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Development of growth and stand structure in Piceaabies stands planted at different initial densitiesUrban Nilsson aa Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, Box49, S‐230 53, Alnarp, Sweden

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To cite this article: Urban Nilsson (1994): Development of growth and stand structure in Picea abies stands planted atdifferent initial densities, Scandinavian Journal of Forest Research, 9:1-4, 135-142

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Scand. J. For. Res. 9: 135-142, 1994

Development of Growth and Stand Structure in Picea abiesStands Planted at Different Initial Densities

URBAN NILSSONSwedish University of Agricultural Sciences, Southern Swedish Forest Research Centre, Box 49, S-230 53 Alnarp, Sweden

Scandinavian Journalof Forest Research

Nilsson, U. (Swedish University of Agricultural Sciences, Southern Swedish Forest ResearchCentre, Box 49, S-230 53, Alnarp, Sweden). Development of growth and stand structure in Piceaabies stands planted at different initial densities. Received Dec. 23, 1992. Accepted April 27,1993. Scand. J. For. Res. 9: 135-142, 1994.

Development of growth and stand structure was studied in four spacing experiments withNorway spruce (Picea abies (L.) Karst.) in northern Sweden. The sites were considered as poorfor Norway spruce. Height and diameter growth were significantly lower in the densestspacings compared to the most widely spaced stands. In the dense spacings the relative growthrates of both small and large trees were reduced. Height-diameter ratio was not influenced byspacing. Stand structure (Gini-cooerficient and skewness) was not significantly affected byspacing. It was concluded that the major limiting factor for growth in the dense spacings wasnot light but mineral nutrients and/or water. The results also support the hypothesis ofunchanged or decreased inequality when competition acts on all trees equally or in proportionto their size. Key words: Picea abies, Sweden, spacing experiment, competition, stand structure,Gini-coefficient

INTRODUCTION

Experiments on spacing have a long tradition inforest research and the influence of spacing on thedevelopment of Norway spruce (Picea abies (L.)Karst.) and Scots pine (Pinus sylvestris L.) planta-tions are well described (e.g. Sjolte-J0rgensen 1967,Elfving 1975, Handler & Jakobsen 1986, Pettersson1992). Trees in dense stands have slower growth thando trees in more open stands because of inter-treecompetition, and this effect is generally greater ondiameter growth than on height growth (Lanner1985). The result of this is an increase in the height-diameter ratio, and this ratio has been used as anindication of the severity of competition (Brand 1986,Gemmel 1988).

However, some spacing experiments do not con-form to the description given above (e.g. Naslund1934, Eklund 1952, Andersson I960): In these experi-ments the influence of spacing on height growth wasfound to be nearly as great as the influence of diame-ter growth. All these experiments have two features incommon. Firstly relatively dense spacings were stud-ied, and secondly they were situated on sites ofrelatively low fertility. A possible explanation for theinfluence of competition on height growth, may bethat light was not a limiting factor for growth. Whenwater and mineral nutrients are limiting factors forgrowth, the tree seems to react with a decrease in

both height and diameter increment (Firbank &Watkinson 1987, Gemmel 1988). Low soil tempera-ture has also been proposed as a possible explanationfor the poor growth of dense Norway spruce standsin northern Sweden (Naslund 1934).

Changes in frequency distributions of plant sizeover time have been used as indicators of competition(Ford 1975, Harper 1977, Cannell et al. 1984, Perry1985). An initially normal distributed population will,when competition is introduced, become positivelyskewed. The development of positive skewness hasbeen attributed to differences in the relative growthrates of small and large trees (Ford 1975, Cannell etal. 1984). Large individuals suppress growth ofsmaller ones more than they are suppressed, and thiseffect is greater than would be expected from theirrelative sizes (Harper 1977, Perry 1985). Distributionswith a few large (dominant) and many small(suppressed) individuals have been attributed tocompetition for light (Ford 1975, Ford & Diggle1981, Cannell et al. 1984, Weiner & Thomas 1986)although a positive skewness may be the result ofvariance in initial exponential growth even withoutcompetitive interactions between individuals (Weiner& Thomas 1986). In addition to a more positiveskewed distribution, size inequality in the stand isamplified, resulting in increases in the Gini-coefficient(Weiner & Solbrig 1984, Weiner 1985). This has also

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136 U. Nilsson Scand. J. For. Res. 9 (1994)

been attributed to competition for light (Weiner &Thomas 1986).

Competition between trees can be "asymmetric",where the large trees suppress growth of small treesmore than they are suppressed, and this effect isgreater than would be expected from their relativesize, or it can be "symmetric", where competition actson all trees equally or in proportion to their size. Ithas been suggested that asymmetric competition isthe result of competition for light, while symmetriccompetition may be likely in situations where light isnot the primary limiting factor (Weiner & Thomas1986, Firbank & Watkinson 1987, Kenkel 1988). Thechanges in stand structure described above are proba-bly effects of assymetric competition (Weiner &Thomas 1986). However, when the competitive inter-action between trees is mainly symmetric the alter-ations in stand structure described above may notdevelop. Weiner & Thomas (1986) hypothesize thatsymmetric competition will result in unchanged oreven decreased size inequality.

In this study four P. abies spacing experiments onsites of low fertility in northern Sweden were investi-gated. The assumption was that competition in thesestands was mostly for mineral nutrients, because thestands were far from canopy closure. The aim of thestudy was to investigate if changes in stand structurecan be used as indications of the origin of competitivestresses. The hypothesis tested was that symmetriccompetition results in unchanged or decreased in-equality.

MATERIAL AND METHODS

The experiments used in this study belong to a seriesof spacing experiments that were laid out in the1950s. Four plantations with P. abies, on low fertilitysites in northern Sweden were selected. Site and standdescriptions are given in Table 1.

The experiment was designed to test the effects ofdifferent initial spacings on the growth of P. abiesand the experimental design was the same for all fourexperiments. Each experiment was organized in 8square plots, with two replicates of four spacings.The arrangement of plots within each block was fixedand the same for all experiments (Fig. 1). The spac-ings were 1.0, 1.5, 2.0 and 2.5 m between treesplanted on a square grid. The size of the plots andnumbers of trees planted on each plot varied withspacing. Each experiment consisted of a total of 1 152trees.

Table 1. Site and stand descriptionsSite index is expected dominant height at age 100, estimatedwith site properties according to Hiigglund and Lundmark(1977)

Site 1 Site 2 Site 3 Site 4

Lat 64°10'N 65°05'N 64°14'N 63°08'NLongHeight above

sea levelSite indexPlanting year

19°36'E200 m

G17Spring1952

20°33' E310m

G15Autumn1951

16°36' E385 m

G18Spring1952

14°33'E330

G22

Block A Block B

2.5 x 2.5 m

375 m2

2.0 x 2.0 m

336 m2

1.5 x 1.5 m

324 m2

1.0 x 1.0 m

288 m2

1.0 x 1.0 m

288 m2

1.5 x 1.5 m

324 m2

2.0 x 2.0 m

336 m2

2.5 x 2.5 m

375 m2

Fig. I. Arrangement of the plots within each site.

At 5, 10, 15 and 20 growing seasons after planting,height (to the nearest cm) of trees in the experimentand diameter at breast height (to the nearest mm) fortrees taller than 1.3 m was measured. After the first20 years the time between measurements varied be-tween sites. Site 1 was measured 35 and 37 years afterplanting, site 2 after 30 and 35 years, site 3 after 30and 36 years and site 4 after 29 years. The measure-ments included height and diameter at breast heightof all trees, except the last measurement on site 1,when heights were measured on a sample consistingof 10% of the total tree number in the stands.

Mortality was low in all sites until ten years afterplanting (Table 2). On sites 1 and 4 mortality contin-ued to be low but it increased on sites 2 and 3. Themain cause of mortality was frost, and no dependenceof spacing on mortality was found.

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Scand. J. For. Res. 9 (1994) Development of growth and stand structure in Picea abies 137

Table 2. Mortality (% of originally planted seedlings) in stands of different initial density

For description of sites see Table 1

Year

planting

51015202930353637

Site 1

S1.0

0.60.60.81.4

1.4

1.8

S1.5

0.43.13.63.6

3.6

3.6

S2.0

0.00.00.80.8

0.8

0.8

S2.5

0.00.01.31.3

1.3

2.5

Site 2

S1.0

0.06.2

15.723.8

46.749.7

S1.5

0.04.5

10.716.1

27.730.8

S2.0

0.03.3

15.820.8

32.534.2

S2.5

0.01.25

12.521.3

27.536.3

Site 3

S1.0

0.00.23.58.1

9.9

11.8

S1.5

0.00.02.78.4

9.8

12.1

S2.0

0.00.00.89.2

13.3

22.5

S2.5

0.00.05.0

12.5

26.3

30.0

Site 4

S1.0

0.00.84.14.95.9

S1.5

0.01.84.95.45.4

S2.0

0.01.7

10.810.810.8

S2.5

0.02.58.7

10.013.8

Analyses were based on living trees. All living treeswere considered to be healthy. In the analyses, treesin the outer row surrounding each plot were excludedto avoid influences from open areas and other spac-ings. Analysis was also made of the 100 tallest treesha"1 in each spacing.

Average height, diameter and basal area perhectare of each spacing were analysed at the differentmeasurement periods. Analysis of variance for a com-pletely randomized block was used to evaluate spac-ing effects on tree and stand development. Thelocation of each spacing was not randomly assignedwithin the blocks. Because the widest spacing in blockA was placed near the densest spacing in block B andvice versa (Fig. 1), differences in site fertility betweentreatments were assumed to be minimized if the aver-age value of each site and spacing was compared(Elfving 1975). Therefore, replicates within each sitewere not used in the analysis of variance. The follow-ing analysis of variance was used for all responsevariables:

ytjk = V- + a, + bj + eijk (1)

where n is the general mean level; as is the spacingeffect (3 df); bj is the site effect (3 df) and eijk is theresidual error. In the model, a, and bj were regardedas fixed effects. In order to assess the interaction term(site x spacing), even though not statistically correct,the following model was used:

y,Jk CkJ eIJk, (2)j J ^ J

where n is the general mean level; a, is the spacingeffect (3 df); bj is the site effect (3 df); CkJ is the blockeffect within site (4df); (ab)^ is the spacing effect in

association with the site effect (9 d) and eijk, is theresidual error. In this model CkJ was regarded as arandom effect and all others as fixed effects. Becausethe interaction term was not statistically significant(p = 0.09-0.74 depending on the variable to betested) and model (1) was more statistically correct,model (1) was used in testing all the main effects.

Analyses of covariance were used to determine theeffects of spacing on the relative diameter growth andthe height-diameter ratio. The following model wasused:

8-vy + eh (3)

where Xy was the covariate (diameter at breastheight); /? was a coefficient and /*, ah bj and eiJk werethe same as for model (1).

The parameters skewness I (x , - .v) 3 / I [ (x , — .x)2]3/

2(i = 1 . . . «)) and Gini-coefficient (Z,Iy|.r, —Xj\l2.xn(n — 1) (J = 1 . . . n; 7 = 1 . . . / i )) were calculatedfor diameter at breast height and height for eachspacing on each site. Analysis of variance accordingto model (1) was used to evaluate spacing effects onskewness and gini-coefficients.

RESULTS

Height growth at the beginning of the experiment didnot differ between treatments, indicating small differ-ences in fertility between plots (Fig. 2). Between age20 and the time of last measurement the closestspaced stands have had slower height growth than themore' widely spaced stands in all four experiments(Fig. 2). Analysis of variance showed effect of spacingon height at the time of last measurement (p < 0.01)

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138 U. Nilsson Scand. J. For. Res. 9 (1994)

400

300

200

g

I 400

300

200

100

0

Sitel . Site 2

Site 3 Site 4

10 20 30 0 10Years after planting

20 30

Fig. 2. Height development of trees planted with differentinitial density. Spacing 2.5 (D), 2.0 ( • ) , 1.5 ( • ) , 1.0(O) m. For descriptions of sites see Table 1.

and height growth between age 20 and the time oflast measurement (p <: 0.01).

Height growth of the 100 tallest trees per hectarewas negatively correlated to stand density (p <0.01),while height at the time of the last measurement wasnot influenced by spacing (p =0.24).

At the time of the last measurement differences inheight-diameter ratio between trees from differentspacings were very small (Fig. 3) and analysis ofcovariance detected no overall significant differenceamong the four spacings (p = 0.5858).

Diameter growth was positively correlated withdistance between trees. At the time of last measure-

1201:

6 8

Diameter class, cm

10 12

Fig. 3. Height-diameter ratio (rel. units) for trees planted atdifferent initial density and of different diameter-classes.Average values of all four sites. Spacing 1.0 ( • ) ; 1.5 (H);2.0(11); 2.5 (D)m.

35 37 30 35 31 36

Years after planting

Fig. 4. Average diameter at breast height for trees plantedat different initial density. Spacing 2.5 ( • ) , 2.0 ( • ) , 1.5( • ) , 1.0 (O) m, For descriptions of sites see Table 1. Forsite 1-3 data from two measurements and for site 4 datafrom one measurement are shown.

ment mean diameters at breast height were larger inthe widest spacing (S2.5) compared to the densestspacing (S1.0) in all sites (Fig. 4). Analysis of vari-ance showed significant effects of spacing on diameterand diameter growth (p < 0.01).

There was a tendency for the 100 tallest trees ha"1

in close spacings to have lower diameter and diametergrowth than the 100 tallest trees ha~' in wider spac-ings, but no significant effects of spacing were found(Table 4).

Height distributions Diameter distributions

1.2

0.9

§ 0.6

0.3

0.0

0.6

gS 0.4

'25 0-2

0.020 35 30

Years after planting

35

Fig. S. Skewness and Gini-coefficients of height (left) anddiameter (right) distributions of stands planted at differentinitial densities. Mean values from all four sites (age 20),from site 2-4 (age 30) and from site 1-3 (age 35). Spacing1.0 ( • ) ; 1.5 (H); 2.0 (H); 2.5 (D) m.

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Scand. J. For. Res. 9 (1994) Development of growth and stand structure in Picea abies 139

0.4

0.3

0.2

O 0.1

0.0

:-i

10 15 20 29-31 35-36

Years after planting

Fig. 6. Gini-coefficients of height distributions of standsplanted with different initial densities at the time for lastmeasurement. Mean values from all four sites (age 5-20),from site 2-4 (age 29-31) and from site 1-3 (age 35-36).Spacing 1.0 ( • ) ; 1.5 (@); 2.0 ( I I ) ; 2.5 ( • )

The closest spaced stands has highest basal area atthe time of last measurement and analysis of variancedetected statistically significant effects of spacingO<0.01).

There was a trend of increasing Gini-coefficients inboth diameter and height distributions with decreasingdistance between trees at the time of last measurement(Fig. 5), but analyses of variance detected no statisti-cally significant differences (Table 3). At age 20 and atage 30 no trend of increasing inequality with decreas-ing spacing was found (Table 3). Gini-coefficients ofheight distributions increased from 5 to 15 years afterplanting, after that a stagnation and a slight decreasein Gini-coefficients were found (Fig. 6).

The closest spacing (S.1.0) has the most positivelyskewed diameter and height distribution at the time

Table 3. Effects of spacing and site on the development of Picea abies plantationsF is the variance ratio; p is the probability that effects are not different; Error MS is the error mean square and Df is thedegree of freedom

Characteristic

Height at the time forlast measurement

Height growth betweenage 20 and the timefor last measurement

Mean Dbh at the timefor last measurement

Mean Dbh growth(2-5 years of growthdepending on site)

Basal area at the timeof last measurement

Skewness of diameterdistributions

Skewness of diameterdistributions

Gini-coefficients ofdiameter distributions

Gini-coefficients ofheight distributions

Standcomponent/Ageclass

All trees

Tallest 100 ha"'

All trees

Tallest 100 ha"1

All trees

Tallest 100 ha"1

All trees

Tallest 100 ha"1

All trees

All trees/Age 29-31All trees/Age 35-36

All trees/Age 20All trees/Age 29-31All trees/Age 35-36

All trees/Age 29-31All trees/Age 35-36

All trees/Age 20All trees/Age 29-31All trees/Age 35-36

Spacing

F

8.6

1.7

27.6

15.6

12.4

2.33

27.7

3.0

27.7

1.01.7

0.70.31.1

1.23.3

1.51.24.0

P

< 0.001

0.24

<0.01

<0.01

<0.01

0.14

<0.01

<0.01

<0.01

0.470.26

0.60'0.830.43

0.370.10

0.280.400.07

Site

F

3.1

11.2

45.0

15.6

5.8

11.7

45.0

3.3

3.3

7.54.0

13.34.92.4

12.45.5

22.26.4

25.0

P

0.08

<0.01

<0.01

<0.01

0.02

0.002

<0.0!

0.07

0.07

0.020.08

0.010.050.17

<0.01<0.04

<0.010.03

<0.01

Df

15

15

15

15

15

15

11

15

15

1111

151111

1111

151111

ErrorMS

14.8

28.9

29.1

28.9

29.1

86.5

0.8

4.6

1.1

0.120.07

0.220.060.07

0.0020.002

0.00070.0010.0006

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140 U. Nilsson Scand. J. For. Res. 9(1994)

Table 4. Height and diameter growth of the tallest 100 trees ha~] planted in stands with different initial densityAge at the time of last measurement of height and diameter is indicated within parenthese. Diameter and height growth isthe summed growth over a period of time before the last measurement, which is different in length depending on site andis indicated within parentheses

Characteristic Spacing Site 1 Site 2 Site 3 Site 4

Height, cm

Diameter, mm

Height growth,cm

Diametergrowth, mm .

S1.0S1.5S2.0S2.5

S1.0S1.5S2.0S2.5

S1.0S1.5S2.0S2.5

S1.0S1.5S2.0S2.5

454(35)544(35)565(35)693(35)

58(35)69(35)69(35)89(35)

263(20-35)315(20-35)390(20-35)473(20-35)

4.8(35-37)8.8(35-37)9.5(35-37)12.5(35-37)

•730(35)744(35)802(35)790(35)

94(35)102(35)118(35)125(35)

433(20-35)434(20-35)436(20-35)409(20-35)

17.2(30-35)15.4(30-35)15.5(30-35)19.2(30-35)

778(36)672(36)725(36)732(36)

105(36) •91(36)101(36)98(36)

489(20-36)479(20-36)517(20-36)577(20-36)

12.0(31-36)19.6(31-36)17.2(31-36)20.2(31-36)

684(29)738(29)732(29)742(29)

86(29)100(29)93(29)97(29)

312(20-29)311(20-29)310(20-29)331(20-29)

_

---

of last measurement (Fig. 5) but analysis of variancedetected very little influence from spacing on skew-ness of height and diameter distribution at this age(Table 3).

When relative diameter growth was studied in rela-tion to diameter at breast height at the beginning ofthe growth period it was found that trees in theclosest spaced stand had lower relative diametergrowth compared to trees in the most widely spacedstand (Fig. 7). This holds true for all diameter classes.Analysis of covariance indicated overall statisticallysignificant differences among spacings for relativediameter growth (p < 0.01).

DISCUSSION

In this study effects of competition were found wellbefore canopy closure. Basal areas in the stands wereonly between 2-6 m2 ha"1, while basal area forstands of P. abies in northern Sweden normally ex-ceeds 10m2ha~' when canopies are closed. Becausethe stands were far from closed and because of thepyramidal shape of P. abies, it may be concluded thatlow light level was not the main factor causing lowergrowth rate in close spacings.

Both diameter growth and height growth wasaffected by spacing, i.e. intraspecific competition re-duced both height- and diameter growth. A conse-quence was that the height-diameter ratios were notaffected by spacing. This finding is in contrast tomost other studies of spacing experiments (e.g. Sjolte-Jorgensen 1967, Handler & Jakobsen 1986, Pet-tersson 1992). The generally found insensitivity ofheight growth to initial spacing is probably due tocompetition for light and may be explained by thehigh priority given to height growth in order to placeneedles in the most advantageous position (Caldwell1987). The lack of effect on height growth may alsobe explained by sink-source relationships and thetiming of height and diameter growth (Lanner 1985).Before canopy closure, when no competition for lightis taking place height growth is not prioritized overdiameter growth. An explanation of the effect onboth height and diameter growth found in this study,is probably that nutrients and water and not light arethe limiting factors for growth.

There was a tendency for greater positive skewnessand higher Gini-coefficients in close spacings at thetime of last measurement, but the differences weresmall and not statistically significant. At age 20 and

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Scand. J. For. Res. 9 (1994) Development of growth and stand structure in Picea abies 141

40 80Diameter at breast height, mm

120

Fig. 7. Relative diameter growth rate(%) for trees of different initial diameterat breast height in the closest spacing(S1.0; O) and the widest spacing (S2.5;

• ).

30 there was no tendency for increased inequalitywith decreased spacing. The influence of spacing wasless on skewness than on the Gini-coefficients. Even ifthe biological interpretation of positive skewness islimited, because positive skewness may arise fromvariance in initially exponential growth rate (Mohleret al. 1978, Watkinson 1985, Weiner & Thomas1986), skewness is considered to be amplified bycompetition (Cannell et al. 1984, Perry 1985). Thereason for the small effect in this study is probablythat, in the closest spacings, relative growth rates ofdominating trees were suppressed as well as the rela-tive growth rate of small trees.

Competition between trees in a stand can be eithersymmetric where competition acts on all individualsequally or in proportion to their size, or asymmetricwhere large trees suppress the growth of small treesmore than they are suppressed, and this effect isgreater than would be expected from their relativesize (Harper 1977, Weiner & Thomas 1986, Weiner1990). Asymmetric competition is suggested to be aresult of competition for light and the development ofdominant and suppressed individuals (Ford 1975,Cannell et al. 1984).

Symmetric competition, on the other hand, may bethe result of competition for water and nutrientswhere plants under competitive stress will grow in thesame way as plants under less competitive stress, onlymore slowly (Weiner & Thomas 1986, Firbank &Watkinson 1987, Kenkel 1988, Weiner 1990). Weiner& Thomas (1986) and Weiner (1990) hypothesizethat symmetric competition without mortality resultsin lower or unchanged inequality at higher densityafter a given period of growth. In the present study,no effect of spacing on inequality was found at age 20and age 30 when statistically significant effects ofspacing on growth were found. Furthermore, compe-tition for light was probably not the major limitingfactor for growth in these stands (see above). At thetime of last measurement there were effects of spacingon Gini-coefficients and coefficients of variation ofboth height and diameter distributions but the differ-ences were small and not significant. It is thereforeconcluded that data from this study may, (if theassumption that competition was mostly for waterand nutrients holds true) support the hypothesis thatsymmetric competition leads to lower or unchangedinequality.

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142 U. Nilsson Scand. J. For. Res. 9 (1994)

In this study Gini-coefficients of height distribu-tions increase for all spacings during the first 15 yearsafter planting. After that no increase or a slightdecrease in Gini-coefficients was found. The mostimportant cause is probably that growth of stuntedseedlings is increased and their higher relative heightgrowth rates will result in height distributions more.concentrated toward the mean values.

In summary, indications of two sided competitionin this study were: (/) Height growth as well asdiameter growth were affected by intraspecific compe-tition, (») the relative growth rate of both large andsmall seedlings was reduced, (///) because of decreasedheight growth, height-diameter ratios in close spac-ings were the same as for more widely spaced stands.Inequality was not larger in close spacings comparedto more widely spaced stands. Therefore, it is con-cluded that this study supports the hypothesis thatsymmetric competition results in decreased or un-changed inequality and that stand structure may beused as an indication of the origin of competitivestresses.

ACKNOWLEDGEMENT

This work was financially supported by the SouthernSwedish Forest Research Program. The authorthanks Dr. P.-M. Ekö for statistical advice and Dr.Richard Bradshaw for useful comments on themanuscript. Furthermore, Prof. Harry Eriksson haskindly provided data for this study.

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Andersson, S.-O., 1960. Norrlands skogsvårdsförbundsexkursion till Jämtland 28 och 29 Juni 1960.Norrlands Skogsvårdsförbunds tidskrift. 4: 389-492 (InSwedish).

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