analysing effects of thinning on stand volume

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Institute of Chartered Foresters, 2008. All rights reserved. Forestry, Vol. 82, No. 1, 2009. doi:10.1093/forestry/cpn047

For Permissions, please email: [email protected] Advance Access publication date 28 October 2008

Analysing effects of thinning onstand volume growth in relation

to site conditions: A case studyfor even-aged Sitka spruce (Piceasitchensis(Bong.) Carr.)

J.P. SKOVSGAARD

Forest and Landscape Denmark, University of Copenhagen, 11 Hrsholm Kongevej, DK-2970 Hrsholm,Denmark

E-mail: [email protected]

Summary

This study challenges all-else-equal assumptions in the analysis of thinning effects and morespecifically those regarding local variation in forest site productivity. It is demonstrated thatconsiderable variation may occur in site productivity in apparently homogeneous forest stands,whether thinned or unthinned, and that this may not be reflected in height, but rather in basal

area growth. To account for such variation, the suggestion is to include an unbiased pre-treatmentmeasure of site productivity, such as total volume production prior to first thinning, as a covariatein the analysis. In a wider perspective, the study further challenges two general site productivityhypothesis, namely, the general assumption that stand volume growth correlates well with standheight (the site index hypothesis, i.e. that site productivity can be estimated based on stand height)and the assumption that, for a given species, total volume production at a given stand height isidentical for all site classes (Eichhorns rule). The thinning response pattern for even-aged Sitkaspruce is quantified for different site types based on two thinning experiments in Denmark and sixcomparable experiments in Great Britain, Norway and Sweden. The effect of thinning from belowon stand volume growth is strongly site dependent, but heavy thinning usually leads to a reductionin volume growth as compared with an unthinned stand growing under similar site conditions. Forsome sites, typically those with stable and ample water supply and a deeply developed root system,

stand volume growth is maximized through light thinning. For some other sites, typically those witha high ground water table and a shallow root system, thinning reduces stand volume growth andconsiderably so with increasing thinning grade. There seems to be a gradient in the reduction, sothat it is lessened with increasing rooting depth and with increasing stability of water supply. Due tosite-dependent variation in the relation between height growth and stand volume growth as well assite-dependent variation in thinning response pattern, it is recommended to establish and maintainunthinned observation plots in managed stands to ensure reliable growth and yield prognoses.Such plots can serve as a reference for thinning practices as well as a standard for potential volumegrowth. This recommendation could also hold for other species and in other parts of the world.


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Introduction

For centuries, the possible effect of thinning onthe volume growth of forest stands has been amain theme in forestry and in forest science.Thinning experiments have been installed to help

quantify this and other effects of different thin-ning practices and, in turn, help forest managersoptimize stand management according to local,national and international criteria.

A major result from thinning experiments isthe general notion that thinning does not influ-ence stand volume growth significantly for arange of thinning grades or stocking densities,whereas heavier thinning beyond this range re-duces volume growth (Wiedemann, 1932, 1937,1951; Langster, 1941; Mller, 1944, 1951,1954). This thinning response hypothesis has had

a major influence on thinning practices for even-aged monospecific forest types in many parts ofthe world and details of the thinning response havebeen quantified for many different tree species,site types, stand ages and thinning regimes (e.g.Assmann, 1961, 1970; Evans, 1992; Florence,1996; Henriksen, 1988; Kramer, 1988; Lewis andFerguson, 1993; Nyland, 1996; Pretzsch, 2001,2002; Savill et al., 1997; Wenk et al., 1990).

Often, a main objective for quantifying the thin-ning response for different thinning practices is todetermine the range of stocking densities that en-

sure maximum use of the site potential for timberproduction and, most notably, the minimum stock-ing needed to produce the largest possible indi-vidual tree sizes without reduction in stand volumegrowth. In addition to volume growth, timber qual-ity, timber price assumptions, harvesting costs, riskof windthrow and future regeneration options areusually also considered when determining desirablestocking density at any time during the rotation.

The analysis of thinning response patterns maybe approached in different ways, depending onthe specific purpose of the analysis, the design

and nature of experiments, the type of mensu-rational data, data quantity and quality andmore personal characteristics such as pedagogicviews and scientific schooling (Skovsgaard andVanclay, 2008). While absolute figures for vol-ume growth with specific thinning practices(characterized nominally or otherwise) remainimportant locally, this approach often does notallow for more general interpretations of experi-

mental results. To attain an indisputable standardto characterise and quantify thinning grade (theamount of the stand removed in thinnings) andto facilitate comparison of thinning responsesacross species, sites and ages, unthinned controlplots may be used as a reference.

For given site conditions and a given spacingor initial stem number at stand establishment, theunthinned stand or control plot will support thehighest possible basal area of live trees at any stageof stand development. All else being equal, thethinning grade of any stand or plot may thus begauged on a relative and somehow objective scaleby its residual basal area relative to the basal areaof an unthinned stand or plot. Similarly, standvolume growth may be scaled relative to that ofan unthinned stand or plot. A diagram of standvolume growth vs thinning grade on these rela-

tive scales provides a simultaneous quantitativeand qualitative assessment of thinning responsefor any period of stand development.

Traditionally, the residual basal area at whichstand volume growth is reduced by 5 per cent com-pared with the highest volume growth for any re-sidual basal area at the same time is regarded asthe limit for desirable stocking densities in terms ofvolume production (the argument is that 5 per centis the statistically detectable change considering theexpected mean error for volume growth estimates,Assmann, 1950). There may be a lower as well as

an upper such limit or critical basal area. Disregard-ing measurement and sampling errors, this evalu-ation may be biased only if the basal area of theunthinned control plots is less than the potential.

In addition to thinning grade, more sophisti-cated analytical summaries may include other di-mensions or aspects, such as age, spacing at standestablishment, thinning type (what type of treesare predominantly removed, either based on soci-ological or mathematical principles), time of firstthinning, or thinning interval (the time betweenthinnings or the sequence of ages or stand heightsat which the stand is thinned).

Due to experimental design, the nature of ex-perimental data and variations in site and standconditions, several shortcomings of thinningexperiments may hamper a proper statisticalapproach to the analysis of thinning response pat-terns (see also Woollons et al., 1994). Potentialshortcomings include variation in topography,variation between plots in exposure to weather


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ANALYSING THINNING RESPONSE IN RELATION TO SITE CONDITIONS 89

conditions, insufficient replication of treatments,absence of unthinned control plots, large within-plot variations (large plots), large edge effectson plot development (small plots), absence of orinsufficient buffer zone around each plot, varia-tion in original spacing, estimates of stand char-

acteristics based on different number of trees fordifferently thinned plots of identical size, lack ofspatial or temporal independence in data, lack ofinformation on stand conditions and stand devel-opment prior to initiation of the experiment andlocal variation in site productivity within the ex-periment. Often, the trade-off between availablestand area of suitable quality for experimentationand the desirable number of treatments and repli-cations strongly influences some of the shortcom-ings. These, as well as atypical stand developmentfor the unthinned control plots, may bias the

evaluation of thinning response.Regarding variation in site productivity within

an experiment, stand height, basal area and totalvolume production of an unthinned plot may pro-vide a treatment independent indication of siteproductivity (except for the rare occasion wheregrowth stagnates due to high stocking density).This may aid the assessment of homogeneity insite and stand conditions prior to first thinningand, with multiple well-distributed control plots,during the subsequent experimental phase. Also,data for the unthinned, pre-experimental stage

may be used to adjust for the influence of localvariation in site productivity when assessing theeffect of thinning on stand volume growth.

Based on two Danish thinning experiments ineven-aged Sitka spruce (Picea sitchensis (Bong.)Carr.), this paper demonstrates a possible wayto handle local variation in site productivity inthe analysis of thinning effects. Focusing on thethinning response pattern of Sitka spruce, resultsfrom these experiments are discussed and com-pared with six similar experiments in Great Brit-ain, Norway and Sweden. As an integral part of

the analyses, three hypotheses on the effects of siteconditions and thinning practice on forest growthare challenged. These are outlined below.

The thinning response hypothesis

The thinning response hypothesis for Sitka sprucespecifically addresses critical basal area and goes

as follows: For even-aged Sitka spruce thinningsfrom below to maintain a residual basal area of50 to 65 per cent of the basal area of an otherwiseidentical unthinned stand on a similar site, reducestand volume growth by 5 per cent compared toits highest value for any residual basal area at the

same time. Lighter thinnings influence stand vol-ume growth less, whereas heavier thinnings leadto larger reductions.

The thinning response hypothesis for Sitkaspruce originates from a previous analysis of partsof the material in this investigation (Henriksen,1961). Originally derived for only one site type,this hypothesis has been widely used a rule-of-thumb in thinning prescriptions for Sitka sprucein Denmark (e.g. Mller, 1965; Henriksen, 1988),and generally tending more towards the 50 thanthe 65 per cent interpretation of critical basal

area.

The site index hypothesis

The site index hypothesis addresses the use ofstand height as an indicator of forest productivityand goes as follows: There is a direct and simplecorrelation between the height growth of a foreststand and its volume growth.

This general hypothesis originated withthe construction of the first yield tables thatused stand height to classify site (Baur, 1877,

1881) and subsequent research on variation insite productivity (Eichhorn, 1902, 1904; Gehr-hardt, 1909, 1921). For numerous tree species,including Sitka spruce, classification of siteproductivity by stand height has become oneof the most widely applied principles of forestgrowth.

The site index hypothesis is tested as part of thedata presentation. To avoid any spurious correla-tion due to the fact that stand height is used in thecalculation of volume, the site index hypothesisis tested based on the correlation between stand

height and stand basal area immediately prior tofirst thinning, using 95 per cent statistical signifi-cance as the test criteria.

Eichhorns rule

The third hypothesis has become known as Eich-horns rule and goes as follows: The total volume


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C-grade plots were thinned to a residual basalarea of 3640 m2 ha1, and each thinning re-moved 46 m2 ha1. The D-grade plots werethinned to a residual basal area of 3235 m2ha1, and each thinning removed 47 m2 ha1.In the thinned plots, the basal area level relative

to the A-grade plots decreased through the periodof observations from 85 to 65 per cent in the Bplots, from 65 to 50 per cent in the C plots andfrom 55 to 40 per cent in the D plots.

At stand establishment an admixture of multi-ple-stemmed mountain pine (Pinus mugo var. ro-tundata (Link) Hoopes) was planted as a nurse.Due to lack of mensurational observations onthe mountain pine, this was not considered in theanalysis. It is known, however, that mountainpine soon gets overgrown by Sitka spruce andthat the volume of mountain pine as an admix-

ture to Sitka spruce is only a few cubic metres perha and thus inferior. The spacing between rowsof Sitka spruce was 2.5 m at stand establishment,with a spacing of 1.51.6 m or less in rows. Theprovenance is unknown. When experiment MBwas initiated, all plots were fully stocked and un-thinned.

The experiment was initiated, thinned andmeasured for the first time autumn 1935 at theages of 24 years (block 1) and 26 years (block 2).Until autumn 1951 block 1 and 2 were intact, butsubsequently broke up due to attacks by the great

spruce bark beetle (Dendroctonus micans Kug.)(Ghrn et al., 1954; Henriksen, 1958, 1961).The analysis comprised the 16-year observationperiod between 1935 and 1951.

The situation at the initiation of the experiment(Figure 1) demonstrates a considerable variationin the sites inherent potential for tree growth. Atthis stage, there was no clear correlation betweenbasal area and stand height. Data also indicatethat differences in site index by height originateat an early stage and are mainly due to differencesin early growth.

Except for the A-grade plots, the replicationplots for each thinning regime sampled the rangeof site conditions in terms of average sand depth(Figure 2). The basal area and volume produc-tion from stand establishment to first thinning de-creased with increasing sand depth. Early heightgrowth was uncorrelated, while stand height atthe end of the observation period was negativelycorrelated with sand depth.

Experiment MF

Experiment MF (Skovsgaard, 1997a) includednine plots with thinning grades A, DB or D. Inthe DB plots, thinnings were heavy (D-gradelevel) until stand height reached 14 m and were

then reduced (B-grade level). The analysis com-prised two A plots, two DB plots and four Dplots. An additional A-grade plot was not in-cluded in the analysis due to an extremely het-erogeneous stand development in this plot. Allplots were surrounded by an unthinned buffer,except two of the D plots where the buffer wasalso thinned. Nett plot sizes of the included plotsvaried between 0.1554 and 0.4063 ha with anaverage of 0.3231 ha.

The basal area of the unthinned A plots in-creased to 63 m2 ha1. The D plots were thinned

to a residual basal area of 1822 m

2

ha

1

, andeach thinning removed 14 m2 ha1. From a standheight of 14 m, thinning grade in the DB plotswas reduced, and the basal area subsequently in-creased to 30 m2 ha1. The basal area level in theD plots (as well as in the DB plots) decreasedthrough the observation period from 55 to 35 percent, relative to the A-grade plots.

At stand establishment, an admixture of mul-tiple-stemmed mountain pine was planted asa nurse, but subsequently overgrown by Sitkaspruce. Due to lack of mensurational observa-tions on the mountain pine, this was not consid-ered in the analysis. The spacing for Sitka sprucewas 2.70 1.35 m at stand establishment. Plant-ing stock was believed to be of Nystrup origin,i.e. from seeds collected in or around experimentMB. When experiment MF was initiated, all plotswere fully stocked and unthinned.

The experiment was initiated, thinned andmeasured for the first time in autumn 1957 at theage of 28 years. Until the wind storm on 2425November 1981, the whole experiment was in-tact. The most recent measurement prior to thestorm is from autumn 1979. The analysis com-prised the 22-year observation period between1957 and 1979.

The situation at the initiation of the experi-ment (Figure 3) demonstrates that there was aconsiderable variation in the sites inherent po-tential for tree growth. At this stage, there was nocorrelation between basal area and stand height.Stand height at the end of the observation period


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correlated well with stand height at the initiationof the experiment. Also for this site, data indicate

that differences in site index by height originateat an early stage and are mainly due to differencesin early growth.

Comparing experiment MB and MF

Experiment MB and MF were both located oncoastal sand, but on quite different site types.

In both cases, the site potential for tree growthis high. The experiments were grown during

different, but overlapping periods of time (in-dicating partially similar and partially differentclimatic and environmental conditions). The twostands were most likely very similar in regard togenetic properties, and both had an admixtureof mountain pine at stand establishment. Foridentical nominal thinning regime, the actuallypractised thinning differed significantly betweenexperiments.

Figure 1. Experiment MB. Scatter plots of mensurational stand characteristics at installation of the experi-ment (subscript BEG.) and at the end of the observation period (subscript END). Symbols lie on actual datapoints and denote the thinning grade that was subsequently applied in that plot (bold and italics: block 1;ordinary font: block 2). Variables: Hdom= stand height (average height of the 100 thickest trees per ha),G= basal area at 1.3 m above ground level, V= stand volume (total volume production), subscript B.T. =before thinning, and subscript A.T. = after thinning. The correlation (r) between stand height and basal areaat initiation of the experiment was for block 1 and 2 together: r=0.251N.S., for block 1: r= 0.952*, forblock 2: r=0.501N.S..


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As a first result, the lack of correlation betweenstand height and stand basal area immediatelyprior to first thinning in both of these experimentsrefute the site index hypothesis and, in turn, Eich-horns rule for Sitka spruce growing under siteconditions like these. The fact that the experi-

mental areas were small (compared with a wholegrowth region) not only fortifies this conclusionbut also limits its general validity.

Methods

The analysis of the thinning response focuses onthe effect of thinning on mean annual volume in-

crement during the observation period and meanannual volume increment for the whole rotation.In this case, the rotation period is from stand es-tablishment to the end of the observation period.

Due to possible site-specific differences inthinning response, separate analyses were car-

ried out for experiment MB and MF as well asfor block 2 of experiment MB. Moreover, defini-tions of thinning regimes differ in the two ex-periments. Due to the experimental setup andthe large variation in site and stand conditionsin both experiments, more detailed analyses ofthinning response patterns, for example for dif-ferent age periods, thinning intervals or thinningtypes, are not feasible.

Figure 2. Experiment MB. The effect of sand depth on mensurational stand characteristics at installationof the experiment (subscript BEG.) and at the end of the observation period (subscript END). SAND = sanddepth. Other symbols and variables as for Figure 1. The correlation (r) between sand depth and the fol-lowing variables were, for stand height at initiation of the experiment: r= 0.319N.S., stand height at end ofobservation period: r=0.604*, basal area before first thinning: r=0.599 (P= 0.051), and volume beforefirst thinning: r=0.687*.


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To account for between-plot variation at thetime when the experiment was installed, andthereby also for variation in site potential withineach experiment, total volume production immedi-ately prior to first thinning was used as a covariatein covariance analyses. Due to the scarce number

of replications for each treatment, mean annualvolume increment was the dependent variable in asimple linear regression with volume immediatelyprior to first thinning as independent variable andthinning treatment (A, B, C, D or DB) as a classvariable. Although non-linear regression with anasymptote may be biologically appropriate, anal-yses of variance demonstrated that a straight lineis a reasonable proxy in this case.

In mathematical terms, the full model may bespecified as IVT ij=j+jVBEG. B.T. ij+ij, whereIV T denotes mean annual volume incrementfrom age to age T, VBEG. B.T. denotes stand vol-ume immediately prior to first thinning, , and are parameters to be estimated and tested, sub-

script i refers to plot identification (plot number),subscriptj refers to nominal thinning regime, andthe residuals ij are normally and independentlydistributed with zero mean and common variance.

A supplementary analysis with sand depthas the covariate was performed for experimentMB. This analysis comprised height growth aswell as volume growth during the observationperiod. In this case, the model may be specified

Figure 3. Experiment MF. Scatter plots of mensurational stand characteristics at installation of the experi-ment (subscript BEG.) and at the end of the observation period (subscript END). Symbols lie on actual datapoints and denote the thinning grade that was subsequently applied in that plot ( = DB-grade thinning).Other symbols and variables as for Figure 1. The correlation (r) between stand height and basal area atinitiation of the experiment was r= 0.391N.S..


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in mathematical terms as IHT ij=j+jSij+ijorIV T ij=j+jSij+ij, where IH T denotes meanannual height increment from age to age T, Sdenotes average sand depth and other symbolsare as explained above.

The effect of thinning on stand volume growth

in each experiment was evaluated at a stand vol-ume (or site potential) corresponding to the av-erage volume for the unthinned A-grade plots atinstallation of the experiment. These plots wereused in the construction of a site-specific growthmodel for Sitka spruce in Denmark (Skovsgaard,1997a). Results of the analysis therefore relate di-rectly to the growth model and may be used whenadjusting site-specific growth predictions fromthe model to different thinning practices. Alter-natively, LS-means estimates would have been anatural choice (cf. Skovsgaard, 2006).

In a summary of results, the relationship be-tween stand volume growth and thinning gradewas modelled using a quadratic regression, withboth variables scaled relative to the value for theunthinned control plots. The response patternsshowed that a quadratic regression was a suit-able common model for the purpose. Thinninggrade was expressed in terms of residual basalarea at the time when the thinning regimes hadbeen adjusted to the target basal area level and

at the end of the observation period. This relatesimmediately to the operational aspect of thinning(as opposed to, for example, mean basal area fora period), and because thinning grades do changeduring the observation period, this approach tothe evaluation should give an indication of the

range of relative basal areas that have produced aspecific thinning response.

The basal area at which stand volume growthis reduced by 5 per cent compared with its maxi-mum was derived from the quadratic regressionfor each thinning experiment. The thinning re-sponse hypothesis for Sitka spruce is thus testedat a site potential corresponding to the stand vol-ume average for the unthinned A-grade plots atthe installation of each experiment.

Results

Experiment MB

The relation between mean annual volume in-crement and total volume production fromstand establishment to first thinning is shown inFigure 4. Analyses of variance have shown thatthe regressions for actively thinned plots on gen-tle terms may be regarded as parallel straight lines

Figure 4. Experiment MB. The effect of thinning grade on mean annual volume increment (IV) evaluated as afunction of total volume growth before first thinning (VBEG. B.T.). Left: for the observation period, age 2440years (block 1) or 2642 years (block 2). Right: for a full rotation, age 440 years (block 1) or 442 years(block 2). Symbols lie on actual data points and denote the thinning grade that was subsequently applied inthat plot (bold and italics: block 1; ordinary font: block 2). In the graph on the left, the four parallel regres-sion lines correspond to volume growth levels for each thinning grade of block 2 (estimates in Table 1).


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Table 1: Experiment MB. Parameter estimates for mean annual volume increment during the observation period(I

V) as a function of total volume production immediately prior to first thinning (V

BEG. B.T.). The column I

V

2440/2642 quantifies mean annual volume increment for a site potential corresponding to the average volumelevel of the A-grade plots at installation of the experiment (block 2: 256.6 m3 ha1; block 1 + 2: 268.6 m3 ha1).IV2440/2642 is given in absolute figures, relative to volume increment for the A grade and relative to volumeincrement for the B grade (which has the highest volume increment level)

IV2440/2642

Thinning Parameter Estimate sm3 ha1year1 % %

Block 2 Slope 0.099 0.025D grade Intercept 7.97 6.44 33.37 90.9 87.2

C grade Intercept correction relative to D grade 1.12 1.29 34.49 93.9 90.1B grade Intercept correction relative to D grade 4.90 1.29 38.27 104.2 100.0A grade Intercept correction relative to D grade 3.36 1.57 36.73 100.0 96.0

Block 1 + 2 Slope 0.093 0.025D grade Intercept 9.77 6.86 34.75 93.1 90.4C grade Intercept correction relative to D grade 1.89 1.32 36.64 98.2 95.3B grade Intercept correction relative to D grade 3.70 1.35 38.45 103.0 100.0A grade Intercept correction relative to D grade 2.57 1.46 37.32 100.0 97.1

(F-test for volume increment during the observa-tion period, block 1 and 2 together: P= 0.120,no significant interaction (P= 0.533); block 2alone: P= 0.113). In block 2, the increment levelwas highest for the B-grade thinning, whereuponit decreases with increasing thinning grade. In

block 1, the increment level was highest for theC-grade thinning.

Regression estimates for the observation pe-riod appear in Table 1 along with a quantifi-cation of the thinning response. The thinningresponse corresponds to a site potential as indi-cated by the volume level of the A-grade plots atinstallation of the experiment. The result was thesame for block 1 and 2 together and for block 2alone: relative to the unthinned A-grade volumegrowth was stimulated slightly by light thinning,but reduced with more heavy thinning. Regres-

sion analyses for the full rotation yields similarresults, although slightly less pronounced (forblock 1 and 2 together, relative volume incrementfor B grade: 102 per cent, C grade: 98 per cent,D grade: 94 per cent). The percentage values foreach thinning grade varied up to 2 per cent unitswithin the range in site potential that is indicatedby each plots volume production from stand es-tablishment to first thinning.

The combined effect of sand depth and thinningon height and volume growth during the observa-tion period is shown in Figure 5. For height growth,there was no statistically significant effect of thin-ning, but disregarding D-grade plots height growthwas reduced with increasing thinning grade. For

the range of sand depths represented here, thedune sand had a negative effect on height growth,but this apparently appeared quite late in the rota-tion (compare with Figure 2). The dune sand alsohad a negative effect on volume growth. The effectof thinning on volume growth followed a patternsimilar to that in Figure 4, but less clear and consis-tent. The reason for this probably is that long-termsite productivity, in addition to (average) sanddepth, also depends on other factors (for example,terrain slope, ground water level, water movementin the soil, possible water logging, etc.).

Experiment MF

The relation between mean annual volume in-crement and total volume production fromstand establishment to first thinning is shownin Figure 6. There was a very strong effectof thinning on volume growth, and analyses


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Figure 5. Experiment MB. The effect of sand depth and thinning grade on mean annual height increment(IH) and on mean annual volume increment (IV) during the observation period, age 2440 years (block 1)or 2642 years (block 2). Symbols lie on actual data points and denote the thinning grade that was subse-quently applied in that plot (bold and italics: block 1; ordinary font: block 2). The correlation ( r) between

sand depth and height growth was r=0.684*. The correlation between sand depth and volume growthwas r=0.913***.

Figure 6. Experiment MF. The effect of thinning grade on mean annual volume increment (IV) evaluatedas a function of total volume growth before first thinning (VBEG. B.T.). Left: for the observation period, age2850 years. Right: for a full rotation, age 450 years. Symbols lie on actual data points and denote thethinning grade that was subsequently applied in that plot ( = DB-grade thinning). In the graph on the left,the three parallel regression lines correspond to volume growth levels for each thinning regime (estimatesin Table 2).

of variance have shown that the regressionsmay be regarded as parallel straight lines (F-testfor volume increment during the observationperiod: P = 0.0001, no significant interaction(P = 0.739)).

Regression estimates for the observation periodappear in Table 2 along with a quantification of

the thinning response. The thinning response cor-responds to a site potential as indicated by thevolume level of the A-grade plots at installationof the experiment. For this site, heavy thinningconsiderably reduced volume growth. Regressionanalyses for the full rotation yielded similar re-sults, although slightly less pronounced (relative


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Table 2: Experiment MF. Parameter estimates for mean annual volume increment during the observation period(IV) as a function of total volume production immediately prior to first thinning (VBEG. B.T.). The column IV2850quantifies mean annual volume increment for a site potential corresponding to the volume level of the A-gradeplots at installation of the experiment (128.9 m3 ha1). IV2850 is given in absolute figures and relative to volumeincrement for the A grade (which also has the highest volume increment level)

IV2850

Thinning Parameter Estimate s m3 ha1 year1 %

Slope 0.036 0.008D grade Correction to intercept of DB grade 1.69 0.46 12.45 57.7DB grade Intercept 9.50 1.07 14.14 65.6A grade Correction to intercept of DB grade 7.42 0.53 21.56 100.0

volume increment for DB grade: 73 per cent,D grade: 67 per cent). The percentage values foreach thinning regime varied up to 3.5 per cent

units within the range in site potential that is in-dicated by each plots volume production fromstand establishment to first thinning. Note thatthe D-grade thinning in experiment MF wassomewhat heavier than the D-grade thinning inexperiment MB.

Summary of results

The effect of thinning on the stand volumegrowth of Sitka spruce for a full rotation is

summarized in Figure 7 for two site types inthe coastal dunes of Denmark. The thinningresponse is clearly site dependent. On sand-bur-ied moraine (experiment MB), thinnings frombelow to a residual basal area of 5065 per centof the basal area of an unthinned stand reducestand volume growth by 5 per cent comparedwith its maximum for any thinning grade. Onsandy soils with a high and fluctuating groundwater table (experiment MF), a 5 per centreduction is arrived at with only light thin-nings from below to a relative basal area of

9095 per cent. For this site type, the thinningpractices that are currently used in forestry lead to aconsiderable reduction in volume growth.

Discussion

Following a discussion of the site-specific thin-ning response pattern of experiments MB and

MF in Denmark, results from these are com-pared with six similar experiments in GreatBritain, Norway and Sweden. The experiments in

Denmark were observed for a period that com-pares with a full rotation or more. Results fromthe other experiments originate from shorterobservation periods.

Sand-buried moraine (experiment MB)

The effect of thinning grade on stand volumegrowth in experiment MB has been examinedpreviously by Henriksen (1961). He used basalarea immediately prior to first thinning (at instal-

lation of the experiment) as a covariate and ar-rived at a thinning response pattern similar to thatin Figure 4, except that the volume growth levelfor the A grade in block 2 was slightly below thelevel of the C-grade thinnings. Henriksens obser-vation period for block 2 continued for 2 yearsmore (until autumn 1953) than in this analysis,and during those 2 years the volume incrementof the C-grade plots increased by 911 m3 ha1year1 to their highest level during the wholeperiod, while at the same time the volume incre-ment of the unthinned A-grade plot decreased by

11 m

3

ha

1

year

1

to its lowest level during thewhole period.The highly site-specific thinning response

in even-aged Sitka spruce (Figure 7) is prob-ably a consequence of the species sensitivity tochanges in water supply (Peterson et al., 1997).The apparent effect of sand depth in experimentMB may thus relate to the occurrence of flatterrain, a high ground water table and a shal-


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low root system where the sand was deep, slop-ing terrain, good water movement in the soil

and a deeply developed root system where thesand cover was thin (details of topography, soiland root systems were described by Henriksen,1951, 1958). The observed increase in criticalbasal area with increasing sand depth is there-fore more likely a hint of the strong reductionin stand volume growth that was observed withthinning in experiment MF and which may beattributed to the water supply situation (cf. thefollowing discussion of results for experimentMF).

The results for experiment MB demonstrate

that basal area and volume growth decrease withan increasing depth of dune sand above a morefertile and moisture holding sub-soil. This holdsfor young, unthinned stands as well as for olderstands, thinned or unthinned. In consequence, thesite potential for volume growth and the carryingcapacity for stand volume depend directly uponsand depth or other factors that correlate withsand depth, for example, topography, position of

the ground water table, water movement in thesoil, possible water logging, etc. The thinning re-

gime influences how much of the site potential isutilized for tree growth. The large variations inthis experiment demonstrate that the site poten-tial may vary considerably in dune regions, evenwithin short distance (block 1 was 0.6 ha, block2 was 1.4 ha).

Remarkably, sand depth does not appear toinfluence height growth very much and only at alate stage. The background for this is not immedi-ately obvious, but may relate to the co-existenceof deep sand and a high ground water table onthe high parts of experiment MB. So, the changes

in ranking of plots by stand height is most likelythe combined effect of several factors. Henriksen(1958) reported a similar observation for anothersimilar dune site (Vilsbl dune plantation). More-over, for a larger range of sites with a sand coverof thickness between 30 and 200 cm, he foundthat the site index by stand height decreased withincreasing sand depth.

Figure 7. The effect of thinning grade on stand volume growth in even-aged Sitka spruce. The graphs showvolume increment in experiment MB and MF during the period from first thinning to the end of the rota-tion at stand break-up (IVBEG.-END) as a function of relative basal area for each thinning grade (GREL.).

Basal area as well as volume increment are gauged on a scale relative to basal area and volume increment,respectively, of the unthinned control and are corrected for variation in inherent site potential within eachexperiment. Left: evaluation based on basal area at the time when the thinning grades had been adjusted totheir target levels (subscript EARLY). Right: evaluation based on basal area at the end of the observationperiods (subscript END). Experiment MB, block 1 and 2 together: ; block 2: . Experiment MF: . Thethinning response was modelled by a quadratic regression line for each experiment. Densely dotted line:IV= 97.9% (equal to a 5 per cent reduction in stand volume growth compared with the highest incrementfor any thinning grade in experiment MB). Widely spaced dots: IV= 95% (equal to a 5 per cent reduction instand volume growth compared with the highest increment for any thinning grade in experiment MF).


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The effect of sand depth (and other factors)on stand height at the end of the rotation indi-cates that the potential for at least height growthchanges during stand development. Supplemen-tary analyses have shown that height at that stagedoes not contribute further explanation when

total volume production from stand establish-ment to first thinning is already in the model andthat height at the end of the rotation alone pro-vides less explanation than volume at the installa-tion of the experiment.

Sandy soil with a high and fluctuating groundwater table (experiment MF)

Experiment MF had a high and fluctuating groundwater table (Henriksen, 1958; Skovsgaard, 1997a,b) and trees with shallow root systems (Skovsgaard,

1997b; during wind storms since November 1981trees are being uprooted rather than breaking). Thiscombination may be a primary cause of the markedthinning response on this site. The explanationmay be as follows. Due to the reduction in foliagebiomass with heavy thinning, the remaining croptrees consume less water through transpiration. Inconsequence, the ground water table may rise. Thismay lead to drowning of roots, and as some rootsdie, the residual crop trees consume even less water,and more roots may die. As a consequence, volumegrowth is further reduced. The age and season for

thinning onset and for subsequent thinnings mayalso play a role. Supplementary analyses of detailsin the thinning response pattern have clarified thatthe reduction in volume growth is not restrictedto periods with either high or low precipitation(or growth seasons immediately following suchperiods).

At present, there is no information availablethat could further clarify causes for the large vari-ation in volume growth from stand establishmentto first thinning in experiment MF.

Other site types

Four distinctively different site types have beenidentified for Sitka spruce in Denmark in relationto height and volume growth patterns (Skovsgaard,1997a). In addition to the two site types alreadyaddressed, these include sites in the glacial moraineregion and sites on sandy, poor soils without a

more fertile subsoil or ground water within reachof the tree roots. Unfortunately, there are no thin-ning experiments with unthinned control plots inSitka spruce on these other two site types in Den-mark.

A common drawback with many thinning ex-

periments is the lack of an unthinned control orthat only merchantable volume is available in theliterature (see, for example, Lynch, 1980, Joyceand OCarroll, 2002). However, six experimentsin Great Britain, Norway and Sweden are di-rectly suited for comparison with the results ofthis analysis and may give an indication of theexpected thinning response on moraine sites (fora discussion of the potential of some other experi-ments, see Skovsgaard, 1997a).

In two thinning experiments in Wales (brownforest soil, precipitation 2400 mm year1) and

Scotland (peat/podzol/gley, precipitation 1400mm year1), respectively, volume growth signifi-cantly decreased with increasing thinning grade(Hamilton, 1981; Rennolls, 1985). The reduc-tion was largest in the Scottish experiment, andthis supports the hypothesis that the thinningresponse in even-aged Sitka spruce is stronglyrelated to the water balance (or, more specifi-cally, that stand volume growth is sensitive tochanges in the water supply). With reservationsregarding variation in site potential for volumegrowth within each experiment (cf. Hamilton,

1981), the results from these two experiments in-dicate that the volume growth of even-aged Sitkaspruce stands more generally may be sensitive tothinning.

Unpublished data from three thinning ex-periments conducted by the Norwegian ForestResearch Institute in Lomeland and Njskogenat Egernsund in the southwest part of Norwaysupport this indication. Within each of theseexperiments, considerable variations occurredin volume production from stand establishmentto first thinning as well as in site index by stand

height.The experiments in Lomeland (sandy moraine,hilly terrain, precipitation 13001400 mm year1)were experimental replications with two differentprovenances. The observation period goes fromfirst thinning at an age of 4950 years (Hdom10.513.7 m) and 17 years ahead. Analyses withtotal volume production from stand establish-ment to first thinning as a covariate showed that


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a medium to heavy thinning (relative basal area 6070 per cent) reduced volume growth duringthe observation period by up to 13 per cent com-pared with the unthinned control.

The experiment in Njskogen (moraine, flat ter-rain, precipitation 1100 mm year1) was atypical

because of growth stagnation in the unthinnedcontrol plot when the stand was around 40 yearsof age (Hdom 17 m, N 3000 ha1, G 51 m2ha1). During the first 8 years of observation (age3139 years), medium to heavy thinning reducedvolume growth by 45 per cent compared withthe unthinned control.

Unpublished data from a thinning experimentconducted by the Swedish University of Agricul-tural Sciences at Hallandssen in the southwestpart of Sweden (sandy, podzolized moraine, pre-cipitation 1000 mm year1) showed a thinning

response pattern similar to experiment MB withmaximum volume growth with a light thinning.The experiment was installed at an age of 42 years(Hdom 24.3 m) and was abandoned 5 years laterdue to storm damages.

Site conditions for these six experimentsvary and most likely do not occur exactly simi-lar in Denmark. For example, the precipitationis generally higher for the experiments outsideDenmark (annual precipitation in Denmark variesfrom 500 mm (dry locations in dry years) to 1200mm (moist locations in wet years). However, the

reduction in volume growth with heavy thinningis common to all the experiments examined. Themagnitude of the response varies from site to siteand probably also depends on stand age and thelength of the observation period.

It is a general experience that the water balanceis important for the growth and development ofSitka spruce (Henriksen, 1958). The determi-nants in this regard include the magnitude andseasonal distribution of precipitation, plant avail-able water, the development and distribution ofroots and the trees water regime. Most likely, the

balance between these factors also influence thethinning response.

Conclusions

The effect of thinning from below on stand volumegrowth in even-aged Sitka spruce is strongly sitedependent, but heavy thinning usually leads to a

reduction in volume growth as compared with anunthinned stand growing under similar site con-ditions. For some sites, typically those with stableand ample water supply and a deeply developedroot system, stand volume growth is maximizedthrough light thinning. For some other sites, typi-

cally those with a high ground water table and ashallow root system, thinning reduces stand vol-ume growth and considerably so with increasingthinning grade. There seems to be a gradient inthe reduction, so that it is lessened with increas-ing rooting depth and with increasing stability ofwater supply.

On sand-buried moraine sites in the coastaldunes of Denmark (sand depth 2068 cm), thin-nings from below to maintain a residual basalarea of 5065 per cent of the basal area of anotherwise identical unthinned stand on a similar

site reduce stand volume growth by 5 per centcompared with its highest value for any thinninggrade during the same period of time. Thinningsto a residual basal area of 50 per cent reducevolume growth by 510 per cent. On sandy soilsnear to ground water practically any thinningwill reduce volume growth. Based on analysesof foreign experiments on comparable soils, mo-raine region sites probably lie somewhere be-tween these extremes. There are no experimentsavailable that could provide indications of thethinning response pattern on sandy, poor soils

without a more fertile sub-soil or ground waterwithin reach of the roots.This conclusion corroborates the hypothesis

for the effect of thinning on stand volume growthin even-aged Sitka spruce as regards sand-buriedmoraine with a thin to moderately thick cover ofsand, but falsifies the hypothesis for some othersite types. The thinning response hypothesis forSitka spruce should be modified accordingly.

In forestry practice, the decision whether to thinand the limits for an acceptable thinning practiceare determined by economic, silvicultural, envi-

ronmental or other criteria rather than solely byprinciples of growth and yield. So, a detectablereduction in stand volume growth is not by itselfsufficient indication to refrain from thinning oradjust thinning practices.

In a broader perspective, this case study chal-lenges all-else-equal assumptions in the analysisof thinning effects and more specifically those re-garding local variation in site productivity. Two


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FORESTRY102

experiments in Denmark demonstrated that con-siderable variation may occur in site productiv-ity in apparently homogeneous stands, whetherthinned or unthinned. To account for such varia-tion, the suggestion is to include an unbiasedpre-treatment measure of site productivity, such

as total volume production, as a covariate in theanalysis. Large variation in site conditions obvi-ously leads to increased demands on replicationof experimental treatments and on the distribu-tion of plots relative to this variation.

The lack of correlation between stand heightand basal area prior to first thinning of the twoexperiments challenges two site productivity hy-potheses, namely, the general assumption thatstand volume growth correlates well with standheight (the site index hypothesis, i.e. site produc-tivity can be estimated based on stand height) and

the assumption that, for a given species, total vol-ume production at a given stand height is iden-tical for all site classes (Eichhorns rule). Theseproblems are reinforced on sites where standvolume growth depends heavily on thinningpractices. In such cases, one or more additionaldimensions should be included in the estimationof site productivity, and Eichhorns rule shouldbe used only for stands that are thinned accord-ing to similar principles and where, at the sametime, soil conditions are similar.

The conclusions of this analysis have imme-

diate implications for growth predictions forSitka spruce in Denmark. Due to considerablesite-dependent variation in the relation betweenheight and volume growth (Skovsgaard, 1997a),site-specific trajectories for stand height vs totalvolume production may be needed to ensure ac-curate growth predictions. This is most notablythe case for highly productive site types suchas the two experiments in this case study. Theyproduce substantially more volume per heightgrowth unit than anticipated in the current yieldtable (Henriksen, 1958). Reliable estimation of

volume growth is further hampered because aux-iliary stand productivity indicators based on, forexample, the self-thinning rule (i.e. V=kN,where Vis stand volume, Nis stem number, k isa species-, site- or stand-specific coefficient and is a universal constant) or an index of stand den-sity (e.g. N=CDE, where Nis stem number, D isquadratic mean diameter, C is a species-, site- ortreatment-specific coefficient and E is a univer-

sal constant) have proved unreliable for such sitetypes, mainly due to changing rootsoil interac-tions during stand development and the poten-tially extreme effect of thinning on tree and standgrowth (Skovsgaard, 1997a).

The recommendation is that estimates of vol-

ume growth per height growth unit should bebased on measurements in unthinned standsor plots, unless a suitable local growth modelis available and thinning practices do not varybetween stands for which predictions are beingmade. The unthinned stands can serve as a refer-ence for thinning practices as well as a standardfor potential volume growth. Together with aheight growth model, the trajectory for standheight vs total volume production in unthinnedstands may thus be used as a basis for growthpredictions for any stand on a similar site.

If sufficiently tall, unthinned stands are notavailable, the results of this analysis may pro-vide an indication of the potential error by usinginformation on the total volume production ofthinned stands instead. Note that the volume ofdead trees in unthinned, experimental plots ofSitka spruce usually amounts to no more than15 per cent of the total volume production for awhole rotation (this holds for dune sites as wellas other site types).

In some parts of the world, for example in con-tinental Europe, only few data are available locally

for unthinned stands. To ensure reliable growthand yield prognoses, it is recommended to establishand maintain unthinned observation plots in man-aged stands. This recommendation could also holdfor other species and in other parts of the world.

Funding

This analysis was part of a larger investigationregarding the growth and yield of Sitka sprucein Denmark and a subsequent study on the es-

timation of forest site productivity. The initialwork was carried out 19851994, mainly dur-ing periods of employment at Lvenholm ForestEstate, Jutland, the former Royal Veterinary andAgricultural University, Copenhagen, and For-est and Landscape Denmark. The final work andthe preparation of this paper took place 19992000 during a sabbatical stay at Southern CrossUniversity, Australia. The initial work and the


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sabbatical were supported by the Danish Agri-cultural and Veterinary Research Council (grantsno. 13-3977 and 9901207), and the sabbaticalfurthermore by Carlsen-Langes Foundation andVemmetofte Aristocratic Convent.

Acknowledgements

The Norwegian Forest Research Institute and theSwedish University of Agricultural Sciences provideddata and information on four thinning experimentsthat were previously unpublished.

Conflict of Interest Statement

None declared.

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