Bengt A. Olsson

Carbon and nitrogen in coniferous forest soils after clear-felling and harvests of different intensity

Abstract Soil carbon and nitrogen contents were determined at four coniferous forest sites in Sweden (Scots pine and Norway spruce in southern and northern Sweden, respectively) over a period of 15–16 years after clear-felling. Three levels of logging-residue harvesting had been applied: (i) conventional stem harvest (residues left on site); (ii) harvesting all above-ground tree parts except needles; (iii) above-ground whole-tree harvesting (no residues remaining). The forest floor and the top mineral soil layers (0–20 cm) were examined, excluding coarse woody detritus (i.e. dead branches and roots). When measured, 15–16 years following clear-felling, the content of C and N in the humus layer had decreased markedly at all sites, whereas increased amounts were frequently observed in the mineral soil. Total C pools decreased by 22% at the northern site with Norway spruce, and by 17% at the southern spruce site. The corresponding figures for N were 22% and 13%. No changes in the total pool were detected at the southern site with Scots pine, whereas at the northern pine site C decreased by 7% and N increased by 7%. Changes in C and N storage were accompanied by decreases in the C N ratio in the humus layer and increases in the C N ratio in the top 5 cm of the mineral soil. No general effect of harvest intensity on soil C or N pools was shown in analyses over all sites, but there was a site and treatment interaction for N, indicating site-specific effects on this element. At the northern spruce site, whole-tree harvesting as well as removal of woody residues only (i.e. needles left on site) resulted in a greater reduction of N pools in the humus compared with conventional harvest. In contrast, at the southern spruce site conventional harvesting resulted in a greater reduction in total N pools compared with the other treatments. Whole-tree harvesting increased C N ratios in the humus and 0–5 cm mineral soil layers compared with the other treatments. This effect was detected in the humus layer only 8 years after harvest at the southern sites, whereas at the northern sites it was observed in the mineral soil 16 years after felling. The implications of these results on N mineralization rates are discussed.

Effects of different forest harvest intensities on the pools of exchangeable cations in coniferous forest soils

Abstract Effects of harvest intensity on exchangeable cations (1 M NH 4 Cl extracts) were examined in four coniferous forest soils in Sweden, 15–16 years after clear-felling. Logging residues were harvested at three intensities, applied experimentally in a randomized block design ( n = 4): (i) conventional stem-only harvest, (ii) branches and stems harvested but needles left on the ground and (iii) whole-tree harvesting where all above-stump biomass was harvested. Two of the sites were located in northern Sweden, and the other two were in southern Sweden. In each region one site was occupied by a pure Scots pine stand ( Pinus sylvestris L.) and the other by a pure Norway spruce stand ( Picea abies (L.) Karst.). Whole-tree harvesting resulted in a lower level of base saturation, especially in the humus layer. Compared with base saturation levels in the stem-only harvest treatment, the reductions were 19%, 16% and 8% at the southern sites with spruce and pine and at the northern spruce site, respectively. At the northern pine site no significant treatment effect was detected. Analyses across all study sites of element pools in the humus and 0–20 cm mineral soil layers indicated that reductions in base saturation generally were associated with depletions of exchangeable pools of K, Ca and Mg. In addition, effective cation exchange capacity per unit area was in general lower after whole-tree harvesting compared with the other treatments. Effects on exchangeable pools of Mn and Zn were similar to those observed for base cations. Exchangeable pools of acidic cations were generally not affected, but at the southern sites pools of acidic cations increased with harvest intensity. No significant differences in soil pH(H 2 O) were detected between treatments. A relatively high proportion of the Ca left on site in the form of slash was recovered in the soil ca. 15 years after felling. This was not the case for K. Furthermore, the nutrient load recovered in the soil was lower at the southern site with Norway spruce than at the other sites. Apart from these inter-site differences, the results were consistent across all study sites.

Decomposition and nutrient release from Picea abies (L.) Karst. and Pinus sylvestris L. logging residues.

Abstract We analyzed the long-term dynamics of the decomposition of different fractions of forest litters by using models derived from a theory on decomposition and element cycling in organic matter. The analysis of decomposition was done (i) by measuring decomposition rates of and nutrient changes in needles, twigs, and branches in field experiments, and (ii) by estimating parameters used in the models with information derived from these experiments. The analysis showed that variability in decomposition rate decreases with increasing substrate diameter. We also used the models to predict the long-term dynamics of carbon, nitrogen, and phosphorus in logging residues. Our predictions suggest that from a short-term perspective the nutrient-rich needles and twigs are a more important nutrient source for the subsequent forest generation compared with branches. However, in the long run the nutrient concentration of the coarse litter fractions will also be important. The predicted amounts of carbon and nitrogen in logging residues were compared with measured amounts in humus layer. On a productive Norway spruce site remaining logging residues were, 16 years after clear-felling, predicted to increase carbon amounts in the forest floor by 50% and on a low productive Scots pine site by 100%. The corresponding nitrogen amounts in the forest floor should have been 30% higher at the spruce site and 70–80% higher at the pine site.

Influence of harvesting intensity of logging residues on ground vegetation in coniferous forests

Ground-vegetation succession was analysed at four Swedish coniferous sites 8-16 years after clear-felling. At each site, three levels of logging-residue harvesting were applied in a randomized block design: above-stump whole-tree harvesting (no residues remaining); harvesting all tree parts except needles; and conventional harvest (residues left on site). Vegetational changes at each site were analysed by principal component analyses. Successional changes were related to the first principal components, whereas residue treatments were related to components of the second or lower order. Treatment-related differences in species cover, as analysed by a two-way ANOVA, were noted at all sites, 8 as well as 16 years after harvest. The covers of most vascular plant species, notably herbs and graminoids, were generally lower after whole-tree harvesting, while the covers of epigeic lichens and Vaccinium myrtillus were higher. Bryophytes were mostly indifferent to logging residue treatment, except for Pleurozium schreberi which had a lower cover in plots without logging residues. The overall differences were small and resembled those found along natural soil fertility gradients. The presence of woody debris appeared to have a weak, suppressive effect on the cover of the graminoids Deschampsia flexuosa and Luzula pilosa. This effect was only evident on the first study occasion, about 8 years after felling. Our results suggest that the major influence of logging residues left on site was related to their nutrient contribution. The impact tended to decrease over time and, in a long-term perspective it is suggested that treatment differences in species cover will be small, especially in forests developing dense canopies. However, one cannot exclude the possibility that repeated whole-tree harvesting will produce more drastic effects by reducing soil nutrient availability.

Effects of slash removal and stump harvesting on soil water chemistry in a clearcutting in SW Sweden

Chemical properties of the soil water in a clearfelled Norway spruce forest in SW Sweden were monitored using lysimeters during a five‐year period. Measurements were made in plots treated as follows: (i) conventional harvest (CH), with slash and stumps remaining at the site, (ii) whole‐tree harvest (WHT), where only the slash was removed, and (iii) complete‐tree harvest (CTH), where both slash and stumps were harvested. Removal of slash from the clearcutting reduced concentrations of NO3 ‐, NH4 + and K+ in soil water below the rooting zone. In plots mechanically disturbed by stump harvesting, NH4 + concentrations increased strongly during the first year after felling. High levels were noted for two years, whereafter there was an additional two‐year period with elevated NO3 ‐ concentrations and reduced pH, indicating enhanced nitrification and NO3 ‐ leaching. The nitrate concentration drop coincided with a rapid expansion of the ground vegetation, mainly composed of Deschampsia flexuosa L. Trin., Carex pil...

Acidity in four coniferous forest soils after different harvesting regimes of logging slash

The soil acidity of four coniferous sites, two in S Sweden and two in N Sweden, were studied 7–9 years after clear‐felling. Each site had three different slash treatments; (a) all slash harvested, (b) only needles remaining, and (c) all slash remaining. Our results verify that harvesting of slash after clear‐cutting means an acidification of the organic horizon in acid forest soils. On plots subjected to full slash harvest the pH‐value of the forest floor was 0.2–0.4 units lower than that of slash‐covered ones, and at three of the four study sites the exchangeable acidity was significantly higher on the slash‐cleared plots. Plots with only needles remaining were generally intermediate in acidity. The change in exchangeable acidity corresponded to approximately 10% of the theoretically estimated proton load caused by slash harvesting. The rate of proton flux associated with whole‐tree harvesting varied between 7–65 meq m‐3 yr‐1, which roughly corresponds to the amounts deposited in precipitation. Harvestin...

Nutrient status in needles of Norway spruce and Scots pine following harvesting of logging residues.

Nutrient concentrations in current and 1-year old needles from two Picea abies (L.) Karst and two Pinus sylvestris L. stands in Sweden were determined 8–10, 16–18 and 22–24 years after clear-felling and experimental manipulation of harvesting intensity. On all sites, three levels of harvest intensity had been applied in a randomized block design (n=4); (i) conventional stem-only harvesting, where all logging residues (i.e. tops, branches and needles) were evenly distributed on the ground, (ii) harvesting all above-ground tree parts except needles and (iii) above-ground whole-tree harvesting (no residues left on site). At stand age 8–10 years, nitrogen concentrations in the current year needles in plots where all residues or needles only were retained were higher than in whole-tree harvested plots, whereas concentrations of K, Ca and Mg were lower. The latter response was interpreted as a dilution effect. P:N, K:N, Ca:N, Mg:N, Mn:N and Zn:N were in general higher after whole-tree harvesting treatments than after the treatments where all residues or only needles had been left on site. At stand age 16–18 years, no significant differences in nitrogen concentrations were observed between treatments, but the levels of Ca, Mg and Mn in both current and 1-year-old needles were lower after whole-tree harvesting than after the treatments where logging residues remained on site. By contrast, potassium levels in the foliage were highest in treatments where only the needles were left on site, whereas the lowest levels were observed for treatments where all residues was left. At stand age 22–24 years, the treatment effects had diminished, except for the effects on Ca and K on the southern Norway spruce stand. It is concluded that the nutrient release from logging residues enhances nutrient uptake in trees of the succeeding forest generation, but this effect does not occur simultaneously for all elements.

Relationships Between Tree and Soil Properties in Picea abies and Pinus sylvestris Forests in Sweden

The exchange of elements between plants and the soil in which they are growing creates reciprocal control of their element composition. Within plants, the growth rate hypothesis from ecological stoichiometry implies a strong coupling between C, N, and P. No similar theory exists for predicting relationships between elements in the soil or relationships between plants and the soil. We used a data set of element concentrations in needles and humus of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) forests in Sweden to investigate the extent to which relationships between elements (C, N, P, S, K, Ca, Mg, Fe, Mn, Al) can be observed within and between plants and soils. We found element composition to be more strongly controlled in needles than in humus. Elements that are covalently bound were also more strongly controlled, with no apparent differences between macro- and micronutrients. With the exception of N/C, there were surprisingly few relationships between elements in needles and humus. We found no major differences between the two tree species studied, but investigations of additional forest types are needed for firm conclusions. More control over element composition was exercised with respect to N than C, particularly in needles, so it might be advantageous to express nutrient concentrations relative to N rather than on a dry weight or carbon basis. Variations in many ecosystem variables appeared to lack ecological significance and thus an important task is to identify the meaningful predictors.

Dynamics of soil C, N and Ca in four Swedish forests after removal of tops, branches and stumps as predicted by the Q model

Abstract We used the Q model to examine the dynamics of carbon (C), nitrogen (N) and calcium (Ca) in the litter/soil system in different scenarios of harvesting intensities, S (stems only), SSl (stems and slash, i.e. tops, and branches including needles) and SSlSt (stems, slash and stumps including coarse roots). Empirical data from long-term field experiments in Sweden, two sites with Norway spruce and two with Scots pine with different levels of productivity, were used to calibrate the model against the stem-only treatment. The highest initial reduction in soil C, N and Ca stores was predicted for SSlSt, and the reduction was more pronounced at low productive sites than at the high productive ones. Most of the decline in soil C and Ca stocks was offset by the litter production in the following forest stand. N showed an initial phase of immobilisation in stumps and coarse roots, while N was immediately released from tops and branches, which contained N-rich needles. Removal of stumps and coarse roots in combination with slash resulted in a similar load of inorganic soil N as for the S treatment, whereas the SSl treatment with stumps left in the soil initially reduced the inorganic soil N pool.

Effects of wood ash, vitality fertilizer and logging residues on needle and root chemistry in a young Norway spruce stand

Abstract The nutrient status of a 6-year-old Norway spruce stand in south-west Sweden was examined 4 and 5 years after application of soil ameliorative treatments. In addition to control plots, three treatments were tested: granulated wood ash at 4150 kg dry weight (dw) ha−1, a nitrogen-free, dolomite-based vitality fertilizer (SkogVital) at 2×1500 kg ha−1 and the fine fraction of harvest residues (mostly needles) at 15,050 kg dw ha−1. The treatments had no short-term effect on height increment. Nitrogen (N) was probably the growth-limiting element, but phosphorus (P) and potassium (K) were also in suboptimum supply. The vitality fertilizer increased the foliar concentration (or the element/N ratio) of P, calcium (Ca) and zinc, while ash application increased the foliar Ca/N ratio. The vitality fertilizer also increased magnesium and Ca concentrations in roots and decreased aluminium concentrations, whereas logging residues and wood ash only increased manganese concentrations in roots. Thus, the vitality fertilizer had a more marked effect on tree nutrient status than the granulated wood ash.