Heléne Lundkvist

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.

Changes in forest-floor chemistry caused by a birch admixture in Norway spruce stands.

Abstract The aim of this study was to determine how soil chemistry and the distribution of fine roots (

Effects of organic matter removal on the soil food web: Forestry practices meet ecological theory

Abstract We examine the long-term effects of removing forest harvesting residues (above-ground whole-tree harvesting; WTH) in the soil food web, using data from two different experiments on slash removal and addition in Sweden. Removal of harvesting residues had negative effects on the abundances of animals at higher trophic positions and on more mobile animals. Predators such as gamasid mites, spiders and staphyliniid and cantharid beetles decreased by 30–50% after WTH at both sites. Microbidetritivorous dipterans and fungivorous Collembola decreased at the pine site, but not at the more productive spruce site. Enchytraeids and nematodes were usually not affected by WTH. No differences in fungal and bacterial biomass were found. The removal of organic matter had fairly long-term, of the order of decades, effects on the soil food web. The effects were quantitative rather than qualitative. No functional groups disappeared after WTH. Our findings do not support the hypothesis that soil food webs are entirely donor-controlled. The more pronounced effects of WTH at higher trophic positions indicates that the major interactions in the food webs we have studied are reciprocal, i.e., that consumers to some degree control the density of their resources.

Long-term effects of logging residue addition and removal on macroarthropods and enchytraeids

The long-term effects of logging residue addition and removal on soil macroarthropods and enchytraeids were examined in a Scots pine Pinus sylvestris L. stand in central Sweden. The study was performed 15-18 years after the treatments had been applied after clear-cutting in 1976. In comparison with plots receiving roughly twice the normal amount of residues, removal of logging residues (above-ground whole-tree harvesting) resulted in decreases in the total numbers of Collembola (springtails), gamasid mites, spiders, predatory insects and dipterous larvae, whereas no significant effects on enchytraeids and diplopods could be detected. Few effects on single species of Collembola and Gamasida were found. The composition of the soil fauna community, as well as food web structure, were significantly affected by whole tree harvesting, but the effects were quantitative rather than qualitative-most organism groups decreased, but the relative importance of different groups did not change markedly. Over the 4 years studied, community predictability (specifically, constancy) at the levels of higher taxa, functional groups and species did not differ substantially between the treatments. Predictabilities of higher taxa and functional groups were higher than predictability of species of Collembola and gamasid mites. Values of community predictability were similar to those found in other studies of forest soil fauna. It is concluded that whole tree harvesting may result in long-term decreases in the abundances of many soil animal groups. The possible impact of decreased abundances of fungivores and predatory arthropods on nutrient cycling and site productivity is discussed. It is argued that the direct effects of these changes on nitrogen mineralization are likely to be small. However, the possibility that the soil fauna may be involved in a positive feedback loop towards lower site productivity means that the observed long-term decreases in several organism groups should be of concern, at least on sites dominated by internal nutrient dynamics.

Soil organisms and carbon, nitrogen and phosphorus mineralisation in Norway spruce and mixed Norway spruce – Birch stands

Abstract We examined how soil organisms and C, N and P mineralisation are affected by admixing deciduous tree species, silver birch (Betula pendula) and woollen birch (B. pubescens), in managed Norway spruce (Picea abies) stands. Pure spruce and mixed spruce–birch stands were examined at four sites in southern and central Sweden. Soil macroarthropods and enchytraeids were sampled in litter and soil. In the uppermost 5 cm of soil humus we determined microbial biomass and microbial respiration; we estimated the rate of C, N and P mineralisation under laboratory conditions. The densities of Coleoptera, Diptera and Collembola were larger in mixed stands than in spruce stands. Soil fauna composition differed between mixed and spruce stands (as revealed by redundancy analysis). Staphyliniidae, Elateridae, Cecidiomyidae larvae and Onychiuridae were the families that increased most strongly in mixed stands. There were no differences in microbial biomass and microbial respiration, nor in the C, N and P mineralisation rates, between mixed and spruce stands. However, within mixed stands microbial biomass, microbial activity and C mineralisation were approximately 15% higher under birch trees than under spruce trees. We propose that the presence of birch leaf litter was likely to be the most important factor causing differences in soil fauna composition. Birch may also influence the quality and the decomposition rate of humus in mixed stands. However, when the proportion of birch trees is low, the short-term (decades) effect of this species on decomposition is likely to be small in mixed stands on acid forest soils.

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.

Needle chemistry in young Norway spruce stands after application of crushed wood ash

Nutrient concentrations in current and 1-year-old needles were analyzed annually for 5 years after application of hardened wood ash in 1–4-year-old Norway spruce (Picea abies (L.) Karst.) stands within a range of climate and fertility gradients. At each site, 3000 kg ha−1 hardened wood ash of two types, Nymölla and Perstorp, was applied in a randomized block design. Wood ash Nymölla contained 12 kg ha−1 P, 30 kg ha−1 K, 891 kg ha−1 Ca, 72 kg ha−1 Mg and wood ash Perstorp contained 12 kg ha−1 P, 60 kg ha−1 K, 486 kg ha−1 Ca, and 60 kg ha−1 Mg. The ash was intended to compensate for nutrients removed at the preceding harvest when logging residues were collected and removed from the site (whole-tree harvesting). The climate gradient included four climate zones throughout Sweden and each of these included a fertility gradient of three sites classified according to their ground vegetation type. There were no effects on nutrient concentrations in the needles 1 year after the application of wood ash. Five years after ash application, the concentrations of P, K and Ca in current and 1-year-old needles were higher than in the control plots. The results were consistent over all stands, irrespective of climate zone and fertility status. P and K concentrations were higher in spruce needles from plots treated with Perstorp wood ash, whereas Ca concentrations were higher in those of Nymölla treated plots. Analyses across all study sites revealed a treatment effect in terms of increased ratios of P:N, K:N and Ca:N in 1-year-old needles. The ratio P:N tended to increase with time in the Perstorp wood ash treatment compared with the control. The needle concentrations of Mg and S were not affected by the ash applications. The increase in needle nutrient concentrations after application of hardened wood ash suggests that wood ash recycling could be used in order to replace nutrients removed at whole-tree harvesting.

Decomposition of heterogeneous substrates; an experimental investigation of a hypothesis on substrate and microbial properties

Abstract The production-to-assimilation ratio (microbial efficiency) of a microorganism feeding on a substrate depends both on the organism and the substrate. An experiment was set up to investigate whether the microbial efficiency can be defined as the product of a microbial property (an efficiency factor) and a substrate property (substrate quality). By following changes in carbon and organic nitrogen content of birch, aspen and spruce wood, which were degraded by either the brown-rot fungus Poria oleracea or the white-rot fungus Phanernchaete chrysosporium over a 130 day period, we showed that this seems to be the case. It was also found that the substrate quality decreased in the order birch > aspen > spruce. which corresponded to increasing lignin concentrations. However, aspen and spruce appeared much closer in quality than birch and aspen although the latter two are much closer in lignin concentrations. For any given substrate P. oleracea had a microbial elliciency which was ca 50% higher than that of Ph. rhrysosporium. In addition, the experiment showed that when both nitrate and ammonium were available as N sources Ph. chrysosporium preferentially used nitrate whereas P. oleracea started using nitrate only when all ammonium in the growth medium had been exhausted.