Effects of biomass removal in thinnings and compensatory fertilization on exchangeable base cation pools in acid forest soils

Forest Ecology and Management 122 (1999) 29±39

Effects of biomass removal in thinnings and compensatory fertilization on exchangeable base cation pools in acid forest soils Bengt A. Olsson* Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, P.O. Box 7072, S-750 07 Uppsala, Sweden

Abstract The effects of whole-tree harvesting and fertilization at ®rst thinning on the exchangeable pools of base cations in the soil were examined on four sites ®ve years after thinning. One site with Norway spruce (Picea abies (L.) Karst.) is located in southwestern Sweden and three other sites with Scots pine (Pinus sylvestris L.) are located in southeastern, south-central and northern Sweden. On all sites, the stands were thinned, and the following treatments were applied in a randomized block design (n ˆ 3): (i) stem-only harvest, logging residues left on site; (ii) whole-tree harvesting; (iii) whole-tree harvesting combined with compensatory fertilization (N±P±K were applied one year after thinning in the form of inorganic fertilizers, and the amounts were equal to the nutrient content of the harvested logging residues); (iv) stem-only harvesting in combination with N (pine sites) or NP (spruce site) fertilization (ammonium nitrate, 150 kg N haÿ1, superphosphate, 30 kg P haÿ1) and (v) whole-tree harvesting and nitrogen fertilization as in treatment (iv). There were no general treatment effects revealed from analysis across all sites. Effects of harvesting intensity were detected on two sites, indicating reduced pools of exchangeable K, Mg and Ca after whole-tree harvesting. The reductions were less pronounced than the effects observed in other studies from experiments with whole-tree harvesting at clear-felling of mature forests. Effects of application of nitrogen alone were more frequently observed than effects of harvesting of logging residues. In most cases, base cation pools were lower after N treatment than after other treatments, and this effect was probably due to exchange and translocation phenomena. However, Ca pools had increased after NP treatment on the spruce site due to high content of Ca in P fertilizer. No effect of compensatory fertilization with N±P±K was detected. A comparison between exchangeable nutrient pools before thinning and ®ve years after indicated that K was generally lost from the soil pro®le. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Base cations; Nitrogen fertilization; Norway spruce; Picea abies; Pinus sylvestris; Scots pine; Soil acidi®cation; Thinning; Wholetree harvesting

1. Introduction The use of logging residues for energy purposes has increased in Sweden during recent decades. Today, the supply of forest fuels is ca. 10 Twh annually, or 12% of *Corresponding author. Tel.: +46-0-18-671911; fax: +46-0-18673430; e-mail: [email protected]

all bioenergy used in Sweden (Anonymous, 1996). The major part of this resource comes from clearfellings, but utilization of residues from thinnings occurs and is expected to increase (Brunberg and Hillring, 1996). Several studies have indicated that whole-tree harvesting can be a greater threat to sustainable forest production than stem-only harvesting is, owing to the

0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 0 3 0 - 4

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B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

increased removal of nutrients from forest sites by whole-tree harvesting (e.g., Weetman and Webber, 1972; MaÈlkoÈnen, 1976; Kimmins, 1977; Wells and Jorgensen, 1979; Lundkvist, 1993). Reductions in forest growth from whole-tree harvesting may primarily result from reduced availability of nitrogen since this element normally limits forest growth in most boreal and temperate forests. Consequently, nitrogen application may correct for growth reductions, and this was demonstrated to occur for Douglas ®r stands (Compton and Cole, 1990). Depletion of base cations from forest soils may also have profound effects in the long run since this process is normally associated with soil acidi®cation. In areas receiving high levels of nitrogen deposition, low base cation availability may lead to nutrient imbalance in forest trees. Nykvist and RoseÂn (1985) and Olsson et al. (1996) have shown that whole-tree harvesting at ®nal fellings leads to a reduction in base saturation, particularly in the humus layer. In their study of four coniferous forest sites, Olsson et al. (1996) found that reductions in base saturation following whole-tree harvesting was mostly associated with reduced pools of exchangeable Ca and Mg and, to a lesser extent, with reductions in K. Na pools were not affected. From a national perspective, increased soil acidity and depletion of base cation pools is accentuated in southern parts of the country because both forest growth and acid deposition increase towards the south (Nilsson, 1993). Looking back over recent decades, forest soils in southern Sweden have lost signi®cant base cation reserves, leading to acidi®cation of even the subsoil (Falkengren-Grerup et al., 1987; HallbaÈcken, 1992). Furthermore, long-term budgets indicate that weathering and deposition of base cations do not keep pace with losses through leaching and whole-tree harvesting (RoseÂn, 1989; Olsson et al., 1993). In general, we expect that base cation depletion in forest soils from harvesting will be proportional to the harvesting intensity. Thus, whole-tree harvesting at thinnings should lead to smaller reductions in base cation pools than wholetree harvesting at clear-fellings does. Between 1984 and 1986, four ®eld experiments were established in Sweden to study the effects of whole-tree harvesting at ®rst thinning on forest growth. These experiments also included treatments with regular nitrogen fertilization and compensatory fertilization with N±P±K. Field experiments using the

same design were also established in Finland and Norway. Jacobson et al. (1996) examined tree growth responses on all sites ®ve years after the treatments and found little or no reduction in growth after wholetree harvesting compared with the growth response after stem-only harvesting. Fertilization with nitrogen could compensate for these losses. Egnell and Leijon (1997), who studied four other forest stands in Sweden, also found little or no reduction in forest growth 6±10 years after whole-tree harvesting at thinnings. The aim of the present paper is to examine the effects of slash removal and compensatory fertilization on the pools of exchangeable base cations in the soil at the sites of the four Swedish experiments studied by Jacobson et al. (1996). Procedures for soil sampling, extraction and chemical analysis were the same as in previous studies (Olsson et al., 1996) in order to make it possible to compare the effect of slash removal at thinnings with effects of whole-tree harvesting at clear-cutting. 2. Materials and methods The four study sites represent a wide range of climate conditions. The most productive site is the È rsaÊs Norway spruce (Picea abies (L.) Karst.) stand in O in southwestern Sweden, located on a silty-sandy till. Ê mot are Two experimental sites in Vetlanda and A Scots pine (Pinus sylvestris L.) forests with similar levels of productivity located in southern and central Sweden, respectively. The study site at LakatraÈsk is a pine stand with low productivity located in northern Sweden. More details about the study sites are given in Table 1. Stand characteristics are given in Table 2. All sites were subjected to ®ve treatments applied in a randomized block design with one replicate within each block (n ˆ 3). Each plot (25  25 m2) was surrounded by a 5 m border strip. All plots were thinned and the thinning grade was ca. 30% of the basal area (Table 2). The following treatments were applied: c: Control/conventional harvest; thinnings with logging residues left on site. The minimum bolt length was 3 m, and the top diameter over bark was 6 cm. w: Whole-tree harvesting; thinning and removal of all above-ground tree biomass.

B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

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Table 1 Characteristics of the experimental sites Site a Location Altitude (m asl) Mean annual temperature (8C) Precipitation (mm yearÿ1) Forest type Soil type Soil texture Clay content (%) Humus form Bulk density of mineral soil (<2 mm, g lÿ1) 0±10 cm 10±20 cm a

È rsaÊs 181 O 0

184 Vetlanda 0

Ê mot 219 A 0

204 LakatraÈsk

57823 N 138050 E 170 6.0 1000 Mesic dwarf-shrub with low herbs Podsol silty-sandy till 0 Mor-moder

57825 N 158180 E 145 6.1 680 Mesic-dry dwarf-shrub

61802 N 168120 E 350 3.8 750 Mesic dwarf-shrub

668190 N 218200 E 120 1.8 670 Mesic-dry dwarf-shrub

Podsol sandy-silty till n.d. Mor

Podsol sandy-silty till 0.2 Mor

Podsol sandy-silty till 0 Mor

908 940

620 534

575 552

519 431

Notations used for the locations after Jacobson et al. (1996).

ammonium nitrate in the year thinnings were made. In addition, 30 kg P haÿ1 was applied in the form of superphosphate on the spruce site and 1 kg B haÿ1 was applied on the northern pine site. wN: Whole-tree harvesting with nitrogen fertilization; thinnings were made as in treatment (w), and fertilizers were applied as in treatment (cN).

wF: Whole-tree harvesting and compensatory fertilization; thinnings and harvests were made as in (w), but N, P and K were applied one year after logging. The dose of N, P and K applied on each site was equal to the nutrient amounts removed in the logging residues. Ammonium nitrate, superphosphate and potassium chloride were used as fertilizers. cN: Conventional harvest with nitrogen fertilization; thinnings were made as in the control plots, and 150 kg N haÿ1 were applied in the form of

The ®rst soil sampling was carried out shortly after thinnings in August through October, and ®ve years later, a second soil sampling was made during the

Table 2 Stand characteristics before and after thinnings and experimental treatments. Nutrient content of harvested logging residues was determined one year after thinning in the wF treatment (data from Jacobson et al., 1996) Site

È rsaÊs 181 O

184 Vetlanda

Ê mot 219 A

204 LakatraÈsk

Tree species Site index (H100, m) a Stand age at thinning (year) Stem density after thinning (haÿ1) Stem wood removed (m3 haÿ1) Logging residues dry mass (Mg haÿ1) nutrient content N (kg haÿ1) P K Ca Mg

Picea abies G 36 30 1290 57

Pinus sylvestris T 26 36 1120 49

Pinus sylvestris T 27 30 1400 56

Pinus sylvestris T 19 71 1030 43

a

11.53 84 9 28 40 10

7.05 37 4 14 19 3

8.94 41 4 16 22 4

H100 refers to height (m) of the predominant trees at stand age 100 years (HaÈgglund and Lundmark, 1977).

4.59 21 3 9 14 2

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B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

same season. In each experimental plot, 25 samples were taken from the litter layer (L), humus layer (FH) and the mineral soil (0±20 cm divided into four 5 cm layers) to make one composite sample for each plot and soil layer. A corer (é 107 mm) was used for sampling the organic layers, and a steel corer (é 27 mm) was used for sampling the mineral soil. Litter and humus samples were sifted through a 5 mm mesh net, and the mineral soil was passed through a 2 mm mesh net. Fresh homogenized samples were dried at 858C overnight for dry matter determinations. Soil extracts were prepared by adding 5 g (organic horizons) or 20 g (mineral soil) of air-dried (308C) samples in 250 ml polyethylene bottles and adding 100 ml of 1 M NH4Cl. Extractions were made for 2 h. Chemical analysis of Na, K, Ca and Mg in the ®ltered extracts was performed with an ICP Atomic Emission Spectrophotometer (Jobin Yvon JY-70 Plus). The amounts of nutrients per unit area in the organic horizon were calculated from the actual dry mass of the sifted organic layers per unit area (given by the 25 samples per plot) multiplied by the nutrient concentration per unit dry mass. Calculations of nutrient amounts in the mineral soil were made based on bulk densities of the ®ne soil fraction (<2 mm). To account for stoniness, the bulk density of the ®ne soil fraction was obtained from material excavated from soil pits (0.5  0.5 m2, depth 0±20 cm). Thus, a low value of bulk density re¯ects high stoniness (Table 1). One soil pit was excavated on the border line of each plot. Mean values (n ˆ 15) of bulk densities for each site and soil layer (0±10, 10±20 cm) were used for the calculations. Statistical analyses of treatment differences within sites were made using one-way ANOVA (the GLM procedure, SAS Institute, 1989) and an LSD test to detect differences between particular treatments. Contrast (p < 0.05) was used to detect the effects of the slash, nitrogen and compensatory fertilization that were combined in different treatments. Statistical analyses on treatment differences across sites were carried out using a nested ANOVA. In this model, blocks were nested within sites. 3. Results The pools of exchangeable base cations were fairly Ê mot, whereas the more prohigh at Vetlanda and A

È rsaÊs had lower reserves. The northern ductive site at O site at LakatraÈsk had very low pools of base cations (Fig. 1). This pattern was mostly determined by the Ca pools of the different sites. On the other hand, Na È rsaÊs than at other sites, pools were much higher at O which could be expected from its location, which is not far from the west coast of Sweden. K and Mg pools were particularly low at LakatraÈsk. 3.1. Treatment effects There were no general differences between treatments in exchangeable pools of base cations in the pro®le ®ve years after harvesting (nested ANOVA, Table 3). Statistical tests on treatment differences for single sites and different soil layers revealed few signi®cant effects, and those that were revealed were mostly for contrast analyses (ANOVA, Table 4). Few signi®cant treatment differences were found when statistical tests were performed on the change over time in exchangeable base cation pools in the humus, the mineral soil (0±20) and the total soil pro®le (Table 5). Thus, possible problems of statistical inference may arise from the large number of statistical tests, but the consistency of the pattern supports a real effect. Contrast analyses on soil data ®ve years after treatments detected signi®cant effects from the Ê mot site only. whole-tree harvesting factor at the A Compared with conventional harvesting, removal of logging residues resulted in lower pools of K in the 5± 10 cm layer and in the total soil pro®le, lower pools of Mg in the 0±10 cm horizon and lower pools of Ca in the 0±5 cm horizon (Table 4, Fig. 1). Contrast analyses on changes in base cation pools over time detected a greater loss of Mg in the humus layer at the Vetlanda site. The effect of nitrogen fertilization appeared to be more profound than the effects of the slash and the compensatory fertilization, since signi®cant treatment effects of N occurred at all sites, whereas no effect of the compensatory treatment was revealed. An overall trend was that nitrogen fertilization resulted in lower È rsaÊs site, exchangeable pools of K and Na. At the O exchangeable Na and K pools were lower in the humus layer in N-treated plots, and negative effects also occurred for Na in the 15±20 cm layer. Negative effects of N fertilization were also revealed for K at

B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

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Fig. 1. Exchangeable amounts of K, Ca, Mg, Na (kg haÿ1) and sum of base cations (K ‡ Ca ‡ Mg ‡ Na, kmolc haÿ1) in the soil profiles of the four study sites five years after first thinning. Mean values (n ˆ 3) and SD bars. Treatments are: c ˆ control/stem-only harvesting; w ˆ whole-tree harvesting; wF ˆ whole-tree harvesting and compensatory fertilization (N±P±K); cN ˆ stem-only harvesting and regular nitrogen fertilization; wN ˆ whole-tree harvesting and regular nitrogen fertilization.

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B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

Fig. 1. (Continued )

Table 3 Results of statistical analyses (p-values) across four study sites (nested ANOVA) on exchangeable pools of base cations in the soil (FH layer and the uppermost 20 cm of the mineral soil) Source

df

K (kg haÿ1)

Ca

Mg

Na

BC a (kmolc haÿ1)

Site

3

0.0001 *** c

0.0001 *** c

0.0001 *** c

0.0001 *** c

0.0001 *** c

Block (site)

8

0.7841 ns b

0.1742 ns b

0.5957 ns b

0.9119 ns b

0.2378 ns b

Treatment

4

0.2719 ns b

0.5271 ns b

0.4414 ns b

0.3283 ns b

0.5233 ns b

12

0.8267 ns b

0.9573 ns b

0.9220 ns b

0.7001 ns b

0.9574 ns b

Interaction Treatment±site a

Equivalent sum of K, Ca, Mg and Na. Not significant. c p < 0.001. b

B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

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Table 4 Effects of whole-tree harvesting, nitrogen fertilization and compensatory fertilization on exchangeable base cation pools for different sites and soil layers. Results from ANOVA analysis on nutrient pools five years after thinning and treatments. p-Values are given when p<0.05 P Site Soil layer K Ca Mg Na BC È rsaÊs O

ns

0±5 cm 5±10 cm 10±15 cm 15±20 cm P All layers

A a: 0.043 (w c>wN cN) N b: 0.006 ns ns ns ns ns

Vetlanda

Humus 0±5 cm 5±10 cm 10±15 cm 15±20 cm P All layers

N b: 0.023 ns ns ns ns ns

Ê mot A

Humus 0±5 cm 5±10 cm 10±15 cm 15±20 cm P All layers

LakatraÈsk

Humus 0±5 cm 5±10 cm 10±15 cm 15±20 cm P All layers

Humus

ns

N b: 0.042

ns

ns ns ns ns ns

ns ns ns N b: 0.039 ns

ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns WTH c: 0.034 N b: 0.047 ns WTH c: 0.025

(N b: 0.0577) WTH c: 0.014 ns ns ns ns

ns WTH c: 0.01 WTH c: 0.033 ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns ns ns ns ns ns

ns N b: 0.0426 ns ns ns ns

ns ns ns ns ns ns

N b: N b: N b: N b: N b:

0.026 0.046 0.012 0.033 0.032

a

Indicates significant difference between treatments from ANOVA analysis (with result from LSD test). Indicates significant effects of nitrogen as determined by contrast analysis (no effects of compensatory fertilization were detected). c Indicates significant effects of whole-tree harvesting treatments as determined by contrast analysis (no effects of compensatory fertilization were detected). b

Ê mot (10±15 cm) and for Vetlanda (humus), for K at A Na at LakatraÈsk (0±5 cm). To some extent, nitrogen fertilization had a similar effect on the divalent base cations as it had on the monovalent ions. Exchangeable pools of Ca in the Ê mot tended to be negatively affected humus layer at A by N fertilization (p ˆ 0.058). At LakatraÈsk, the general increase in Mg pools over time in the mineral soil was lower in treatments with N. On the other hand, nitrogen fertilization resulted in higher pools of È rsaÊs site, exchangeable Ca in the mineral soil at the O as is indicated from comparisons of Ca pools ®ve years after treatments (Table 4) as well as from analyses of changes in Ca pools over time (Table 5).

3.2. Changes in nutrient pools over time Some overall trends were revealed for the change in base cation pools over the ®ve-year period. Generally speaking, when data from two sampling times are used to give the change in nutrient pools over time there is a gain in information, but sampling and analytical errors from the ®rst sampling would be introduced. For example, a reduction in nutrient pools in the humus layer that appears to be compensated for by an increase in the mineral soil can be the result of inconsistent delimitation of the humus layer from the mineral soil between sampling times. Errors of this type make comparisons

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B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

Table 5 Change in exchangeable pools of base cations after thinning and treatments (five years). Mean values are given for the humus, the mineral soil (0±20 cm) and the total profile (n ˆ 3). p-Values are given when p < 0.05 Site

Soil layer

Treatment c

Difference w

wF

ÿ2.54 ÿ3.69 ÿ6.23

ÿ1.81 ÿ3.82 ÿ5.63

ÿ6.33 ÿ13.4 ÿ21.8

ÿ6.49 ÿ6.21 ÿ12.7

ns ns ns

cN

wN

ÿ1

È rsaÊs O

Humus 0±20 P cm

K (kg ha ) ÿ0.74 ÿ6.38 ÿ7.12

Vetlanda

Humus 0±20 cm P

ÿ2.64 ÿ6.49 ÿ9.13

ÿ8.54 ÿ4.98 ÿ13.5

ÿ4.12 ÿ8.80 ÿ12.9

ÿ6.64 ÿ6.75 ÿ13.4

ÿ6.55 ÿ16.6 ÿ23.2

ns ns ns

Ê mot A

Humus 0±20 P cm

ÿ8.41 2.43 ÿ5.98

ÿ5.19 ÿ5.25 ÿ10.4

ÿ6.05 2.95 ÿ3.09

ÿ6.89 ÿ2.09 ÿ8.98

ÿ7.98 ÿ1.21 ÿ9.20

ns ns ns

LakatraÈsk

Humus 0±20 P cm

1.28 0.93 1.44

0.51 ÿ0.25 0.25

0.99 ÿ0.08 1.32

1.72 ÿ9.06 ÿ7.34

2.25 ÿ5.75 ÿ3.00

ns ns ns

ÿ9.41 ÿ13.4 ÿ22.8

19.8 ÿ9.37 10.5

ÿ7.27 14.6 ÿ9.57

ÿ6.49 20.2 14.2

ns N a: 0.014 ns

ÿ40.1 71.0 30.9

ÿ13.7 ÿ1.56 ÿ15.3

ÿ7.05 6.67 ÿ0.38

Ca (kg haÿ1) 1.60 ÿ5.64 ÿ4.04

È rsaÊs O

Humus 0±20 cm P

Vetlanda

Humus 0±20 cm P

ÿ0.49 ÿ22.4 ÿ22.9

Ê mot A

Humus 0±20 cm P

ÿ37.1 75.9 38.8

LakatraÈsk

Humus 0±20 P cm

È rsaÊs O

Vetlanda

ÿ38.6 141 102

ÿ12.2 ÿ31.3 ÿ43.5

ns ns ns

ÿ19.2 118 99.0

ns ns ns

2.36 42.2 44.5

ÿ0.39 34.8 34.4

14.3 9.56 23.9

4.52 16.1 20.7

9.13 10.4 19.5

12.1 8.02 20.1

16.8 6.70 27.6

ns ns ns

Humus 0±20 cm P

Mg (kg haÿ1) ÿ0.77 1.15 0.38

ÿ2.05 ÿ0.15 ÿ2.21

1.43 0.67 10.5

ÿ3.92 0.48 ÿ9.57

ÿ3.92 ÿ0.44 14.2

ns ns ns

Humus 0±20 cm P

ÿ0.17 ÿ2.17 ÿ2.34

ÿ3.82 4.52 0.70

ÿ1.85 ÿ4.10 ÿ5.94

ÿ0.76 ÿ0.61 ÿ1.37

ÿ1.74 ÿ5.78 ÿ7.52

WTH b: 0.042

Ê mot A

Humus 0±20 cm P

ÿ2.72 5.65 2.93

ÿ1.25 0.68 ÿ0.57

ÿ0.67 1.52 0.85

ÿ2.82 9.51 6.69

ÿ2.36 4.55 2.19

ns ns ns

LakatraÈsk

Humus 0±20 cm P

1.35 1.08 2.43

0.37 0.92 1.29

0.63 0.89 1.81

1.02 ÿ0.09 0.93

1.37 0.37 2.04

ns

ns N a: 0.0142 ns

B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

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Table 5 (Continued ) Site

Soil layer

Treatment c

Difference w

wF

cN

wN

ÿ1

È rsaÊs O

Humus 0±20 cm P

Na (kg ha ) 0.26 ÿ3.58 ÿ3.33

ÿ0.89 ÿ4.91 ÿ5.80

0.71 ÿ2.45 ÿ1.74

ÿ1.79 ÿ6.85 ÿ9.54

ÿ0.92 ÿ0.31 ÿ1.23

ns ns ns

Vetlanda

Humus 0±20 cm P

ÿ0.29 7.49 7.20

ÿ0.63 8.65 8.01

ÿ0.38 6.91 6.52

ÿ0.33 7.34 7.01

ÿ0.41 7.91 7.50

ns ns ns

Ê mot A

Humus 0±20 cm P

ÿ0.78 ÿ2.09 ÿ2.87

ÿ0.25 ÿ1.38 ÿ1.63

ÿ0.62 ÿ4.44 ÿ5.06

ÿ0.13 ÿ3.56 ÿ3.69

ÿ0.87 0.07 ÿ0.79

ns ns ns

LakatraÈsk

Humus 0±20 cm P

0.20 1.24 1.44

0.26 0.57 0.83

0.26 1.14 1.43

0.34 1.12 1.47

0.42 0.54 0.83

ns ns ns

BC (kmolc haÿ1) 0.009 ÿ0.742 ÿ0.506 ÿ0.987 ÿ0.497 ÿ1.73 ÿ0.118 ÿ2.56

1.09 ÿ0.617 0.476 ÿ0.959

ÿ0.925 0.130 ÿ1.88 ÿ0.598

ÿ3.92 0.797 ÿ0.031 ÿ0.936

ns ns ns ns

0±20 cm P

ÿ1.14 ÿ1.25

4.16 1.60

ÿ0.340 ÿ1.30

0.429 ÿ0.169

ÿ2.12 ÿ3.06

ns ns

Ê mot A

Humus 0±20 P cm

ÿ2.32 4.22 1.90

ÿ0.129 1.97 1.84

ÿ0.257 1.74 1.49

ÿ2.34 7.58 5.24

ÿ1.40 6.25 4.85

ns ns ns

LakatraÈsk

Humus 0±20 cm P

0.280 0.899 1.18

0.544 0.641 1.50

P

È rsaÊs O

Humus 0±20 cm P

Vetlanda

Humus

0.866 0.644 1.51

0.746 0.210 0.955

1.03 0.242 1.51

ns ns ns

a

Indicates significant effects of nitrogen as determined by contrast analysis (no effects of compensatory fertilization were detected). Indicates significant effects of whole-tree harvesting treatments as determined by contrast analysis (no effects of compensatory fertilization were detected). b

between sites less reliable, but they should not affect general trends revealed for speci®c elements. From this viewpoint, the results indicate an overall net loss of K in the soil pro®le that was greatest at the sites in southern Sweden (Table 5). Changes in exchangeable pools were less consistent for the other elements. The trend for Na was similar to the trend for K. At the two northern sites, exchangeable pools of Ca and Mg increased, and both negative and positive trends were observed for Ca and Mg at the southern sites.

4. Discussion The results indicate that harvesting of logging residues at ®rst thinning has a small negative effect on the exchangeable pools of base cations from a short-term perspective since frequent effects from logging residues were detected only for the pine site Ê mot. This outcome would be expected from the at A moderate amounts of nutrients left in the residues. In comparison with the observations by Olsson et al. (1996) on more long-term (15 years) effects of whole-

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B.A. Olsson / Forest Ecology and Management 122 (1999) 29±39

tree harvesting at ®nal cuttings, in the present study, nutrient amounts in the logging residues were only about one-third or less of those for comparable sites, and signi®cant effects from whole-tree harvesting on soil nutrient pools were negligible. However, the relative effects of whole-tree harvesting at thinnings might be expected to exceed those at ®nal felling because in the latter nutrient losses from logging residues may be relatively high owing to limited vegetation uptake (Likens et al., 1994). Apart from the quantitative differences between the present study and the study by Olsson et al. (1996), some qualitative differences appeared. In the present study, whole-tree harvesting tended to affect K and Mg pools more than Ca pools. In the study by Olsson et al. (1996), the negative effect of whole-tree harvesting was relatively greater for Ca and somewhat less pronounced for Mg, whereas both negative and positive effects were observed on K pools. In particular, K is more mobile in the soil and is more readily released from decomposing logging residues than Ca is (e.g., Abbott and Crossley, 1982; Edmonds, 1987; Fahey et al., 1991). Thus, it is possible that the weak effect of whole-tree harvesting on soil pools of K and Mg detected in this study appears to be a short term phenomenon and that a similar effect on other cations, most notably Ca, could be expected in the long run. Compared with the other treatments, the nitrogen fertilization had the most pronounced effects on the soil pools of the base cations. At the pine sites, the regular nitrogen fertilization resulted in lower pools of base cations. The amounts of ammonium nitrate applied were substantial compared with the nutrient load in the other treatments. Therefore, it is likely that the observed effects of N were due to exchange reactions and downward translocation of base cations in the soil. Another possible explanation is that nitrogen fertilization may increase the requirement for other nutrients in the soil. On the other hand, at the spruce site the nitrogen and phosphorus fertilization resulted in increased pools of exchangeable Ca (P was only applied on this site). This was likely an effect of the substantial amounts of Ca (75 kg Ca haÿ1) in the superphosphate fertilizer. No signi®cant effect was detected for the compensatory fertilization of N±P±K. One reason for this could be that fairly low amounts of N, P and K were

applied. Furthermore, Ca losses due to harvesting of logging residues were not fully compensated with the P fertilizer and no Mg was applied. Thus, the results of this study do not contradict the hypothesis that this compensatory treatment had a positive effect. In conclusion, the treatments of this study had consistent but no remarkable effects on the exchangeable pools of base cations. Regular nitrogen fertilization had greater impact on exchangeable base cation reserves in the soil compared to the effects of other treatments, and nitrogen application alone generally resulted in reductions in exchangeable base cations. To mitigate nutrient depletion in the soil following whole-tree harvesting, nitrogen applications should therefore be combined with applications of other macro-nutrients. Acknowledgements I thank Lars GoÈran OlseÂn for conducting the ®eldwork and Jim Adams, Kerstin AhlstroÈm, Cecilia Jansson, Irene Persson and Berit Solbreck for conducting the laboratory work. I thank HeleÂne Lundkvist and Staffan Jacobson for valuable comments on earlier versions of the manuscript. The Forestry Institute of Sweden is acknowledged for access of ®eld experiments. Erich Schultz (Proper English) is acknowledged for linguistic revision. This work received ®nancial support from the National Swedish Board for Industrial and Technical Development (NUTEK). References Abbott, D.T., Crossley Jr., D.A., 1982. Woody litter decomposition following clear-cutting. Ecology 63, 35±42. Anonymous, 1996. Energy in Sweden. Swedish National Board for Industrial and Technological Development. Info 332-1996 (In Swedish). Brunberg, B., Hillring, B., 1996. SkogsbraÈnsleuttag idag och i morgon. K. Skogs- o. Lantbr. akad. Tidskr. 135(13), 11±24 (In Swedish). Compton, J., Cole, D.W., 1990. Impact of harvest intensity on growth and nutrition of second rotation Douglas fir. In: Dyck, W.J., Mees, C.A. (Eds.), Long-term field trials to assess environmental impacts of harvesting, Proceedings, IEA/BE Workshop, pp. 151±162. Edmonds, R.L., 1987. Decomposition rates and nutrient dynamics in small-diameter woody litter in four forest ecosystems in Washington, USA. Can. J. For. Res. 17, 499±509.

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