Irradiance in thinned Norway spruce (Picea abies) stands and the possibilities to prevent suckers of broadleaved trees

Forest Ecology and Management, 20 {1987) 307-319

307

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Irradiance in T h i n n e d N o r w a y Spruce ( P i c e a abies) Stands and the Possibilities to P r e v e n t S u c k e r s of B r o a d l e a v e d Trees TORD JOHANSSON

Swedish University of Agricultural Sciences, Department of Forest Yield Research, S- 770 73 Garpenberg (Sweden) (Accepted 30 October 1986)

ABSTRACT Johansson, T., 1987. Irradiance in thinned Norway spruce (Picea abies) stands and the possibilities to prevent suckers of broadleaved trees. For. Ecol. Manage., 20: 307-319. Both incoming shortwave radiation (Rg) and photosynthetically active radiation (PAR) in percentage of full daylight were measured at the same time by point and strip sampling in four plots (0.1 ha) of Picea abies (L.) Karst. The standard deviations (%) of Rg and PAR were, respectively, 11.1 and 9.8 at 64 points, 15.7 and 13.9 at 32 points, and 24.7 and 23.8 at 16 points per plot. A period of at least 40 s per strip (30 m rain- 1) gives a CV (coefficient of variation) of 30%. There is no significant difference between relative irradiance (RI) estimated by the point method (64 points) and by the strip method ( 8 strips). Curves of RI (Rg and PAR) and basal area (m 2 ha- 1), diameter sum (m ha- 1) and density ( stems ha- ' ) of fifteen trials with different thinning programmes are presented. Irradiance (R~) in heavily thinned stands was 3-14% of irradiance on an open place. The irradiance, R~, in extra-heavily thinned stands is 12-27%, and in unthinned stands, 1-3% that of an open place. The Rg curve lies above the PAR curve in all cases. Some practical implications of the study are presented. Heavy thinning of Norway spruce stands gives RI (Rg) values < 10% at basal area of >/25m2 ha-1 which is necessary to minimize development of suckers of broadleaved trees.

INTRODUCTION

Light is an important factor of a plant's vitality and growth rate. Several authors have investigated the role of light versus growth rate of plants of different species (Fricke, 1904; Zon and Graves, 1911; Bates, 1925; Grasovsky, 1929; Shirley, 1929, 1932; Buell and Gordon, 1945; Oosting and Kramer, 1946; Jarvis, 1964 ). The minimum requirement of light for species of conifers has been investigated in laboratory experiments by Bates (1925), who concluded that the requirement of 0.7-2.3% of short- wave solar radiation at noon for the survival 0378-1127/87/$03.50

© 1987 Elsevier Science Publishers B.V.

308 of plants is not the same as that experienced in the field, where plants are killed by an average of 10% of daylight total energy at noon. Shirley (1932) reported that light values below 17% of diffuse radiation on cloudless days result in unreliable plant establishment. Light intensities below 5 % of intensity at noon on cloudless days prevent maple-basswood reproduction and ground cover from invading spruce-fir forest (Buell and Gordon, 1945). Jarvis (1964) reported that the compensation point (incidence of balance between respiration and photosynthesis) of Quercuspetraea (Matt.) Liebl. seedlings was 2% of daylight. Johansson (1986) reported that the heights of suckers of birch stumps were lower at a photon flux density of 25 ]~E m -2 s- 1 than at 200 ttE m -2 s- i. Measurements of light have been made in many ways (Anderson, 1971; Kubin, 1971 ) and by using many different instruments ( eg. Burns, 1927; Gast, 1930; Wassink and van der Scheer, 1951; Logan, 1955; Fairbairn, 1958; Monteith, 1959; Rediske et al., 1963; Federer and Tanner, 1965; Biggs et al., 1971; Burnell and Merritt, 1974; Jarvis and Leverenz, 1983). The results of the experiments mentioned above emphasize the difficulties in the methodology of measuring light, illustrating such factors as: how to include sunflecks; whether measurements should be continuous or instantaneous; over what period should radiation be summed, e.g., annual radiation, radiation during the growth period or radiation during shorter periods. Some of the questions may be clarified by the purpose of the experiment, whereas others are more difficult to answer. Photographic computation of light conditions in stands has also been attempted (Hill, 1924; Evans and Coombe, 1959; Anderson, 1964; Brown and Worley, 1965 ). Recently, an advanced computer technique has been developed for this approach (Olsson et al., 1982). Many reports deal with measurements of irradiance in stands of different species (Cieslar, 1904; Salisbury, 1936; N~igeli, 1940; Wellner, 1948; Miller, 1959; V~zina, 1964; V~zina and P~ch, 1964; Gay et al, 1971; Kellom~iki and Oker-Blom, 1983; Masahiro and Niki, 1983; Baldocchi et al., 1984; Kuusipalo, 1985). One of the most frequently occurring softwood species in Scandinavia is Norway spruce (Picea abies (L.) Karst). Several workers have made light measurements in Norway spruce stands (Knuchel, 1914; Atkins and Stanbury, 1930; Baumgartner, 1952; Ovington and Madgwick, 1955; V~zina, 1961; Mitscherlich et al., 1967; Schomaker, 1968). Most of the investigations were aimed at answering how light intensity varies within and between stands, either instantaneously, during the day, or throughout the season. The plots examined were often small (10-50 m 2) and the stands contained both the dominant trees and a understory of different species of soft and hardwoods and tall herbs. In the present study the plots are large (1000 m 2) and different thinning grades are examined. To study a lot of measured plots in a short time it was necessary to have a simple method to measure irradiance. With one sensor it was important to measure a whole plot rapidly during the period when the 'light climate'

309 outside the stand was homogenous. The method used in this study is similar to but technically more simplified and easier to use than the others mentioned. In Sweden, spruce stands contain twofold more broadleaved trees, especially birches (Betula pubescens Ehrh. and B. pendula R o t h ) , than do pine stands (Folkesson and Johansson, 1981 ). Attempts are generally made to minimize the number of sprouting birch stumps after thinning in order to avoid an understory of broadleaved trees developing later on in the stand or on the clearcut area. This report is an attempt to investigate the relation between commonly used parameters to describe a stand - - such as basal area, density (stems per ha) or diameter sum - - and the irradiance in the stand at various thinning grades. The reliability of two kinds of measurements of irradiance are also compared. Practical recommendations in cutting broadleaved trees without giving rise to a lot of suckers are then possible to make. The recommendations are based on the irradiance in a spruce stand after thinning and the light requirement to prevent production of suckers. MATERIAL AND METHODS Two different experiments in Norway spruce stands were conducted. The first compared two sampling methods (points and strips ), and the reliability of the methods depending on the number of measured points; the second, correlation of irradiance in W m -2 a n d / r E m -2 s -1 on basal area (m 2 h a - l ) , density ( stems h a - 1) or diameter sum ( m h a - 1).

Sampling methods Two thinning trials in Norway spruce (Picea abies ( L. ) Karst. ) were chosen from the thinning programme being studied at the D e p a r t m e n t of Forest Yield Research. Both trials were located in homogeneous stands of spruce. In Trial 1, with different thinning treatments, two plots with respective basal areas and densities of 17 m 2 ha -1 and 680 stems ha -1, and 38 m 2 ha -1 and 1940 stems ha-1, were used. The stand, planted on a ditched ground, was 38 years old. In Trial 2, two plots with respective basal areas and densities of 23 m 2 ha-1 and 716 stems ha-1, and 35 m 2 ha-1 and 674 stems ha-1, were used. This stand, 80 years old, had always been a forest area. Each plot was surrounded by a 10-m broad strip of spruces treated in the same way as the plot. The plots were rectangular ( 20 × 50 or 25 × 40 m ) . Trial 1 had been thinned 5 years previously, and Trial 2, 15 years, before the observations described here. There were no understory trees on the plots. Shortwave solar radiation Rg ( W m -2) was measured using an integrator (Li-510) and a solarimeter (Kipp and Zonen, Delft, Holland). Photosyntheticaly active radiation, PAR, (/rE m -2 s-1) was measured using a sensor (LiCor 190 M ) . The radiation in each plot was estimated in four sampling

310 TABLE 1 Main characteristics of the plots Thinning program/ Plot number

Basal area Density (m 2 (stems ha 1) ha-')

Volume (m 3 ha 1)

Dbh (cm)

Height Crown length (m) (m) (%)'

Unthinned 1 2 3

37.7 37.8 40.5

2690 3580 1820

315 243 292

13.3 11.6 16.8

15.8 12.3 14.7

9.3 8.4 12.7

59 68 86

Heavy thinning 4 5 6 7 8 9 10

22.3 23.0 23.5 24.9 27.3 30.7 31.5

760 716 2110 8300 955 1330 760

200 204 146 188 246 221 569

19.3 19.8 11.9 19.5 19.0 17.2 18.1

18.0 18.9 11.8 15.5 18.2 14.7 19.7

11.6 10.5 7.3 13.3 12.1 12.0 10.4

64 56 62 86 66 82 53

Extra-heavythinning 11 12 13

11.2 18.0 20.4

610 680 650

763 123 194

15.3 18.4 19.9

13.4 13.9 19.0

8.5 12.0 11.4

63 86 60

Thinning ~ o m t h e t o p 14 15

22.7 34.0

1690 1514

183 317

13.0 16.9

15.4 18.3

9.0 9.7

58 53

Estimations were made immediately after the plots were thinned. 'Percent of height.

arrangements: 64, 32, and 16 points per plot; and eight strips per plot. The points were randomly chosen with a starting point 0-2 m from the baseline and 0-2 m from the outer edge parallel to the observation lines. Measurements were taken by sensor and solarimeter mounted on a 3-m-long stretcher carried by a two-man team; each man also carried a strap-held integrator. For all measurements, point and strip, the stretcher was held at 50 cm height above ground. Measurements by sensor and solarimeter were taken concurrently. Point measurements were taken for periods of 60 s duration, strip measurements at periods of 20, 40, and 60 s per strip. The strips were 20 m long with a 3-m distance between any two strips. All estimations (point and strip ) were made from June to August at 10:00-14:00 (cf. Anderson, 1966). Light measurements were only made when the sky was cloudless.

Correlation of light intensity and silvicultural parameters Fifteen trials used in the Department's thinning programme were chosen for estimations of irradiance using the strip method as above. The plots (0.1 ha)

311 50

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30-

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~'o

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Figure 1. The relationship of the coefficientof variation, CV, using different periods of observation per 20-m strip, (incoming shortwave solar radiation R~ (--) and photosyntheticallyactive radiation, PAR (- - -)). in each trial have different canopy structures depending on the thinning programme ( Table 1 ). No destructive operations were allowed at the plot; because of that, no estimation of leaf area index (LAI) was possible. Four grades of thinning were used: unthinned; heavy thinning; extra-heavy thinning; and thinning from the top. Heavy thinning contains three to six cycles of thinning, with a cycle of 5-10 years and with 20-60% of basal area being removed in the first thinning. Extra-heavy thinning is one thinning with 60-70% of basal area being removed. The highest trees are removed in thinning from the top in three to six periods, with a removal of 20-50% of basal area at each time. Each strip was estimated for 30-40 s. Both incoming short wave and photosynthetically active radiation were measured. The estimated irradiance on a plot was compared (in percent) with the irradiance observed at the same time on an open place beside the plot. The irradiance is presented as a relative number (percent of unshaded sunlight), RI. Data were analysed by analysis of variance. The level of significance used throughout the work is 1%. The t-Test (Sokal and Rohlf, 1981 pp. 402-412) was used to analyse differences between point and strip sampling when the variances were unequal. RESULTS There were significant differences in irradiance within the four plots (Table 2 ). The lower the number of points observed per plot the lower were the ratios of variance. The standard deviation (%) of R~ and PAR, respectively, were 11.1 and 9.8 at 64 points, 15.7 and 13.9 at 32 points, and 24.7 and 23.8 at 16

312

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313 TABLE 2 Analysis of variance Source of variation

Plots Error Total Plots Error Total

Variance ratios (F) Incoming shortwave solar radiation (Rg) W m - 2

64 points 32 points 16 points 3 21.99"** 3 13.37"** 3 6.21"** 252 124 60 255 127 63 Photosyntheticallyactive radiation ( P A R ) ~ E m - 2 s 1 3 21.24"** 3 15.01"** 3 5.84*** 252 124 60 255 127 63

***Significance at 0.1% level Test of point measurement on 0.1-ha plots for estimating canopy density by incoming shortwave solar radiation (Rg) and photosynthetically active radiation. (PAR) p o i n t s p e r 0.1 ha. A n a n a l y s i s of c o e f f i c i e n t o f v a r i a t i o n , CV, ( ( s t a n d a r d deviat i o n / m e a n ) X 100) s h o w s t h a t t h e m e a n v a l u e of CV a t t h e o b s e r v e d p l o t s is lower t h e longer t h e t i m e u s e d p e r s t r i p (Fig. 1 ). A t 40 s p e r s t r i p (30 m / m i n ) , CV h a s d r o p p e d to 30%. A n a n a l y s i s of t h e d i f f e r e n c e s b e t w e e n i r r a d i a n c e e s t i m a t e d b y t h e p o i n t m e t h o d (64 p o i n t s ) a n d b y t h e s t r i p m e t h o d ( 8 s t r i p s ) s h o w s n o d i f f e r e n c e b e t w e e n t h e t w o t e s t e d m e t h o d s ( T a b l e 3 ). T h e m e a n of R I of 8 e s t i m a t e d p o i n t s p l a c e d in t h e s a m e d i r e c t i o n as a s t r i p w e r e a n a l y z e d TABLE 3 t-Test (computed t-values) of differences between point and strip measurement of four plots for incoming shortwave solar radiation (Rg) and photosynthetically active radiation (PAR) Plot No. 1 2 3 4

64 points-8 strips

8 X (8 points- 1 strip)

Rg

PAR

Rg

PAR

- 0.142 - 0.309 - 1.542 0.095

0.168 0.370 - 1.606 - 0.033

0.042 0.217 0.793 0.054

0.048 0.205 0.689 0.008

to.ol (63) =2.387 to.ol (7) =2.998

Figure 2. The relationshipbetween relativeirradiance (To) of incoming shortwave solar radiation,Rg (--) and photosynthetically active radiation, P A R (- - - ) respectivelyand (a) basal area (m 2 ha-l), (b) density (stems ha I), and (c) diameter sum (m ha i). Plotted RI values of differentthinning grades are measurements of Rg.

314

against the RI of the strip ( in total, 8 pairs per plot). No differences were found (Table 3 ). In the experiment comparing relative irradiance (RI) to parameters such as density, basal area and diameter sum, the correlation is high. Correlation between basal area or diameter sum and RI is better than between density and RI (Fig. 2). Depending upon thinning grade, the value of RI fluctuates. RI is highest for extra-heavily thinned stands. DISCUSSION

The problem of measuring solar radiation beneath a canopy is complicated by the irregular distribution of radiation in both space and time. In principle, the results presented on differences in RI between plots with various basal areas, densities, or diameter sums are in agreement with other investigations (Wellner, 1948; Jackson and Harper, 1955; Gatherum, 1961; V~zina, 1961; Mitscherlich et al., 1967; Greis and Kellom~iki, 1981 ). In the present study an acceptable error of the measurements was 10% (standard deviation). Results from the study show that 64 points have S.D. < 10%. The higher the number of estimated points per plot the lower the value of the standard deviation. Reifsnyder et al. (1971) found in their study of a pine canopy that 412 radiometers would be needed to estimate the instantaneous radiation with a standard deviation of 11%. Gay et al. (1971) reported that at least three pyranometers are necessary to estimate means over short periods of up to a few h. Gatherum (1961) stated that 136 readings per plot on a clearcut area and 1168 readings per plot on the control are needed to attain a reliability within 10% of the mean at 95% probability. Reifsnyder and Lull (1965) concluded in their recommendations that 20-40 readings in a uniform stand are necessary on sunny days, fewer on cloudy days. In the present study the low number of readings (64) compared with references above are a function of the uniform stand size used in the study. The sample size (n) needed to obtain an allowable error ( AE % ) at a fixed level of probability (t) and a fixed coefficient of variation (CV % ) may be estimated (Binkley and Merritt, 1977) as follows: n = t 2(CV) 2(AE-2). In general, the standard deviation at different numbers of observed points was lower at measurements of PAR than of Rg. The CV value is lower the longer the observation time per strip (cf. Fig. 1 ). This is in congruence with results presented by Reifsnyder et al. (1971), who concluded that moving a sensor increases the effective size of the sensor. The strip method is relevant if the speed of the response of the sensor is so slow that it can be considered an instantaneous measurement of the radiation at any point along the moving path. In the present study at least 40 s per strip (30 m m i n - 1) is necessary to provide a low CV (Fig. 1). Neither 64 points compared with 8 strips, nor 8 points compared with 1 strip (in total, 8 pairs), give any significant differences between irradiances at any one plot (cf. Table 3).

315 The correlation between different stand parameters and RI depends upon the parameters used (cf. Fig. 2). PAR does not penetrate the canopy as well as Rg since foliage intercepts more PAR than NIR ( Near Infrared Radiation). This result is supported by Baldocchi et al. (1984), who reported a greater decrease in PAR than in Rg in an oak-hickory stand during the leafing season, and a quotient of 0.2-0.3 between PAR and Rg during late spring to early autumn. In the present study the quotient, PAR/Rg, is 0.4-0.5 as a mean. Anderson (1969) reported from investigations with sunflowers (Helianthus annuus cv. Stripped Jupiter) and wheat (Triticum aestium cv. Robin) that leaves transmitted less PAR than NIR. During the leafing season, PAR is attenuated to a greater extent than Rg (Baldocchi et al., 1984 ). Results of this study confirm that an evergreen stand ( Norway spruce) attenuated PAR more than Rg. Coombe (1957) and Federer and Tanner (1966) have investigated the spectral composition or distribution of light in forests. The transmission of PAR in spruce was lower than in hardwoods. Stoner et al. (1978) reported that measured albedos of PAR were lower than the total solar albedos due to the high reflectance of stems within NIR. Studies of the correlation between basal area and irradiance on spruce have been reported by Vdzina (1961), Mitscherlich et al. (1967), Schomaker (1968) and Greis and Kellom~iki (1981). Schomaker (1968) reported 10% RI (wavelengths > 280 nm) in a Norway spruce stand with a basal area of 23 m 2 ha -1 In the present study, comparing basal area gives an RI (Rg) of 10%. V~zina (1961) reported RI (wave lengths 300-3000 nm) of 2.4% (59 m 2 h a - l ) , 2.6% (37 m 2 ha -1) and 7.3% (38 m 2 h a - l ) . The difference in RI between the two later observations depends upon different stem densities (1124 and 815 stems ha- 1, respectively ). The highest basal area in the present study ( 40.5 m 2 h a - 1) gives an RI (Rg) of 3%, whereas 37 m 2 ha -1 gives an RI (Rg) of 3.5%. Mitscherlich et al. (1967) reported about 4 and 3% RI (300-3000 nm) at 35 and 40 m 2 h a - 1, respectively, compared with the same figures in the present study (cf. Fig. 2 ). Jackson and Harper (1955) presented a study on shortleafpine (Pinus echinata Mill. ) with a correlation between RI (300-3000 nm) and basal area. The curve line is on a higher level, but the shape of the curve is the same as in the present study. The same applies to Wellner's study (1948) on Western white pine (P. monticola D. Don. ). Reports of the correlation between RI (3003000 nm) and diameter sum on spruce are sparse. Wellner (1948) presented a study on Western white pine where the RI was 10% at the diameter sum 250 m ha '. In the present study, the RI value on spruce at the same diameter sum is 4%. In Miller's (1955) report, the curve of stem density or diameter sum is on a higher level than this study due to pine stands allowing more light to penetrate than spruce stands. In Miller's (1955) collection of different studies, the shape of a curve through the points shows that RI values relative to diameter sums have values corresponding to those in this study. Correlations between density and RI have been sparsely investigated. Vdzina

316 (1961) presented RI values from Norway spruce of 2.4% (1700 stems h a - l ) , 2.6% (1124 stems ha -I) and 7.3% (815 stems h a - l ) . In the present study comparable RI (Rg) values are 3.5%, 6% and 10%, respectively. Single Rgvalues of plots in the present study are as mentioned above. In regression of many values the Rg values are higher the lower the stem density. In stands with the strongest thinning treatment, extra-heavy, RI (Rg) is 12-27%. RI (Rg) in stands treated with heavy thinning gives RI of 3-14% compared with 1-3% RI of unthinned stands. RI (Rg) in stands thinned from the top varies between 5 and 11%. Fairbairn (1961) presents measurements of light intensity in the Bowmont forest plots in Scotland (cf. Hummel, 1947) where these were subjected to thinning of different grades. In stands treated with thinning grade 'D' ('a heavy low thinning') values of RI (Rg) were 8.26-13.10% and with the thinning grade 'C', 5.39-8.90%. The prevention of sucker production following the thinning of broadleaved trees in important. Heavy thinning of a stand gives light conditions which are in several cases low enough to prevent sucker production. RI (Rg) ~<10% is necessary to minimize development of suckers (cf. Bates, 1925; Buell and Gordon, 1945; Jarvis, 1964; Johansson, 1986). RI in extra-heavily thinned stands is too high to prevent development of suckers. Stands thinned from the top give uncertain RI values in this study. A practical measure of density in forestry is the basal area of a stand. To prevent suckers of broadleaved trees the basal area after a heavy thinning in an uniform spruce stand ought to be at least 25 m 2 h a - 1. Density is easy to measure but in this case provides no practical recommendations as regards RI dependence. Currently, diameter sum is not a useful measure in Scandinavian forestry; foresters simply don't use it. The measurement by strips and a stretcher is a practical measure which result in values, if the stand is uniform, with a standard deviation of 10-15%. It is easier and faster to estimate irradiance with the strip method than by the point method. The presented curves of irradiance in stands of Norway spruce make it possible to calculate irradiance as influenced by stand characterization and forest operations. The curves only give means of the maximum irradiance, i.e. with the sun at zenith and no clouds.

ACKNOWLEDGEMENTS

I wish to thank Jan-Erik Lundh, forest engineer, for valuable help in the experimental work and calculations of the results, Miss Eva Ostberg and Miss Britt-Marie Nilsson for typing the manuscript, and Mrs. Britt Sundberg for typing the figures. Financial support has been given by the Swedish Council for Forestry and Agricultural Research and the National Board of Forestry.

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