The effect of water and nutrient availability on the productivity of Norway spruce in northern and southern Sweden

Forest Ecology and Management 119 (1999) 51±62

The effect of water and nutrient availability on the productivity of Norway spruce in northern and southern Sweden Johan Bergha,*, Sune Lindera, Tomas Lundmarkb, BjoÈrn Elfvingc a

Department for Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7042, SE-750 07, Uppsala, Sweden b Vindeln Experimental Forests, Swedish University of Agricultural Sciences, SE-922 91, Vindeln, Sweden c Department of Silviculture, Swedish University of Agricultural Sciences, SE-901 83, Umea, Sweden Received 3 October 1997; accepted 28 October 1998

Abstract Results from two yield optimisation experiments in young stands of Norway spruce in northern and south-eastern Sweden are presented after 10 and 9 years' treatment, respectively. The aim of the experiments was to demonstrate the potential yield of Norway spruce, under given climatic conditions and non-limiting soil water, by optimising the nutritional status of the stands, at the same time as leaching of nutrients to the groundwater was avoided. A complete mix of nutrients was supplied either once a year or daily during the growing season, in combination with irrigation. On the basis of repeated foliar analysis and predicted growth response the proportions and amounts of nutrients applied were adjusted annually. After 10 years' treatment at the northern site, fertilisation had increased the annual stem volume production from 3 m3 haÿ1 aÿ1 in unfertilised stands to 14 m3 haÿ1 aÿ1. Although site quality initially was high at the southern site, nutrient optimisation increased annual stem volume yield from 12 in control stands to 29 m3 haÿ1 aÿ1 in irrigated±fertilised stands. Water had a positive effect on stem growth in southern experiment, but no effect in the north. In all stands in which the availability of nutrients and soil water was optimised by combined fertilisation and irrigation, the yield of stemwood was similar to or surpassed the best yields obtained by conventional silvicultural means. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Boreal; Fertilisation; Irrigation; Picea abies; Yield optimisation

1. Introduction The upper limit for forest biomass production on a particular site is set by the amount of incoming radiation (cf. Jarvis and Leverenz, 1983; Linder, 1985). Actual production is, however, determined by the amount of radiation intercepted by the canopy *Corresponding author. Tel.: +46-18-672525; fax: +46-18673376; e-mail: [email protected]

during the active period of growth and, to a lesser extent, by the ef®ciency of conversion of intercepted radiation into biomass (e.g. Linder, 1987; McMurtrie et al., 1994). Depending on region, the growth period may be determined by temperature (high latitudes) or water availability (arid, semi-arid). It is, however, clear from several long-term forest experiments, on a range of species growing in contrasting environments, that current rates of biomass production in most forest ecosystems are far below their potential level

0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00509-X

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J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

and that manipulation of nutrients, water or both can result in dramatic increases in yield (cf. Tamm, 1991; Linder et al., 1996). In the temperate and boreal zones, nutrient availability is the main limiting factor, but in large parts of the Mediterranean and southern hemisphere, forest growth is primarily limited by water. Clear evidence of the interaction of water and nutrients in terms of their effects on general growth patterns has been obtained from long-term experiments with Pinus radiata in Australia (Linder et al., 1987; Raison et al., 1992), a plantation of Eucalyptus globulus in Portugal (Pereira et al., 1989, 1994), and from a current experiment with Pinus taeda in North Carolina, USA (Albaugh et al., 1997). In all stands in which the availability of nutrients and soil water was optimised by means of combined fertilisation and irrigation, the stemwood yield was similar to or surpassed the best yields obtained by conventional silvicultural means. Earlier combined irrigation and fertilisation experiments in forest stands attempted to create non-limiting conditions, in terms of water and nutrient availability, but without attempting to de®ne and maintain an optimal nutrient balance in the stand or to avoid leakage of nutrients to the groundwater (e.g. Aronsson and Elowson, 1980; Linder et al., 1987; Pereira et al., 1989, 1994). In the latter part of the 1980s, a new type of nutrient optimisation experiment was established in young stands of Norway spruce in northern (Flakaliden) and south-eastern (Asa) Sweden (Linder, 1990; Linder and Flower-Ellis, 1992). The principal aim of these experiments was to eliminate mineral nutrients and water as growth-limiting factors, at the same time as leaching to the groundwater was avoided. On the basis of foliar analysis and predicted growth response, the amount of element to be applied was estimated relative to set target values of individual nutrient elements in the needles (cf. Linder, 1995). Subsequent analysis of nutrients in foliage and soil water showed to what extent the targets had been met; the information was then used to adjust the proportions and amounts of nutrients to be added in the next season. Results from these experiments on the production of stemwood during the ®rst 10 years, are presented. Since the two sites represent a climatic gradient between a boreal and a cold-temperate climate in Scandinavia, clear differences in growth constraints

can be seen between them. These constraints will be exempli®ed and the differences between the two sites discussed. The experiments are part of a long-term study on the effects of abiotic and biotic factors on the structure (e.g. Flower-Ellis, 1993; Nilsson and HaÈllgren, 1993; Stenberg et al., 1995) and the ecophysiology (e.g. Linder, 1995; Strand, 1995, 1997; Stockfors and Linder, 1997a, b) in young stands of Norway spruce. 2. Materials and methods 2.1. Site description The Flakaliden experiment (648070 N, 198270 E, 310±320 m a.s.l.) was laid out in 1986 in a young Norway spruce (Picea abies (L.) Karst.) stand, planted in 1963 with 4-year-old seedlings of Norway spruce of a local provenance, after clear-felling, prescribed burning and soil scari®cation. At the start of the experiment the mean height of the trees was 3.0 m, diameter at breast height (DBH, 1.3 m) was 36 mm, and the number of trees was ca. 2400 per hectare (Appendix A). According to a classi®cation scheme based on site properties (HaÈgglund and Lundmark, 1977), the yield class for spruce before the start of treatment was estimated at G17±G19 (top height: height of the largest trees, m, at 100 years). Two nutrient optimisation treatments were included. The ®rst treatment (IL) was a complete nutrient solution, which was injected into the irrigation water and supplied every day during the growing season (June±mid-August). The second treatment (F) was a solid fertiliser mix, which was applied in early June each year. Controls (C) and plots with irrigation (I) were also included. For further details regarding the treatments, see Linder (1995). The treatments, which began in 1987, were replicated four times, and each replicate consisted initially of a double plot, made up of two 50 m 50 m plots. Each plot contained a net plot (1000 m2) surrounded by a buffer zone. All non-destructive measurements were made on the net plot, and destructive sampling was restricted to the buffers. Some of the plots were later used for new treatments, but without reducing the number of true replicates (Fig. 1). The total area of the experiment was 8.25 ha.

J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

53

Fig. 1. Schematic map of the Flakaliden research site. The main experiment consisted of untreated control plots (C) and treatments with irrigation (I), annual fertilisation with solid fertilisers (F), and `daily' fertilisation combined with irrigation (IL). Each treatment was replicated four times. Shaded plots were part of the main nutrient optimisation experiments. Other treatments (non-shaded plots) were nitrogen fertilisation supplemented with wood ash (A), and fertilisation with all essential nutrient elements except phosphorus (F-P) or magnesium (FMg). One plot was used for a water exclusion treatment (D), where summer precipitation was reduced by 65%.

The Flakaliden site has a harsh, boreal climate, characterised by short, cool summers with long days and long cold winters with short days. The monthly mean air temperature at the site varies from ÿ8.78C in February to 14.48C in July (Fig. 2), and snow usually covers the frozen ground from mid-October to early May. Mean annual precipitation is approximately 600 mm, of which more than onethird falls as snow. The growing season (daily mean temperature > ‡58C) lasts 120 days. The weather at the site was monitored using a standard weather station, and hourly values were calculated and stored. The Asa experiment (578080 N, 148450 E, 225±250 m a.s.l.) was commenced 1 year after the experiment at Flakaliden. The site was planted in 1975, after clearfelling and soil scari®cation, with 2-year old rooted seedlings of Norway spruce. At the start of the experiment (1987), the mean height of the trees was 3.5 m, DBH 39 mm, and the number of trees ca. 2400 per

hectare (Appendix B). Before the start of treatment, the yield class for spruce, estimated according to HaÈgglund and Lundmark (1977), was G32±G34. The experimental design was identical to Flakaliden, except that the experiment was unblocked and there were only single 50 m 50 m plots. One control (C) plot and two solid fertiliser (F) plots were not included in the statistical analysis as they were separated from the main experimental area and differed in initial site conditions. Asa has a milder climate than Flakaliden (Fig. 2) and the growing season is more than 2 months longer, 190 days compared to 120 days at Flakaliden. The longer growing season and the higher daily incident radiation at Asa, resulted in 40% higher total incident radiation during the season. The mean temperature during the growing season was higher for Asa, 11.58C compared to 10.28C at Flakaliden. Soil may freeze periodically in winter, but the entire soil pro®le is usually ice-free in early spring. Mean annual preci-

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J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

de®cit of 10 mm of water. For further details regarding the treatments, see Linder (1995). 2.3. Measurements and growth estimates

Fig. 2. Monthly mean values of air temperature (a), and incident radiation (b) at Asa (solid line) and Flakaliden (broken line), respectively. Presented values are mean values based on 5 years' (1990±1994) measurements. The arrows indicate the start and end of the growing season at Asa (open arrows) and Flakaliden (filled arrows), respectively.

pitation is ca. 700 mm, but is frequently low in early summer. 2.2. Treatments An `optimal' nutritional status to be attained in the foliage of the trees was de®ned in terms of target needle concentrations for each individual nutrient element. The initial target values for macro-nutrients were derived from studies of `optimal' nutrition of Norway spruce in laboratory and ®eld experiments. The amount and composition of the nutrient solution and fertiliser mix was determined each year. The decision was based on nutrient analysis of foliage and monitoring of nutrients in the soil water, collected by means of suction lysimeters. The amount of irrigation needed was determined in relation to the amount of available water in the rooting zone (0±40 cm), as measured by tensiometers. The target was to maintain soil water between ®eld capacity and a maximum

At Flakaliden complete inventories were carried out in 1986, 1991 and 1996, where DBH and height of every tree on the subplots was measured (ca. 8000 trees). Between years of complete inventory, DBH and height of ca. 100 trees per treatment were measured each autumn, except 1992. These measurements, which were used to estimate the annual development of height, DBH, basal area and volume, were later `corrected' against the results of the complete inventories. Different volume functions were used for estimating the volume growth of individual trees, depending on tree size and on the geographical location of the site (NaÈslund, 1947; Andersson, 1954; Brandel, 1990). At Asa, tree height and DBH were measured each year, except 1988. DBH was measured on all trees in the net plots, but height on a reduced number of trees selected from the diameter distribution in the stands. Regressions between volume and DBH, based on data from the standing sample trees, for which both diameter and height had been measured, were derived to assign a volume to the remaining diameters. Trees for biomass studies were harvested at Flakaliden in 1986 and 1992, and at Asa in 1988 and 1993. Twelve trees per treatment, covering the range of tree sizes, were harvested and processed. The sampling procedure was described in detail by Flower-Ellis (1996). The stems from the harvest in 1992 were divided into ®ve sections, each represented by a disk. The volume of each section was estimated by Smalian's formula (Smalian, 1840). Disks taken at breast height were used to determine wood basic density. Each disk was divided into two parts, the inner part representing wood formed before, and the outer wood formed after, the start of treatment. The basic density of the wood samples was determined by a water displacement method described by Olesen (1971). 2.4. Statistical analysis To test if there were any signi®cant differences between the treatments, an analysis of variance (Anova) was made, using SAS statistical software

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55

(version 6.09, SAS, 1994). Treatment, block and initial volume were included as variables in the Anova-model for Flakaliden volumes. To account for the initial differences in volume the volume on each plot in 1986 was included as a covariate. Since the Asa experiment was not blocked, only treatment and initial volume were used as variables. In the analysis of basic density the density before and after treatment was compared, within each treatment, in a paired t-test. 3. Results There was a fast response to fertilisation in the stands at Flakaliden (Fig. 3). Already after 3 years' treatment, both height and diameter growth in fertilised (F and IL) stands had doubled in comparison to the non-fertilised (C and I) stands (Table 1). The difference in height and diameter growth between the treatments remained at a similar level during the entire period of study, but the difference in volume growth continued to increase with time (Fig. 3). After 10 years' treatment, the volume growth in fertilised stands (F and IL) was almost four times higher than that in stands without fertilisation (C and I), and standing volume had almost tripled (Table 1). There was no effect of irrigation on growth during the ®rst 10

Fig. 3. The annual increment of stem volume (CAI) in stands of young Norway spruce in the nutrient optimisation experiment at Flakaliden. The treatments were control (open circle), irrigation (open triangle), solid fertilisation (filled circle), and combined irrigation and fertilisation (filled triangle). The treatments commenced in 1987.

Fig. 4. The annual increment of stem volume (CAI) in stands of young Norway spruce in the nutrient optimisation experiment at Asa. The treatments were control (open circle), irrigation (open triangle), solid fertilisation (filled circle), and combined irrigation and fertilisation (filled triangle). The treatments commenced in 1987.

years' treatment (Fig. 3, Table 1). Mortality was low (<0.2%) during the studied period, and was mainly caused by suppressed trees being broken by heavy loads of snow (Appendix A). Even at the more fertile site, Asa, the response to fertilisation was pronounced (Fig. 4). During the ®rst 4 years, volume production increased by 60% in fertilised (F and IL) compared to non-fertilised (C and I) stands, and there was no signi®cant effect of irrigation (Table 1). During the second 4-year period, however, non-irrigated stands (C and F) did not grow as well as those receiving extra water (Fig. 4, Appendix B). Solid fertilisation (F) still caused a 60% increase in volume yield compared to unfertilised stands, but when it was combined with irrigation (IL), the volume yield was more than doubled. During the period 1991±1995, the volume increment in irrigated±fertilised (IL) stands was 50% higher (p < 0.001) than that in fertilised stands without irrigation (F). The difference in yield between the irrigated±fertilised stands (IL) and the other treatments (C, I and F) continued to increase throughout 8 years of study. At Flakaliden, fertilisation had a pronounced effect on stand structure in terms of the range of tree sizes found (Fig. 5), with a wider range of heights and diameters in the fertilised stands. There was no difference in the shape of the distribution curves between

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J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

Table 1 Stocking, height, diameter, basal area, and volume (over bark) in young stands of Norway spruce in nutrient optimisation experiments at Flakaliden and Asa Treatment

Year

Stocking (stems haÿ1)

Mean height (m)

1986 1991 1996

2595 2570 2563

2.8 3.8 4.9

4.9 6.3 7.5

I

1986 1991 1996

2408 2403 2395

3.0 4.1 5.1

F

1986 1991 1996

2168 2155 2143

IL

1986 1991 1996

Basal area (m2 haÿ1)

Volume (m3 haÿ1)

33 53 70

2.6 6.0 10.1

7.3 19.0 34.8

4.8 6.1 7.4

38 56 73

3.0 6.2 10.4

8.5 19.4 36.1

3.1 5.1 7.3

5.0 7.0 9.0

39 77 114

2.9 10.5 22.6

8.2 35.8*** 95.7***

2345 2338 2330

3.0 5.0 7.2

5.1 7.2 9.0

36 75 112

2.6 10.6 23.6

7.4 35.8*** 98.8***

1987 1991 1995

2357 2357 2350

3.4 5.9 7.9

5.1 7.7 10.0

38 71 92

3.2 10.3 17.1

9.5 40.7 80.5

I

1987 1991 1995

2380 2380 2370

3.3 5.8 8.5

4.7 7.4 10.0

36 68 96

2.8 9.5 18.5

8.0 35.5 89.6

F

1987 1991 1995

2585 2585 2585

3.7 6.7 8.8

5.1 8.0 10.1

40 86 112

3.6 15.8 26.8

10.3 63.1*** 131.4**

IL

1987 1991 1995

2243 2243 2235

3.6 6.7 9.7

5.1 8.2 11.4

43 91 130

3.5 15.2 30.7

10.3 60.8*** 163.4***

Flakaliden C

Asa C

Top height (m)

Mean diameter (mm)

The treatments were untreated controls (C), irrigation (I), annual fertilisation (F), and `daily' supply of nutrients combined with irrigation (IL). Tree age at the start of the treatments was 28 years at Flakaliden and 15 years at Asa. Significant differences in volume, as an effect of treatment, were tested against the control plots (** ˆ p  0.01, *** ˆ p  0.001).

C and I treatments or between F and IL. In spite of the presence of a rather large number of small trees in F and IL stands, these trees represented a minor part of the standing volume. In the F and IL stands, trees with a DBH above 100 mm accounted for almost 90% of the standing volume. The relationship between DBH and height was identical within all plots before treatments started, but after 10 years' treatment, the relationship had changed, and was different for non-fertilised (C and I) and fertilised (F and IL) trees (Fig. 6). Below a DBH of 120 mm, fertilised trees were taller in relation to their diameter, and above a DBH of

120 mm they were shorter than non-fertilised trees of the same diameter. At both Flakaliden and Asa, wood basic density was signi®cantly (p  0.001) affected after 5 years' fertilisation, (Table 2). At Flakaliden there was a 15% decrease in the basic density of wood produced after the fertilisation treatment started, but no effect of irrigation was seen. At Asa, the largest decrease in wood basic density was found in (IL) trees (17%), with a lower (12%) decrease in (F) trees. Irrigation alone resulted in a decrease in wood density by 7%, which was similar to the difference between fertilised trees with (IL) and without (F) irrigation.

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Fig. 6. The relationship between diameter and height in young Norway spruce trees in the nutrient optimisation experiment at Flakaliden after 10 years' treatment. Data were combined for control (C) trees and trees from irrigated (I) plots (broken line) and for trees from fertilised stands (F and IL), with and without irrigation (solid line).

Fig. 5. The distribution of diameter (a) and height (b) of young Norway spruce trees in the nutrient optimisation experiment at Flakaliden. The treatments were control (open circle), irrigation (open triangle), solid fertilisation (filled circle), and combined irrigation and fertilisation (filled triangle). The results presented are after ten years' treatment. The total number of trees per treatment was approximately 1000.

4. Discussion There was a strong and signi®cant increase in volume yield at both Flakaliden and Asa (Figs. 3

and 4, Table 1), con®rming earlier ®ndings that nutrient availability is a major constraint on forest production in Sweden (Tamm, 1991). The decrease in wood basic density as an effect of treatment (Table 2) does not change this result. At Flakaliden, yield was still increasing in all treatments after 10 years, but was set back during the last 2 years as an effect of exceptionally large cone crops. In 1995, 85% of the trees, independent of treatment, carried large numbers of cones. As estimated by growth-ring analysis, increment losses of up to 40%, caused by cone production, have been found in Norway spruce (e.g. Chalupka et al., 1975).

Table 2 Wood basic density (kg dmÿ3) at breast height in young Norway spruce trees in nutrient optimisation experiments at Flakaliden and Asa Treatment

C I F IL

Flakaliden

Asa

Initial density (1963±1986)

Treatment density (1987±1992)

Initial density (1973±1987)

Treatment density (1988±1993)

0.4008 0.404 0.384 0.386

0.413 0.411 0.326*** 0.327***

0.409 0.392 0.378 0.372

0.414 0.365*** 0.331*** 0.310***

The treatments were untreated controls (C), irrigation (I), annual fertilisation (F), and `daily' supply of nutrients combined with irrigation (IL). A paired t-test was used to test for significant differences in wood density between treatments in the period before and after the commencement of treatment (*** ˆ p0.001).

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Water had no effect on growth at Flakaliden, which is in agreement with results from a long-term irrigation±fertilisation experiment in young stands of Scots pine in central Sweden (Linder, 1987, 1990). In spite of a much shorter growing season at Flakaliden, the current annual increment in fertilised (F and IL) stands was higher after 10 years' treatment than that in the control stands at Asa (Figs. 3 and 4), and does not ®t any yield table for Norway spruce in northern Sweden (cf. Eriksson, 1976). This clearly demonstrates that the main constraints on forest yield in northern Sweden is nutrient availability, together with the short, cool growing season. At Asa, there was also a pronounced effect of fertilisation, but no effect of irrigation during the ®rst 4 years (Fig. 4). During the last 5-year period, however, there was a signi®cant difference in growth between fertilised (F) and irrigated and fertilised (IL) stands. A strong effect of water was also found in a similar experiment on the west coast of Sweden, where irrigation resulted in a 50% increase in accumulated stem growth over a 5-year period (Nilsson, 1997). The current annual increment in irrigated± fertilised stands surpassed Swedish yield tables (cf. Eriksson, 1976) as well as the highest yield class for Norway spruce in Great Britain (Hamilton and Christie, 1971). When nutrients and water were not limiting (ILtreatment), the difference in yield of stemwood, between Flakaliden and Asa can be ascribed to the climatic differences (cf. Fig. 2). The length of the growing season at Asa was 190 days compared to 120 at Flakaliden and Asa had 40% more incident radiation during growing season than Flakaliden. Actual production in a forest stand is determined by the amount of light absorbed during the growing season (cf. Jarvis and Leverenz, 1983; Linder, 1985; Cannell, 1989), hence by the amount of leaf area in the stand. A major increase in leaf area as an effect of fertilisation has been reported from long-term experiments in stands of Scots pine in central Sweden (Linder and Axelsson, 1982) and Norway spruce (Albrektson et al., 1977; Axelsson and Axelsson, 1986). Linder and Axelsson (1982) also found that the increase in canopy size could not explain the even higher increase in volume growth, indicating a shift in carbon allocation within the trees from belowground to aboveground growth. To test whether this response

Fig. 7. The relationship between annual stemwood production and the estimated amount of absorbed photosynthetically active radiation (APAR) during the growing season in young Norway spruce stands at Flakaliden. Estimates are presented for 5 years: 1990 (*), 1991 (5), 1994 (~), 1995 (}), and 1996 (&) for control (C) stands (open symbols) and irrigated±fertilised (IL) stands (filled symbols). The fitted linear regressions are; control: y ˆ 0.2177x ÿ 13.454 (R2 ˆ 0.866; p ˆ 0.021), and irrigation± fertilisation: y ˆ 0.5618x ÿ 85.228 (R2 ˆ 0.997; p < 0.0001).

could be con®rmed in the Flakaliden experiment, the amount of intercepted radiation during ®ve growing seasons was plotted against measured stemwood production (dry mass) in the (C) and (IL) stands (Fig. 7). The leaf area of the stands was estimated from regressions derived from the biomass harvests and from annual measurements of diameter. The amount of incident radiation was measured values for the period May±August each year. The amount of absorbed radiation was estimated by Lambert±Beer's law (Monsi and Saeki, 1953) and an extinction coef®cient estimated for the stands in question (Stenberg et al., 1995). There was a strong linear relationship between absorbed radiation and annual production of stemwood (dry mass) in both treatments (C: R2 ˆ 0.866, p ˆ 0.021; IL: R2 ˆ 0.997, p  0.0001). The slope of the regression lines indicated that there was a major difference between treatments in the conversion ef®ciency of absorbed radiation into stemwood. Fertilised stands converted more of the absorbed energy into stemwood than did unfertilised stands: this agrees with earlier reports on altered patterns of carbon allocation in conifers as an effect of fertilisation

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59

methods. There is, however, a need to develop better predictive tools, which will allow realistic analysis of future yields, product quality, and economy in such systems. More intensive production methods should not aim at increasing productivity alone, but must assign a high importance to achieving sustained yields, as well as to the total carbon balance of the managed ecosystems.

(e.g. Linder and Axelsson, 1982; Axelsson and Axelsson, 1986). The long-term productivity of a site is generally re¯ected by the site index (SI), estimated by means of site curves from observed top height at a given age. However, after a sudden improvement in fertility, resulting from fertilisation and irrigation, the natural relationship between top height and age is changed, as well as the `natural' distribution of tree sizes within the stand (Fig. 5). This makes it dif®cult to use conventional methods to estimate the improved site index. Another complicating factor is that for dominant trees the relationship between stem diameter and height was affected by fertilisation (Fig. 6). A change in stem form as an effect of fertilisation has earlier been reported for two species of pine by Snowdon (1981, 1985) and for Norway spruce by Mead and Tamm (1988). The fact that smaller trees in the fertilised stands (F and IL) were taller than trees of the same diameter in unfertilised (C and I) stands agrees with ®ndings by Nilsson and HaÈllgren (1993). They studied the effect of light competition between trees in the (IL) stands at Flakaliden, and found that shading signi®cantly increased height development and reduced diameter growth. The experiments have shown that, by improving nutrient and water availability, it is possible to more than double the production of ®bre and wood products by the introduction of more intensive silvicultural

Acknowledgements The nutrient optimisation experiments at the Flakaliden research site and Asa were established with support from The Swedish Forestry Research Foundation (SSFf) and The Swedish Council of Forestry and Agricultural Research (SJFR). Major support for operational costs and research was later obtained from SJFR, The Swedish Environmental Protection Agency (NV), The Swedish National Board for Industrial and Technical Development (NUTEK), and the former Swedish State Power Board (Vattenfall AB). We are most grateful to Bengt-Olov Wigren, Ulla Nylander, Gunnar Karlsson, Magnus Pettersson, and Jan Parsby, for their skilful technical help in the ®eld. Many thanks are also due to Jeremy Flower-Ellis for continuous help and valuable discussions during the study and preparation of the manuscript.

Appendix A Stocking, height, diameter, basal area, volume (over bark), and mortality in young stands of Norway spruce in a nutrient optimisation experiment at Flakaliden. Top height refers to average height of the, by diameter, 100 largest trees per hectare. The treatments were untreated controls (C), irrigation (I), annual fertilisation (F), and `daily' supply of nutrients combined with irrigation (IL). Tree age at the start of the treatments was 28 years. Treatment

Plot

Year

Stocking (stems haÿ1)

Mean height (m)

Top height (m)

Mean diameter (mm)

Basal area (m2 haÿ1)

Volume (m3 haÿ1)

C

4

C

6

C

10

C

14

1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996

2660 2630 2630 2580 2510 2480 2400 2400 2400 2740 2740 2740

2.6 3.6 4.6 2.6 3.6 4.8 3.0 4.2 5.4 3.1 4.0 4.9

4.8 6.2 7.4 4.5 6.0 7.5 5.0 6.3 7.8 5.4 6.6 7.4

30 48 65 30 50 69 36 57 76 37 57 68

2.2 5.1 9.2 2.1 5.2 10.1 2.8 6.4 11.0 3.37 7.3 10.2

6.2 15.2 30.7 5.6 15.4 34.1 7.9 20.2 39.4 9.6 22.6 35.0

Dead trees (stems haÿ1) 30 30 70 100

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J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

Appendix A (Continued ) Treatment

Plot

Year

Stocking (stems haÿ1)

Mean height (m)

Top height (m)

I

3

I

8

I

12

I

16

F

2

F

5

F

11

F

15

IL

1

IL

7

IL

9

IL

13

1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996 1986 1991 1996

2550 2540 2530 2170 2160 2140 2250 2250 2250 2660 2660 2660 2120 2120 2110 2110 2060 2030 2190 2190 2190 2250 2250 2240 2170 2170 2170 2330 2330 2320 2350 2330 2310 2530 2520 2520

3.3 4.3 5.2 3.0 4.1 5.0 2.8 4.0 5.1 2.9 4.1 5.2 3.3 5.4 7.6 2.9 4.8 6.9 2.9 4.9 7.2 3.2 5.2 7.4 3.0 5.1 7.3 2.8 4.6 6.8 3.1 5.1 7.1 2.9 5.1 7.4

5.2 6.6 7.8 4.9 6.2 7.7 4.2 5.8 7.0 4.8 5.9 7.3 5.5 7.6 9.6 4.6 6.7 8.6 4.7 6.8 8.8 5.0 6.7 8.7 5.3 7.5 9.1 4.5 6.7 8.7 5.2 7.4 9.4 5.2 7.4 8.8

Mean diameter (mm) 42 57 72 37 55 72 35 55 74 37 56 73 43 83 120 36 71 108 36 75 113 41 79 116 37 77 115 33 70 106 38 76 113 35 77 116

Basal area (m2 haÿ1) 3.8 7.1 10.9 2.7 5.6 9.2 2.5 5.6 10.0 3.2 6.7 11.3 3.4 11.8 24.4 2.4 8.9 20.4 2.4 9.7 21.5 3.4 11.5 24.3 2.6 10.3 22.7 2.3 9.2 21.1 2.9 11.0 24.0 2.7 11.8 26.5

Volume (m3 haÿ1) 11.0 22.7 39.2 7.5 17.3 32.4 6.7 17.0 34.0 8.8 20.5 38.9 10.0 42.2 107.9 6.7 29.8 84.0 6.8 32.0 88.3 9.5 39.2 102.7 7.5 35.3 96.1 6.2 29.8 84.3 8.2 37.8 102.3 7.8 40.5 112.6

Dead trees (stems haÿ1) 10 20 10 30

10 50 80

10

10 20 40 10 10

Appendix B Stocking, height, diameter, basal area, volume (over bark), and mortality in young stands of Norway spruce in a nutrient optimisation experiment at Asa. Top height refers to average height of the, by diameter, 100 largest trees per hectare. The treatments were untreated controls (C), irrigation (I), annual fertilisation (F), and `daily' supply of nutrients combined with irrigation (IL). Tree age at the start of the treatments was 15 years. Treatment C

Plot

Year

Stocking (stem haÿ1)

1

1987 1991 1995

2490 2490 2490

Mean height (m) 3.2 5.6 7.3

Top height (m)

Mean diameter (mm)

Basal area (m2 haÿ1)

Volume (m3 haÿ1)

4.6 7.8 10.1

34 62 83

2.6 8.3 14.4

7.3 30.7 64.4

Dead trees (stem haÿ1)

J. Bergh et al. / Forest Ecology and Management 119 (1999) 51±62

61

Appendix B (Continued ) Treatment

Plot

Year

Stocking (stems haÿ1)

Mean height (m)

Top height (m)

C

6

C

13

I

2

I

5

I

9

I

11

F

3

F

4

IL

7

IL

8

IL

10

IL

12

1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995 1987 1991 1995

2380 2380 2380 2200 2200 2180 2320 2320 2310 2640 2640 2630 2380 2380 2380 2180 2180 2160 2410 2410 2410 2760 2760 2760 2930 2930 2930 2090 2090 2070 2190 2190 2190 1760 1760 1750

3.7 6.3 8.3 3.3 5.9 7.8 3.4 6.1 9.0 3.6 6.1 8.7 3.3 6.0 8.5 3.0 5.2 7.9 3.8 6.7 8.7 3.6 6.7 8.8 3.2 6.2 9.7 4.0 7.1 10.5 3.9 7.0 9.7 3.5 6.5 9.0

5.1 8.8 10.5 5.5 8.6 11.0 4.6 7.9 10.7 5.0 8.4 10.8 4.7 8.4 10.7 4.5 7.4 10.2 5.3 9.0 10.7 4.9 8.5 10.5 4.8 8.8 12.4 5.5 9.7 12.8 5.2 8.9 12.3 4.9 8.6 11.5

References Albaugh, T.J., Allen, H.L., Dougherty, P.M., 1997. Leaf area and growth responses of loblolly pine to nutrient and water additions. For. Sci., submitted for publication. Albrektson, A., Aronsson, A., Tamm, C.O., 1977. The effect of forest fertilisation on primary production and nutrient cycling in the forest ecosystem. Silva Fenn. 11, 233±239. Andersson, S.-O., 1954. Funktionen und Tabellen zur Kubierung kleiner BaÈume. Medd. Stat. SkogsfoÈrsoÈksanst. 44 (12), 1±29 (in Swedish with German summary). Aronsson, A., Elowson, S., 1980. Effects of irrigation and fertilisation on mineral nutrients in Scots pine needles. Ecol. Bull. (Stockholm) 32, 219±228.

Mean diameter (mm) 40 73 97 40 78 96 35 67 95 38 67 93 38 71 99 32 68 97 41 87 114 39 85 110 33 78 115 47 96 135 45 92 131 45 98 137

Basal area (m2 haÿ1) 3.4 10.6 18.4 3.7 11.9 18.3 2.6 8.8 17.4 3.4 10.0 19.4 3.1 10.2 19.4 2.3 8.9 17.7 3.6 15.3 26.2 3.5 16.3 27.3 2.8 14.9 32.4 4.1 16.0 31.5 3.9 15.5 31.3 3.3 14.3 27.7

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20 10 10

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