Increment losses by full-tree harvesting in Norway spruce (Picea abies)

Forest Ecology and Management, 24 (1988) 283-292

283

Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

Increment Losses by Full-Tree Harvesting in Norway Spruce (Picea abies) H. S T E R B A

Institut [i~r Forstliche Ertragslehre, Universittit f~r Bodenkultur~ Peter Jordanstrasse 70, A1190 Wien (Austria) (Accepted 21 October 1987)

ABSTRACT Sterba, H., 1988. I n c r e m e n t losses by full-tree harvesting in Norway spruce (Picea abies). For. Ecol. Manage., 24: 283-292. In three experiments, pre-commercial t h i n n i n g has been carried out in three different ways. After felling, the trees in one t r e a t m e n t were immediately removed, in a n o t h e r t r e a t m e n t they were removed one growing season later, so as to leave the needles in the stand, a n d in a t h i r d t r e a t m e n t the felled trees were left in the stand. T h r e e years after the establishment of the experi m e n t s it t u r n e d out t h a t the basal area i n c r e m e n t differed by 12% between the t r e a t m e n t where the felled trees were left in the s t a n d as a whole, a n d the two other treatments. Since in one experiment the n u t r i e n t supply of nitrogen and phosphorus was increased significantly in the t r e a t m e n t where the felled trees were left in the stand, it is concluded t h a t the i n c r e m e n t differences between the t r e a t m e n t s can be interpreted as the fertilizing effect of the slash when left in the stand.

INTRODUCTION

Pre-commercial thinnings in Norway spruce (Picea abies (L.) Karst) thickets, i.e. stands with a dominant height of less than 10 m, are highly recommended in Austria because it has been proved that they induce an acceleration of diameter growth, thus leading to lower height/diameter ratios of stems, which increases the stability of stands against snow damage (see Chroust, 1968; Pollanschfitz, 1974; Merkel, 1975; Mildner, 1967). Furthermore, a sufficient number of stems with height/diameter ratios less than 80 is required to allow selective thinning at a dominant height of about 15 m, which again would lead to a higher monetary yield at the end of the rotation. On the other hand, the use of pre-commercial thinnings as fuel wood is growing more and more important through the introduction of appropriate chippers. Because operational methods are intended to lower harvesting costs, fulltree harvesting is often carried out and the total biomass, including branches,

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© 1988 Elsevier Science Publishers B.V.

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twigs, and needles, is removed from the stands. Krapfenbauer (1968) and Kreutzer (1980) have warned of nutrient removal by full tree harvesting. Krapfenbauer and Buchleitner ( 1981 ) have shown that the gain of the biomass is increased by just 40% through full-tree harvesting whereas the removal of nitrogen is increased by 500% and that of phosphorus by 800%. Glatzel et al. (1985) have shown that full-tree harvesting may annually induce an acidification of soils by 1.8 Kmol H + ha-1 year-1. They noted that full-tree harvesting would be most dangerous in young stands because accumulation of nutrients in needle biomass is highest in thickets. Eschenlohr (1921) showed that already 500 years ago farmers in Memmingen refused to collect the residue after clear-cuts, because they considered them to be fertilizer for the next stand. Subsequently, experiments by Fabricius (1938) showed that the height growth of Norway spruce was increased by 20% when the cultivation was covered with the slash from nearby clear-cuts. Krapfenbauer (1983) regrets that there have been experiments on the fertilizing effect of branches and needles in cultivations only but none in thinnings, where the main crop would take direct advantage of the slash. So fulltree harvesting in pre-commercial thinnings can be understood, on the one hand, as nutrient removal and, on the other hand, as a renunciation of the fertilizing effect of the slash. The aim of this work, then, is to investigate the increment after pre-commercial thinnings which had been performed by removing different fractions of the biomass of the felled trees from the stands. MATERIAL AND METHODS

Description of the study areas The study was conducted in three experiments, the situation of which is shown in Fig. 1 in relation to the growth districts of Austria (sensu Mayer, 1971 ); Table 1 gives a site description of the three experiments. Each of the three experiments took place in pure or nearly-pure thickets of Norway spruce. Two of these three experiments were planned as completely randomized designs with four replications (plots of 300-600 m 2) of every treatment. Only at Bad Zell could there not be found any Norway spruce thicket of enough extension to allow for 12 plots, so this experiment was designed as a randomized-block design, with two blocks, containing the three treatments with two replications each. The two blocks were named after the owners of the stands, Kern and Grillnberger.

Experimental layout The experiments started immediately after the thinning, when approximately 50 spruce trees within a circle situated in the center of each plot were

285

experiment

@

name

Bad Zell

@

Wilhelmsburg

@

Hartberg

-

-

mixed forests of

s p r u c e , f i r and b e e c h in t h e e x t r a a l p i n e , lower m o n t a n e belt

JllllJIIIrllrllPIIIm,xed . t .... of t h e bn eoer tchhe r fn. . . foothills ~

of the Alps

Subillyriangrowthregion of

the eastern

mixed oak forest area

Fig. 1. Situation of the three experiments in relation to the growth districts of Austria, sensu Mayer (1971).

numbered and marked at breast height, thus achieving a security strip between plots of different treatments. - In t r e a t m e n t 'Full-tree green', the whole trees were removed from the plot, including all branches, twigs, and needles. - In t r e a t m e n t 'Full-tree dry', the felled trees were removed from the plot after one growing season, so t h a t by this time they had lost most of their needles. - In t r e a t m e n t 'Control', the felled trees were left in the stand. Only in the Wilhemsburg experiment was the stem wood removed, thus leaving branches and needles in the plot. Three years later the marked trees were remeasured. From these data the increments of height and of basal area were calculated. To estimate the volume increment, a volume equation was used which had been developed from data from another experiment on pre-commercial thinning in a Norway spruce thicket. The following volume equation is based on 80 stems which were felled and measured exactly for stem volume at the start of the experiment and again 5 years later: v=0.5503 * dbhlS% h °9256

286 TABLE 1 Site description of experiments Experiment name Wilhelmsburg

Hartberg

Bad Zell Kern

County

Lower Austria

Styria

Altitude (m a.s.1.) Annual precipitation (ram) Mean temp. ( ° C ) Topography Bedrock

380

350

880 8.0 north slope Flyschsandstone gleyic cambisol

750 8.2 flat loess and diluvial loam gleyic cambisol

Soil type1 Decomposition layer2 (cm)

0.5

2.2

Grillnberger Upper Austria 600

780 7.2 north slope south slope gneiss dystric cambisol 6.9

3.6

1From Anonymous (1974). 220 samples.

where v is the stem volume (dm3), dhb the diameter at breast height (dm), and h the height (din). The residual standard deviation of this equation was _+1.5 dm 3.

Situation at the beginning of the experiments All experiments started when the thinning was completed. In the Wilhelmsburg and Bad Zetl experiments a geometrical thinning was executed, felling every second row of stems. Since the Hartberg experiment had developed from natural regeneration, thinning there was a selective thinning from below. Table 2 gives the stand characteristics of the experiments before and after thinning, thus describing the way of thinning. With the equations recalculated from tables by Krapfenbauer and Buchleitner (1981) the removal of the dry biomass had been roughly estimated for the different experiments and the different treatments in Table 3, from which it can be seen that between 8 and 40 t / h a of biomass was removed and that N contained in the needles, assuming a concentration of about 1%, was equivalent to fertilization with 30-120 kg N per ha, depending on the experiment. In two experiments the nutritional status of the trees immediately after thinning but before applying the different treatments could be described by determining the element concentration of the top whorl of 10-15 trees per plot.

287 TABLE 2 Stand characteristics before and immediately after thinning Norway spruce thickets Experiment

Wilhelmsburg Hartberg Bad Zell Grillnberger Kern

Before

After

Number of stems ha -1

Mean height (m)

Mean dbh (cm)

Number of stems ha -1

Mean height (m)

Mean dbh (cm)

5014 14400

7.5 6.6

7.5 5.2

2407 4244

7.6 8.3

7.5 7.1

5350 4267

6.4 5.4

6.9 5.4

2700 2250

6.4 5.5

7.0 5.5

TABLE 3 Estimated oven-dry biomass removed in various stand components during thinnings Experiment

Biomass removal (t/ha)

Wilhelmsburg Hartberg Bad Zell Grillnberger Kern

Stemwood

Slashwood

Needles

19 20

5 7

6 12

15 6

5 2

6 3

TABLE 4 Top whorl foliar element concentrations {% ) determined in late autumn Experiment

N

P

K

Ca

Mg

Wilhelmsburg Hartberg Bad Zell Grillnberger Kern

1.46 1.18

0.14 0.14

1.22 0.98

0.46 0.37

0.12 0.15

1.25 1.16

0.19 0.18

1.00 0.87

0.33 0.42

0.11 0.10

Deficiency limita

1.31

0.11

0.33

0.01

0.07

aAccording to Stefan, 1985.

These concentrations did not differ significantly among the future treatments in the Wilhelmsburg and Bad Zell experiments. Possible differences in the nutritional status among the future treatments in the Hartberg experiment could be excluded because, according to a site mapping of Zukrigl (1973), the

288

whole experiment was located on one site class. To describe the nutritional status of this experiment, top-whorl foliar element concentrations were determined at the time of the re-observation of this experiment, 3 years after treatment. From Table 4 it can be seen that in all experiments except Wilhelmsburg the supply of nitrogen seems to be deficient.

Computational and statisticalprocedures After re-observation of the stems of all experiments, the increments of height, basal area, and volume were calculated for each marked tree. To increase the accuracy of the mean increment of the treatments, height, basal area and volume at the beginning of the experiment were used as covariants. The relationship between volume increment and volume at the beginning of the experiment is shown in Fig. 2 for one plot in the Grillnberger experiment. This figure also shows how the mean increment is adjusted to the mean volume of the whole experiment. In this example, the coefficient of correlation was 0.97* with 48 d.f., the equation for the regression being Vi = 3.465 + 0.8749 * v where vi is the 3-year volume increment (dm 3) and v the volume at the beginning of the experiment (dm 3). The residual standard deviation for the increment was _+4.0 dm 3. Mean volume and mean volume increment of this plot plot no

GRILLNBERGER

11

(V i 8 3 - 8 6 )

o

50

40

mean increment of the plot

o

~

"

o~//~

ofo

:o°

increment of the plot, adjusted for the mean volume of the experiment

volume at the beginning of the i J ~ experiment

0 h

i I

4

L

L

i

i

J

o

1o I

z~,

3o

4o

so

60

70

I

80

90

loo(v83)

/

I mean volume of the plot mean volume of the experiment

Fig. 2. Example of method used to adjust increment for the mean volume at the beginning of the experiment.

289 were 20.01 dm 3 and 20.97 dm 3 respectively. Adjusted to the mean volume of the whole experiment (10.26 dm 3) the mean increment yields 12.44 cm 3. The adjusted mean increments of the plots were analysed by simple analysis of variance in the Wilhelmsburg and Hartberg experiments and by an analysis of variance for a randomized block design in the Bad Zell experiment. After this, all experiments were pooled by calculating a two-way analysis of variance. The method of weighted squares of means was chosen because the splitting of the Bad Zell experiment had led to an unbalanced design. Multiple-mean comparisons were performed using Scheff~'s test. All tests used were done at a base of P=0.05. RESULTS All the increments of all plots of every experiment were adjusted to the mean starting situation of the experiment, as described above. The correlation coefficients of the linear regressions used to adjust the mean increments yielded between 0.64 and 0.95 for the basal area increment, 0.36 and 0.84 for the height increment, and 0.48 and 0.89 for the volume increment. The mean adjusted increment values per tree (adjusted to the mean starting situation of every experiment) are given in Table 5, from which it can be seen that, generally, the ranking of the mean values appears in the expected order, so that the increment of the control was highest, whereas the increment of the treatments where the total biomass of the felled trees including needles had been removed was smallest. The calculation of the appropriate analyses of variance resulted in significant differences of basal area increment both in the Bad Zell experiment and in the pooled mean of all experiments, whereas differences of height increment were not significant in any experiment, and those of volume increment were significant in the Bad Zell experiment only. Multiple mean comparisons by Scheff~'s test for linear contrasts show that, for those variables and experiments where the analysis of variance yielded significant differences, only the difference between the control and the mean of the other two treatments was significant. As for the top-whorl foliar element concentrations, no significant differences among the three treatments were found in any of the three experiments. In the Wilhelmsburg and Bad Zell experiments, the change of the nutrient concentrations over the course of this study could be investigated because the nutrients had been determined at the beginning of the experiments and again 3 years later. It is only in the Bad Zell experiment that the changes in N and P differed significantly among the treatments. Differences presented in Table 6 show that the leaving of the whole biomass of the felled trees in the stand improved the N and the P concentrations and thus the nutritional status of the main crop, while removal of the biomass weakened the nutritional status

290 TABLE 5 Mean annual increments related to the mean tree of the experiment Experiment

Wilhelmsburg

Annual increment Variable

Full tree green

Full tree dry

Control

hi

44.6 7.52 4.54 33.7 4.25 2.78

49.7 8.40 5.00 39.8 5.17 3.42

45.3 8.36 4.85 44.8 5.09 3.45

gi vi

61.9 7.86 b 4.52 b 47.6 6.52 b 3.01 b

56.1 7.36 b 4.05 b 55.1 6.29 b 3.02 b

62.2 6.69 a 5.16 a 53.7 8.36 a 3.87 a

hi gi vi

44.3 6.32 b 3.70

48.3 6.80 b 3.98

49.3 7.49 a 4.27

gi vi Hartberg

hi

gi vi Bad Zell Grillnberger

hi

gi vi Kern

Pooling all experiments

hi

Height increment (hi) in cm, basal area increment (gi) in cm 2 and volume increment (vi) in dm 3. Annual increments differing significantly at P = 0.05 are marked with letters; within a row treatments with different letters are significantly different. TABLE6 Changes (percentage in 1986 minus percentage 1983) of top whorl foliar N and P concentration over the 3-year duration of the experiment at Bad Zell Element

Full-tree green

Full-tree dry

Control

Nitrogen Phosphorus

-0.01 b - 0.017 b

+0.05 b - 0.005 b

+0.18 a + 0.006 a

Differences significant at P = 0.05 are marked with letters; treatments with different letters are significantly different. w i t h regard to N a n d P. Again, Scheff~'s test s h o w e d t h a t o n l y the difference b e t w e e n the c o n t r o l a n d the m e a n of the two o t h e r t r e a t m e n t s was significant.

DISCUSSION The three experiments described here show that, already 3 years after the a p p l i c a t i o n of p r e - c o m m e r c i a l t h i n n i n g , the i n c r e m e n t of height, basal area,

291

and stem volume was greatest in plots where the whole biomass of the felled trees had been left in the stand, whereas the increment data were smallest in plots where the whole trees had been removed immediately after felling. When pooling all experiments, the difference in the basal area increment between the control and the mean of the two other treatments was significant, resulting in a 12% loss of increment. Generally, the two treatments involving the removal of the biomass have a lower basal area increment than the treatment where the whole trees were left in the stand (Table 5). The Bad Zell stands were deficient in N before the experiment started. Increment differences among the treatments in this experiment were 19% for volume and 22% for basal area, the greatest changes observed. This was also the only experiment in which changes of the top-whorl concentrations of N and P differed significantly among the treatments. These results indicate a strong relationship between treatment-induced changes of nutritional status and increment. The increment response only 3 years after thinning, combined with the fact that in the Bad Zell experiment N and P concentrations increased in the control, suggests that the observed increment differences are caused by the fertilizing effect of the slash of felled trees (Fabricius, 1938; Krapfenbauer, 1983) rather than by nutrient removal (Krapfenbauer, 1968; Kreutzer, 1980; Krapfenbauer and Buchleitner, 1981 ). But of course it cannot be excluded that future increment losses caused by the nutrient removal may be added to the effects observed in these experiments during the first 3 years. CONCLUSIONS

Pre-commercial thinnings in Norway spruce thickets carried out as full-tree harvesting, thus removing branches and needles of felled trees from the stand, led to significant increment losses of about 12% as a mean of all experiments, and of about 20% in the experiment where these losses were greatest. These differences are possibly induced by the fertilizing effect of the slash of the felled trees when they are left in the stand. ACKNOWLEDGEMENTS

This work was supported by the 'Jubiliiumsfonds der Osterreichischen Nationalbank'. I also gratefully acknowledge the permission of the forest owners Aigelsreiter, Grillnberger, and Kern to conduct the experiments in their forests as well as their help with the execution of the thinnings according to the experimental design.

292 REFERENCES Anonymous, 1974. Soil map of the world. 1:5 000 000. Vol. I, legende; FAO-UNESCO, Paris. Chroust, L., 1969. Skody snehem ve smrkov~ck opozdene vychov~nan:~ch. Lesnictvi, 15: 701-712. Eschenlohr, H., 1921. Die Anf~nge einer geordneten Forstwirtschaft im Hoheitsgebiet der freien Reichsstadt Memmingen. Forstw. Centralb., 43 (8/9): 297-319. Fabricius, L., 1938. Forstliche Versuche. XX. Bodenbedeckung mit Pflanzstoffen. Forstw. Centralb., 60 (1): 1-15. Glatzel, G., Englisch, M. and Kazda, M., 1985. Forschungsarbeiten zum Schwefelhaushalt von WaldSkosystemen. In: E. F(ihrer (Editor), Forschungsinitiative gegen das Waldsterben. Bundesministerium ftir Wissenschaft und Forschung, Wien, pp. 72-81. Krapfenbauer, A., 1968. Rationalisierungsbestrebungen und Standortsproduktivit~t. Allg. Forstztg. Wien, 79(6): 128-130. Krapfenbauer, A., 1983. Von der Streunutzung zur Ganzbaumnutzung. Centralblatt Gesamte Forstwes., 100 (2-3): 143-174. Krapfenbauer, A. and Buchleitner, E., 1981. Holzernte, Biomassen- und N~ihrstoffbilanz eines Fichtenbestandes. Centralblatt Gesamte Forstwes., 98 (4): 193-223. Kreutzer, K., 1980. Der Einfluss moderner Holzernteverfahren auf die ()kologie des Waldes. Verhandlungen d. Ges. ftir 0kologie (Freising-Weihenstephan 1979). Vol. VIII 1980, pp. 229-233. Mayer, H., 1971. Die Waldgebiete und Wuchsbezirke ()sterreichs. Centralblatt Gesamte Forstwes., 88(3): 129-164. Merkel, O., 1975. Schneebruch im Fichtenbestand bei 40 j/~hriger Auslesedurehforstung. Allg. Forstz. Mtinchen, 30 (33/34): 663-666. Mildner, H., 1967. Die Widerstandsf~ihigkeit von Fichtenjungbest~inden gegentiber atmosph~irischen Einwirkungen. Soz. Forstwirts., 17 (2): 57-59. Pollanschtitz, J., 1974. Erste ertragskundliche und wirtschaffliche Ergebnisse des Fichten-Pflanzweiteversuches "Hauerste~g". In: J. Egger (Editor), 100 Jahre Forstliche Bundesversuehsanstalt Wien. Forstliche Bundesversuchsanstalt Wien, pp. 99-172. Stefan, K., 1985. Ergebnisse der Bioindikatornetz-Analysen. In: E. Fiihrer (Editor), Forschungsinitiative gegen das Waldsterben. Bundesministerium f'tir Wissenschaft und Forschung, Wien, pp. 61-71. Zukrigl, K., 1973. Die Vegetation im Revier Katwald der Stadt Hartberg, Oststeiermark (unpublished).