8  1.3 37.6  1.2 39.3  1.4 27.8  1.1 28.7  1.0

(46) (48) (59) (74) (74) (49) (37)

37.0  2.3 46.6  2.3 38.8  2.2 48.0  1.4 50.7  1.6 27.3  1.6

Annual height growth year 4–6 (cm) Scots pine (No. 1) 10.6  0.7 Scots pine (No. 2) 12.6  0.9 Scots pine (No. 4) 13.5  0.8 Lodgepole pine (No. 5) 17.8  0.8 Lodgepole pine (No. 6) 18.3  1.0 Norway spruce (No. 9) 4.2  0.3 Norway spruce (No. 10) 4.4  0.4

(46) (48) (59) (74) (74) (49) (37)

17.4  1.5 22.7  1.6 18.4  1.2 36.0  1.0 34.1  1.0 5.8  0.9

a

P-value (18) (14) (21) (53) (55) (9)

0.086 0.006 0.455 <0.001 <0.001 0.806

(18) (14) (21) (53) (55) (9)

<0.001 <0.001 0.002 <0.001 <0.001 0.058

b

b

The total number of planted seedlings of each provenance in the shelterwood plots and clear-cut area was 96 and 64, respectively. Mean  S.E. for undamaged seedlings are shown. The number of replicates is indicated in parentheses. b Data were omitted due to the low number of undamaged seedlings.

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shelterwoods for all provenances of Scots pine and lodgepole pine. In contrast, Norway spruce, which exhibited a relatively poor annual height growth, produced similar levels of growth in the shelterwoods and the clear-cut (Table 2). The multiple regression analyses indicated that annual height growth in the period 4–6 years after planting was more strongly correlated with distance to the nearest tree than with cumulative global radiation. Further analyses showed that height growth was not consistently related to the amount of radiation (Fig. 3A, C; cf. Fig. 3B, D). In the examples shown, height growth was significantly correlated with global radiation for seedlings planted in 1989 in the northern part of the shelterwoods. However, the slope of the regressions was steeper than expected. Furthermore, height growth was significantly correlated with radiation in less than half of the 14 regression analyses. Generally, a significant relationship between height growth and cumulative global radiation was found only when the distance to the nearest tree was also significantly correlated with radiation. The competition index (Stoll et al., 1994), which was strongly correlated with distance to the nearest tree, was in most cases not a significant factor in the multiple regression models. The relationship between annual height growth and

distance to the nearest tree was significant for all three species (Fig. 4), but this explained only 17–44% of the variation in height growth. The height 3 years after planting was, generally, less well correlated with the distance to the nearest tree than annual height growth during years 4–6 (data not shown). There were significant differences in annual height growth between the three provenances of Scots pine, both in the shelterwoods and the clear-cut (Table 3; cf. Fig. 4A). Similar differences between the provenances were observed for height after three growing seasons (data not shown). However, no significant differences between the provenances were observed when annual height growth was expressed as relative height growth, i.e. as a proportion of the height 3 years after planting. No significant differences in annual height growth were observed between the provenances of lodgepole pine, either in the shelterwoods or the clear-cut (Table 3; cf. Fig. 4B). There were no significant differences between the provenances of Norway spruce in the shelterwoods (Table 3; cf. Fig. 4C). The annual height growth of seedlings planted in 1988 was greater than in those planted in 1989 for all three species in the shelterwoods and for Scots pine and lodgepole pine in the clearcut (Table 3). Furthermore, analysis of variance showed that

Fig. 3. Annual height increment over the period 4–6 years after planting, as a function of cumulative global radiation (A, C) and distance to the nearest shelter tree (B, D) for one provenance of Scots pine (A, B) and one provenance of lodgepole pine (C, D). Seedlings were planted in 1988 (black symbols) and 1989 (white symbols).

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Table 3 Effects of provenance and planting year on annual height growth of Scots pine, lodgepole pine and Norway spruce 4–6 years after planting Shelterwoods a

Clear-cut

F-value

P-value

F-value

Scots pine Provenance Planting year

6.86 4.24

0.001 0.041

5.59 12.20

0.007 0.001

Lodgepole pine Provenance Planting year

0.35 7.56

0.553 0.007

2.62 13.89

0.109 <0.001

Norway spruce Provenance Planting year

0.71 4.35

0.402 0.040

b

a b

P-value

Distance to the nearest tree was used as a covariate. Not assessed.

annual height growth was significantly greater for lodgepole pine than for Scots pine, both in the shelterwoods and in the clear-cut area. 4. Discussion

Fig. 4. Annual height increment of provenances of Scots pine (A); lodgepole pine (B); and Norway spruce (C) as a function of the distance to the nearest shelter tree. Different provenances are indicated by different symbols. The regression equations are y = 1.20x + 4.0 (r2 = 0.356, P < 0.001), y = 1.45x + 6.0 (r2 = 0.277, P < 0.001) and y = 1.73x + 3.9 (r2 = 0.441, P < 0.001) for Scots pine Nos. 1, 2 and 4, respectively; y = 1.48x + 10.7 (r2 = 0.290, P < 0.001) and y = 1.76x + 8.8 (r2 = 0.392, P < 0.001) for lodgepole pine Nos. 5 and 6, respectively; y = 0.46x + 1.7 (r2 = 0.166, P = 0.004) and y = 0.66x + 0.4 (r2 = 0.365, P < 0.001) for Norway spruce Nos. 9 and 10, respectively. For the regressions, data from the 2 planting years were pooled.

The results of the regression analyses suggest that the amount of global radiation experienced by the individual seedlings in the shelterwoods had little direct or indirect effect on height growth. In other forest types, increased height growth with increasing light availability has been found for both naturally generated and planted conifer seedlings (e.g., Klinka et al., 1992; Chen et al., 1996; Chen, 1997; Coates and Burton, 1999; Williams et al., 1999; Wright et al., 1998). However, the variation in cumulative global radiation reaching the ground in the shelterwoods was moderate, especially during the second planting year (Fig. 3A, C); no seedlings in the shelterwoods were exposed to GRT-values below 34%. This may partly explain the apparent discrepancy between the results of this study and other studies of growth responses to light availability. Indeed, the model used in this study for simulating the global radiation below shelterwood canopies includes several simplifications, which resulted in some differences compared to the data collected in situ. However, the deviations observed between simulated and measured daily irradiance in the shelterwoods during clear-sky conditions were acceptable, typically less than 15%, and were probably even lower for the derived seasonal irradiances that include a large number of cloudy days. We found that factors associated with the distance to the nearest tree had a greater influence on height growth than the amount of radiation or factors correlated with the amount of radiation. The increased height growth of individual seedlings with increasing distance to the nearest tree is in agreement with other studies in northern Scots pine forests (e.g., Hagner, 1962; Kuuluvainen et al., 1993; Niemisto¨ et al., 1993; Skoklefald, 1995; Valkonen et al., 2002). For Scots pine, differences in annual height growth between the clear-cut and the shelterwoods can largely be attributed to factors dependent on the distance to the nearest tree, since mean values of height growth in the clearcut (Table 2) were similar to those predicted at a distance of 10 m

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from the nearest tree (Fig. 4). For lodgepole pine, we cannot discount the possibility that height growth was stimulated by some additional factor. Alternatively, lodgepole pine may be less susceptible to summer frosts, etc. in the clear-cut area as indicated by the higher percentage of undamaged lodgepole pine seedlings than Scots pine seedlings in that area (see below). The depression of height growth in the vicinity of large trees may be attributed to belowground competition for mineral nutrients and water (cf. Bjo¨rkman and Lundeberg, 1971). In addition, the interception of precipitation and the production of crown leachates and root exudates by overstorey trees may affect the growth of understorey seedlings and saplings. Studies of the fertilization of young stands of Scots pine in central and north Sweden suggest that the availability of mineral nutrients, especially nitrogen, is limiting for growth (Tamm, 1991; Nohrstedt, 2001). Furthermore, some studies in other forest systems indicate that nitrogen availability may be similar in uncut and partial-cut forests (e.g., Barg and Edmonds, 1999; Kranabetter and Coates, 2004). Therefore, we suggest that competition for nitrogen may be severe in Scots pine shelterwoods growing on poor sandy soils, although no direct evidence for nitrogen availability increasing with increasing distance to the nearest tree was obtained in the present study (cf. Barg and Edmonds, 1999). Trenching experiments indicate that competition for belowground resources is important in northern coniferous forests on infertile soils (Coomes and Grubb, 2000). The height after three growing seasons was generally less well correlated with the distance to the nearest tree than the height growth during the subsequent 3 years. We tentatively speculate that the soil preparation prior to planting reduced the competition for soil resources, especially nitrogen, during the first three growing seasons. Although the presence of shelter trees negatively affected the height growth of Scots pine and lodgepole pine seedlings in the present study, shelter trees can also have positive effects on the growth and survival of seedlings. For example, damage caused by summer frosts (Langvall and Ottosson Lo¨fvenius, 2002) and ¨ rlander and Karlsson, by the pine weevil (Hylobius abietis L.) (O 2000) may be reduced in Scots pine shelterwoods. In the present study, the protective function of shelter trees is indicated by the higher percentage of undamaged seedlings in the shelterwoods (53 and 45% for Scots pine and Norway spruce, respectively) than in the clear-cut (28 and 9% for Scots pine and Norway spruce, respectively). In contrast, the percentage of undamaged lodgepole pine seedlings was lower in the shelterwoods (67%) than in the clear-cut (84%). The planting year had a significant effect on height growth (Table 3). Several factors may contribute to such an effect, e.g., differences in environmental conditions between years, the quality of plant material and reforestation work, and the effect of soil stabilization after mounding. The relative importance of these factors is difficult to determine. Mounding was carried out only once, whereas planting was conducted over 2 successive years. Because the effect of improving the nutrient status of the soil by mounding (Sutton, 1993) decreases with time, we speculate that the effect of planting year is due to soil stabilization.

There were significant differences in height growth between the species. However, different provenances of the same species were largely similar in their response to competition by overstorey trees. The relatively poor height growth in Norway spruce can be attributed, in part, to invisible damage caused by freezing temperatures during the summer, perhaps interacting with high light levels after the frosts (Lundmark and Ha¨llgren, 1987). Such damage should be more pronounced in the clearcut area. Furthermore, Norway spruce is probably not well adapted to the nitrogen-poor sandy soil that formed the substrate in the present study (Levula et al., 2003). Lodgepole pine had a significantly greater height growth than Scots pine, which may be due to lodgepole pine seedlings having a higher relative growth rate than those of Scots pine. The higher relative allocation of biomass to fine roots in lodgepole pine compared with Scots pine may also be important (Norgren, 1996; cf. Ingestad and Ka¨hr, 1985). In conclusion, height growth in conifer seedlings growing below the canopy of shelter trees of Scots pine on nitrogen-poor sandy soils was affected more by factors dependent on the distance to the nearest tree than factors related to the solar radiation exposure on the ground. This implies that, when using artificial regeneration in a shelterwood system, careful consideration should be given to the location of the planting/seeding. Choosing appropriate planting spots with respect to distance to the nearest tree could maximize growth rates. Furthermore, gapshelterwood systems in the form of Continuous Cover Forest System seem to be sufficient for optimising the growth and protection of seedlings. Growth of conifer species will generally be higher in small forest gaps compared to a typical shelterwood. By using a system where gaps are formed in a shelterwood, e.g., in a chequered pattern, it may be possible to exploit the environmental benefits of shelter trees and at the same time create the desirable conditions of a clear-cut for conifer tree seedlings (cf. Palik et al., 2003). Acknowledgements This investigation was part of the ‘Research Programme for the Utilization of the Boreal Forest’, and was funded by the European Regional Development Fund and Bratta˚sstiftelsen. We wish to thank the helpful staff of Vindeln Experimental Forests for field assistance and support. References Barg, A.K., Edmonds, R.L., 1999. Influence of partial cutting on site microclimate, soil nitrogen dynamics, and microbial biomass in Douglas-fir stands in western Washington. Can. J. For. Res. 29, 705–713. Bjo¨rkman, E., Lundeberg, G., 1971. Studies of root competition in a poor pine forest by supply of labelled nitrogen and phosphorus. Stud. For. Suecica 94, 1–16. Bostedt, G., Parks, P.J., Boman, A.R., 2003. Integrated natural resource management in northern Sweden: an application to forestry and reindeer husbandry. Land Econ. 79, 149–159. Canham, C.D., Berkowitz, A.R., Kelly, V.R., Lovett, G.M., Ollinger, S.V., Schnurr, J., 1996. Biomass allocation and multiple resource limitation in tree seedlings. Can. J. For. Res. 26, 1521–1530.

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