We developed site specific component (stem, branch, and foliage) biomass functions for two sites in Sweden (64° and 57° North latitude) where four treatments (control, irrigated, fertilized, irrigated plus fertilized) were applied in the existing Norway spruce stands (Picea abies L. Karst.) for 17 years. We tested for site effects in the component biomass equations and compared site specific biomass estimates to those generated using published functions ( Lehtonen et al., 2004 and Wirth et al., 2004). Site effects were significant for all components and indicated it would be unlikely to generate equations that well estimate biomass across the Norway spruce range as implicitly indicated in our efforts to generate species biomass expansion factors. We rejected our hypothesis that the published functions would well estimate component biomass for control plots. The published functions did not compare well with site specific component biomass estimates for the other treatments; both published functions well estimated stem mass up to stem mass of 25 Mg ha⁻¹, beyond which stem mass was overestimated, and both functions over and under estimated foliage and branch mass. Nor did the published functions compare well with each other, with stem, foliage and branch mass estimate differences of 12, 55, −8% and 11, 77, and 59% for the southern and northern sites, respectively, when averaged over all treatments and years. Adding limiting resources through fertilization increased stem, foliage and branch mass 57, 11, 18% and 120, 37, and 69% at the southern and northern sites, respectively, which would increase carbon sequestration and available stemwood and bioenergy materials. We recommend that more effort is spent in process-based modeling to better predict mass at a given site and ultimately provide better estimates of carbon sequestration and bioenergy material production changes.

Nitrogen is the main limiting nutrient in boreal ecosystems, but studies in southwest Sweden suggest that certain forests approach phosphorus (P) limitation driven by nitrogen (N) deposition. We added N, P or N + P to a Norway spruce forest in this region, to push the system to N or P limitation. Tree growth and needle nutrient concentrations indicated that the trees are P limited. EMF biomass was reduced only by N + P additions. Soil EMF communities responded more strongly to P than to N. Addition of apatite to ingrowth meshbags altered EMF community composition and enhanced the abundance of Imleria badia in the control and N plots, but not when P was added. The ecological significance of this species is discussed. Effects on tree growth, needle chemistry, and EMF communities indicate a dynamic interaction between EMF fungi and the nutrient status of trees and soils. (C) 2018 Elsevier Ltd and British Mycological Society. All rights reserved.

Aims

To estimate the production of external ectomycorrhizal mycelia (EMM) in Norway spruce forests with varying nitrogen (N) and phosphorus (P) levels, and to relate this to the N retention capacity of ectomycorrhizal fungi (EMF) and N leaching.

Methods

Seasonal changes in EMF production (in ingrowth mesh bags) and soil water N (in suction lysimeters) were analyzed after fertilization with N or N combined with P. The EMF N retention capacity was estimated by the addition of isotopically labeled N to the mesh bags.

Results

No relationship was found between the seasonal variation in EMF growth and N leakage from the soil. However, in the mesh bags, the total assimilation of ¹⁵N by EMF was almost halved by N fertilization, while twice as much ¹⁵N leached through.

Conclusions

We found a high specific N assimilation capacity per unit weight of EMF mycelia. This was unaffected by N fertilization, but the total assimilation of N by EMF was drastically reduced due to reduced production of EMM. However, N-retaining processes other than N assimilation by EMF must be taken into account to explain the losses of N after fertilization.

Att klimatet kan komma att förändras påverkar svenskt skogsbruk. Skogen har i sig en direkt inverkan på klimatet samtidigt som skogsbruket kan behöva anpassas till de nya förhållandena. Ett osäkert klimat sätter brukandet av skogen i ett nytt läge som vi inte har någon tidigare erfarenhet av. Scenarier för framtida klimatutveckling är behäftade med stor osäkerhet och de förväntade effekterna på skogen blir således ännu mer osäkra. Trots detta kan man ändå förutsäga några sannolika huvuddrag i effekterna på den svenska skogen vid ett framtida ändrat klimat. En ökad potential för biomassaproduktion kan förväntas, liksom ökade möjligheter att använda nya arter i skogsbruket. Samtidigt ökar sannolikt risken för vissa typer av skador.

Att väga eventuella fördelar i form av ökad produktion och ökade möjligheter i trädslagsval mot ökade risker för skador är viktigt för att ge samhället ett helhetsperspektiv och för att en större grupp ska ha möjlighet att ta till sig frågan. Det är också viktigt att i största möjliga mån kvantifiera eller ge ramarna i ekonomiska termer för hur det förändrade klimatet kan tänkas påverka skogsbruket. Vidare kan det vara styrande för prioritering av fortsatta forskningsarbeten och riskbedömning och för att prioritera åtgärder. Därför har vi försökt utifrån befintlig kunskap idag, konstruera en Tabell över den ekonomiska betydelsen och forskningsbarheten för olika risk/ämnesområden (se Tabell 17 sidan 39). De kanske största effekterna av ett förändrat klimat på ekonomin inom skogsbruket skulle vara om vi lyckas utnyttja den ökade produktionspotentialen. Det förutsätter att vi kan bemästra de negativa effekterna i första hand av en ökad risk för vindfällning, skadeangrepp från insekter och svampar. Mot bakgrund av skogsbrukets stora betydelse som naturresurs och industriell bas, så finner vi att det är viktigt att vi står rustade inför en framtid med såväl ökade hot som nya möjligheter.

I denna skrift försöker vi beskriva och analysera tänkbara effekter av ett förändrat klimat på skogen och bedömt deras implikationer för produktionsskogsbruket. Andra aspekter än produktionsaspekter på skogsbruket har inte behandlats. Analysen sker i fyra steg. Vi inleder med att, så långt nuvarande kunskapsläge tillåter, kvantifiera effekterna på den skogliga primärproduktionen – tillväxten i skogen. I ett andra steg omsätts dessa effekter till effekter på produktionsekonomin i ett bestånd. Därefter analyseras tänkbara effekter på risken för stormfällning i skogen. I ett sista steg breddas diskussionen till en något mera spekulativ bild av tänkbara effekter på skogsbrukets ekonomi.

Recent investigations have shown that annual wood production in Sweden can be increased by 30 million m³ per year in a long-term perspective (>50 years) by using new forest management methods such as new tree species or seedling materials. However, to meet the increased demands during the next 20 years, Sweden will have to rely on silvicultural methods available today. Growth in boreal and cold temperate forest is with only few exceptions limited by nutrients availability, primarily nitrogen, and one way to satisfy the increased demands in a short-term perspective is nitrogen fertilization. A set of thinning and fertilization experiments were started in the 1960’s in Scots pine and Norway spruce stands over the whole of Sweden representing different soil, moisture and vegetation types. We used data from these experiments to examine the long-term effects of repeated fertilization in thinned stands on growth, stand development, and yield. The 34 Scots pine sites and 13 Norway spruce sites included in our analyses had at least four treatment plots (no thinning, repeated light thinnings, repeated light thinnings with repeated N fertilization, and repeated light thinnings with repeated N + P fertilization). In northern Sweden, 100 kg N ha⁻¹ and 150 kg N ha⁻¹ were applied at each fertilization event for Scots pine and Norway spruce stands, respectively. In southern Sweden, 150 kg ha⁻¹ N was applied in Scots pine stands and 200 kg ha⁻¹ N in Norway spruce stands. Phosphorus was applied at the rate of 100 kg ha⁻¹. Several sites also included non-thinned fertilized plots. Pine stands but not spruce stands were responsive (up to 25% more growth depending of the attribute assessed) to repeated fertilization. Surprisingly, the non-thinned pine stands showed strong continuing response to fertilization throughout the 30+ year observation period resulting in higher cumulative volume response than the thinned stands. In thinned stands incremental volume response to fertilization continued but slowly diminished with time indicating that fertilization and thinning effects were less than additive. However, thinning and fertilization effects were additive for diameter growth. Fertilization accelerated stand development with significant shifts in diameter distributions to larger and potentially more valuable trees. Conclusively, repeated nitrogen fertilization is a silvicultural practice that will result in significant and sustained increases in Scots pine production.

The aim of this study was to analyze the effect of repeated fertilizer application on the genetic parameters of Norway spruce. Genetic and environmental variances of growth and phenological traits were estimated to find differences between fertilized and control treatments in broad sense heritability and accuracy of estimated genotypic value. Furthermore, genotype × environment interactions (GxE) between the two treatments were investigated. Two Norway spruce clonal field trials in central Sweden were subjected to both treatments and were measured at various points in time up to a field age of 15 years, to monitor the effects of fertilization. For growth traits, trees in the fertilized treatment exhibited lower environmental variance than those in the non-fertilized treatment; consequently, fertilization yielded higher heritability and greater accuracy of estimated genotypic value. Furthermore, the GxE increased as the effects of fertilization became more pronounced; the genetic correlation between treatments dropped to around 0.5 in the last measured growth period. For phenological traits, no GxE but a slight increase in heritability of prolepsis on the leader shoot was found. The results from this study show that, for the conditions encountered in central Sweden, Norway spruce clones should be tested and selected under the conditions in which they are to be deployed. If repeated fertilizer application is to be adopted under operational conditions, substantial losses in genetic gain for growth can be expected when using current selected clones due to the induced GxE. While the fertilized treatment yielded a higher heritability and accuracy of estimated genotypic value for growth traits than did the control, the Swedish Norway spruce breeding program will not benefit from fertilizing genetic field trials because the increased accuracy of estimated genotypic value is nullified by the GxE.

We estimated how the possible changes in wind climate and state of the forest due to climate change may affect the probability of exceeding critical wind speeds expected to cause wind damage within a forest management unit located in Southern Sweden. The topography of the management unit was relatively gentle and the forests were dominated by Norway spruce (Picea abies (L.) Karst.). We incorporated a model relating the site index (SI) to the site productivity into the forest projection model FTM. Using estimated changes in the net primary production (NPP) due to climate change and assuming a relative change in NPP equal to a relative change in the site productivity, we simulated possible future states of the forest under gradual adjustment of SI in response to climate change. We estimated changes in NPP by combining the boreal-adapted BIOMASS model with four regional climate change scenarios calculated using the RCAO model for the period 2071–2100 and two control period scenarios for the period 1961–1990. The modified WINDA model was used to calculate the probability of wind damage for individual forest stands in simulated future states of the forest. The climate change scenarios used represent non-extreme projections on a 100-year time scale in terms of global mean warming. A 15–40% increase in NPP was estimated to result from climate change until the period 2071–2100. Increasing sensitivity of the forest to wind was indicated when the management rules of today were applied. A greater proportion of the calculated change in probability of wind damage was due to changes in wind climate than to changes in the sensitivity of the forest to wind. While regional climate scenarios based on the HadAM3H general circulation model (GCM) indicated no change (SRES A2 emission scenario) or a slightly reduced (SRES B2 emission scenario) probability of wind damage, scenarios based on the ECHAM4/OPYC3 GCM indicated increased probability of wind damage. The assessment should, however, be reviewed as the simulation of forest growth under climate change as well as climate change scenarios are refined.

Swedish climate policy targets net zero greenhouse gases (GHG) by mid-century, with road transport independent of fossil fuels by 2030, requiring far-reaching changes in the way energy is used. Forest management is expected to support carbon sequestration and provide biomass for various uses, including energy. In this paper, we combine two energy scenarios with four forest scenarios and quantify GHG balances associated with energy-use for heat, electricity, and road transport, and with forest management and production, use, and end-of-life management of various forest products, including products for export. The aggregated GHG balances are evaluated in relation to the 2-degree target and an allocated Swedish CO₂ budget. The production of biofuels in the agriculture sector is considered but not analyzed in detail.

The results suggest that Swedish forestry can make an important contribution by supplying forest fuels and other products while maintaining or enhancing carbon storage in vegetation, soils, and forest products. The GHG neutrality goal is not met in any of the scenarios without factoring in carbon sequestration. Measures to enhance forest productivity can increase output of forest products (including biofuels for export) and also enhance carbon sequestration. The Swedish forest sector can let Sweden reach net negative emissions, and avoid “using up” its allocated CO₂ budget, thereby increasing the associated emissions space for the rest of the world.

Repeated fertilization of forests to increase biomass production is an environmentally controversial proposal, the effects of which we assessed on breeding birds in stands of young Norway spruce (Picea abies), in an intensively managed forest area in southern Sweden. Our results show that fertilized stands had 38% more species and 21% more individuals than unfertilized stands. Compared with stands under traditional management, the further intensification of forestry by repeated applications of fertilizers thus seemed to enhance species richness and abundance of forest birds. We cannot conclude at this stage whether the response in the bird community was caused by changes in food resources or increased structural complexity in the forest canopy due to the skid roads used for the application of the fertilizers. Future studies should focus on structural and compositional effects of fertilization processes during the entire rotation period and at assessing its effects in a landscape context.

Repeated fertilization of young forests is a promising concept to increase the production of wood fiber, but the consequences of intense fertilization regimes on forest birds and their food resources, mediated through changes in the foliar chemistry are inadequately known. We assessed the effects of repeated fertilization in stands of young Norway spruce (Picea abies) on foliar phenolics and arthropods in an intensively managed forest area in southern Sweden in relation to the abundance of arboreal feeding birds breeding in the same stands. We anticipated leaf-sucking arthropods (viz. Hemiptera (aphids)) to react more strongly to changes in foliar phenolics than other invertebrate groups. Overall, we found small effects of fertilization on abundance and composition of different groups of foliar arthropods. However, the abundance of Hemiptera was much higher in early spring in fertilized stands than in unfertilized stands, whereas mites (Anactinotrichida and Actinotrichida) were more abundant in late spring in the fertilized stands. On the contrary, springtails (Collembola) were more abundant in unfertilized stands than in fertilized stands the late spring. The concentrations of two of the most abundant phenolics (hydroxycinnamic acid derivatives) as well as the sum concentration of HPLC-phenolics, were consistently lower in the fertilized stands. Positive effects on arthropod abundance mediated trough changes in the foliar chemistry following fertilization could help to explain why resident Goldcrest (Regulus regulus), which feeds on folivorous arthropods (i.e. aphids) during winter, was found to be more common in the fertilized stands ( Edenius et al., 2011). Our results suggest that in intensively managed forest areas repeated fertilization of young spruce has the potential to enhance the suitability of these stands as winter feeding habitat for coniferous foliage-gleaning insectivorous birds compared to unfertilized stands in the same developmental stage. However, intensification of forest management further impedes habitat quality for more specialized species and generally reduces the diversity of forest birds by simplifying structure and composition of forest stands and shortening the rotation period. Therefore, careful planning of spatial arrangement of treatment units with different management, retention of natural forest and/or structures and legacies such as dead wood and deciduous trees are necessary to promote avian diversity in intensified forest management schemes.

The analysis of forest management strategies at landscape and regional levels forms a vital part of finding viable directions that will satisfy the many services expected of forests. This article describes the structure and content of a stand simulator, GAYA, which has been adapted to Swedish conditions. The main advantage of the GAYA implementation compared to other resources is that it generates a large number of management programmes within a limited time frame. This is valuable in cases where the management programmes appear as activities in linear programming (LP) problems. Two methods that are engaged in the projections, a climate change response function and a soil carbon model, are designed to complement other methods, offering transparency and computational effectiveness. GAYA is benchmarked against projections from the Heureka system for a large set of National Forest Inventory (NFI) plots. The long-term increment for the entire NFI set is smaller for GAYA compared with Heureka, which can be attributed to different approaches for modelling the establishment of new forests. The carbon pool belonging to living trees shows the same trend when correlated to standing volume. The soil carbon pool of GAYA increases with increased standing volume, while Heureka maintains the same amount over the 100-year projection period.

The European Union (EU) does not have a common forestry policy but EU policies can indirectly affect the forest sector. This study departs from the EU "Fit for 55" package of legislation and uses a forest sector model to simulate and analyze three responses in the Swedish forest sector (2020-2100) to policy initiatives addressing climate change and biodiversity: (i) increasing the area of set-asides with 50%; (ii) prohibiting harvest of old forest (>120/140 years of age); and (iii) extending the minimum allowed age for final harvest with 30%. Results indicate that, while all three responses can reduce net carbon emissions compared to business-as-usual, extension of the minimum allowed age for final harvest reduces emissions the most. In general, the effects on net carbon emissions are highly correlated with the level of harvest. Increasing the area of set-asides and prohibiting old forest harvest help preserve old forest better than both business-as-usual and final felling age regulation. Longer-term results are uncertain as policies and technology development can radically change biomass use, product portfolios and displacement effects.

To develop recommendations for tree selection in Continuous Cover Forestry (CCF), access to valid tools for simulating growth at individual tree-level is necessary. To assist efforts to develop such tools, in this study, long-term observation data from two uneven-sized Norway spruce plots in southern Sweden are used to evaluate old and new individual-tree growth models (two established Swedish models, two new preliminary models and included as a reference, a Finnish model). The plots' historical management records and site conditions are the same, but their last thinning treatment differs. Observed diameter increment at tree-level is investigated in relation to treatment. Individual tree growth residuals of tested models are evaluated in relation to tree diameter, treatment, projection length and sensitivity to the predictor mean stand age. Furthermore, the relations between displayed residuals and basal area local competition are analysed. The analyses indicate that active thinning made annual diameter increment independent of tree diameter above a threshold level, while the absence of thinning supported a concave relationship. All tested models displayed a significant linear bias leading to overestimation of small trees' growth and increasing underestimations of larger trees' growth with tree diameter. All distance-independent models displayed residual trends related to local competition.

Individual tree selection (ITS) is one option to manage uneven-sized forest ecosystems. However, scientifically based field guidelines adapted to ITS and economic profitability are rare, often because there is a lack of suitable tree models to use in growth and treatment simulations. The objective of this study is to develop individual-tree distance-dependent growth models focusing on Norway spruce dominated uneven-sized stands. Three models of different complexity, but with the same structural basis, are presented, followed by some examples of growth patterns for the subject trees. The data include 1456 trees (307 sample trees) collected from five sites in southern Sweden. The basic model (S) depends on subject tree size as the predictor, the second model (SD) adds distance to competitors as a predictor, and the third model (SDC) adds crown ratio as a predictor to the structure. R-Adj(2) increases with number of predictors from 0.48 to 0.58 to 0.62. The levels of RMSE improve accordingly from 5.02 cm(2) year(-1) (S) to 4.43 cm(2) year(-1) (SD) and 4.26 cm(2) year(-1) (SDC). The present calibration range and model structures primarily make the models suitable for management simulation of individual-tree selection of Norway spruce in southern Sweden. The format of the models allows for further extension with additional predictors and calibration data with greater coverage.

The size of knots is negatively correlated with bending strength in sawn timber and it is therefore used as a quality grading criterion in national roundwood grading standards. Some standards even use the size of the largest knot as the sole estimate for individual log knottiness. The size of knots is determined by crown horizontal extension, which in turn is dependent on the impact of competing trees. Thus, with knot size models that are competition-dependent, roundwood quality due to knottiness can be simulated for different management al-ternatives. However, these types of models, calibrated on uneven-sized Norway spruce in Fennoscandia, are currently not available. Therefore, the objective of this study is to develop a competition-dependent model framework for prediction of the largest knot size per stem height section, for application within uneven-sized Norway spruce stands. Data from terrestrial laser scanning of an uneven-sized stand in southern Sweden are used to calibrate a modular prediction framework, consisting of interlinked allometric statistical models. Alternative framework sub-models are presented and the preferred model combination can be selected according to context and available input data. The flexible modular format enables further development of separate sub-components for adaptation to growing conditions not covered by the current calibration range.

Whereas there is evidence that mixed-species approaches to production forestry in general can provide positive outcomes relative to monocultures, it is less clear to what extent multiple benefits can be derived from specific mixed-species alternatives. To provide such insights requires evaluations of an encompassing suite of ecosystem services, biodiversity, and forest management considerations provided by specific mixtures and monocultures within a region. Here, we conduct such an assessment in Sweden by contrasting even-aged Norway spruce (Picea abies)-dominated stands, with mixed-species stands of spruce and birch (Betula pendula or B. pubescens), or spruce and Scots pine (Pinus sylvestris). By synthesizing the available evidence, we identify positive outcomes from mixtures including increased biodiversity, water quality, esthetic and recreational values, as well as reduced stand vulnerability to pest and pathogen damage. However, some uncertainties and risks were projected to increase, highlighting the importance of conducting comprehensive interdisciplinary evaluations when assessing the pros and cons of mixtures.

Anthropogenic land-cover change (ALCC) is one of the few climate forcings for which the net direction of the climate response over the last two centuries is still not known. The uncertainty is due to the often counteracting temperature responses to the many biogeophysical effects and to the biogeochemical versus biogeophysical effects. Palaeoecological studies show that the major transformation of the landscape by anthropogenic activities in the southern zone of the Baltic Sea basin occurred between 6000 and 3000/2500 cal year BP. The only modelling study of the biogeophysical effects of past ALCCs on regional climate in north-western Europe suggests that deforestation between 6000 and 200 cal year BP may have caused significant change in winter and summer temperature. There is no indication that deforestation in the Baltic Sea area since AD 1850 would have been a major cause of the recent climate warming in the region through a positive biogeochemical feedback. Several model studies suggest that boreal reforestation might not be an effective climate warming mitigation tool as it might lead to increased warming through biogeophysical processes.

We analyzed ecosystem carbon fluxes from eddy-covariance measurements in five young forests in southernSweden where the previous stand had been harvested by clear-cutting or wind-felled: three stands with Norwayspruce (Picea abies (L.) Karst.), one with Scots pine (Pinus sylvestris) and one with Larch (Larix x eurolepis A.Henry). One of the spruce stands had the stumps harvested, one was fertilized and one without any specialtreatments. These stands returned from positive (sources) to negative (sinks) annual carbon fluxes 8–13 yearsafter disturbance, depending on site productivity and management. This corresponds to approximately 15% ofthe rotation periods at these sites. Extrapolation in combination with chronosequence data suggests thatconventionally regenerated stands reach a neutral carbon balance after approximately 30% of the rotationperiod. The lowest carbon emissions and shortest recovery time was observed in a stand where the stumps of thetrees, in addition to the stems and logging residues, were removed after harvest. This stand not only returned to acarbon sink within this time period but the total carbon gains since disturbance also equaled the total losses afteronly 11 years. These results stress that production stands in southern Sweden are carbon sources during arelatively small part of the rotation period, and that this part can be considerably shortened by measures thatincrease productivity or reduce the amount of woody debris left after disturbance.

Forest fertilization is one of the most efficient methods in forest management to increase the short-term production of biomass. In this context, this chapter provides a brief background of the physiological response of trees to fertilization and increased nutrient availability. Furthermore, we shall describe different fertilization regimes and demonstrate the potentials of fertilization in enhancing biomass production, which will be performed by presenting relevant literature and some unpublished results. This chapter will also elaborate on some ideas for developing fertilization in operational forestry.

Plant growth in northern forest ecosystems is considered to be primarily nitrogen limited. Nitrogen deposition is predicted to change this towards co-limitation/limitation by other nutrients (e.g., phosphorus), although evidence of such stoichiometric effects is scarce. We utilized two forest fertilization experiments in southern Sweden to analyze single and combined effects of nitrogen and phosphorus on the productivity, composition, and diversity of the ground vegetation. Our results indicate that the productivity of forest ground vegetation in southern Sweden is co-limited by nitrogen and phosphorus. Additionally, the combined effect of nitrogen and phosphorus on the productivity was larger than when applied solely. No effects on species richness of any of these two nutrients were observed when applied separately, while applied in combination, they increased species richness and changed species composition, mainly by promoting more mesotrophic species. All these effects, however, occurred only for the vascular plants and not for bryophytes. The tree layer in a forest has a profound impact on the productivity and diversity of the ground vegetation by competing for both light and nutrients. This was confirmed in our study where a combination of nitrogen and high tree basal area reduced cover of the ground vegetation compared to all the other treatments where basal area was lower after stand thinning. During the past decades, nitrogen deposition may have further increased this competition from the trees for phosphorus and gradually reduced ground vegetation diversity. Phosphorus limitation induced by nitrogen deposition may, thus, contribute to ongoing changes in forest ground vegetation.

Forest fertilization with nitrogen (N) has several benefits to society such as increased wood production and carbon sequestration. There are, however, concerns about N leakage, particularly following clear-cutting. The forest-floor vegetation may increase the N retention of forest ecosystems; however, very few studies have quantified the amount of vegetation required. We studied the relationship between vegetation cover and risk of N leakage, estimated by the amounts of ammonium-N and nitrate-N retained on ion-exchange capsules in the soil, during 4 years following the clear-cutting and harvesting of logging residues in a previously fertilized forest in southern Sweden. Previous fertilization increased the amount of nitrate-N captured on the capsules, whereas the amount of ammonium-N decreased. The vascular vegetation cover increased from almost zero to approximately 25% independent of fertilization. The amount of ammonium-N and nitrate-N retained on the capsules was already reduced by 50%-75% at 20% vegetation cover, and by 30%-40% cover, it approached zero, independent of the number of years since clear-cutting. The vegetation may impede tree-seedling establishment, implying a trade-off between seedling growth and N-retention capacity. However, our results indicate that maximum N retention may be achieved at a relatively low vegetation cover, which could be accomplished with less intrusive scarification methods than currently used.

Question

Does the abundance of keystone forest floor species change in response to changes in the forest structure?

Location

Sweden

Methods

We used data from the Swedish National Forest Inventory to investigate changes in the abundance of three common species, as well as the total abundance of all understorey vascular plants (the field layer) in forests in the boreal and temperate parts of Sweden. GLMs and GAMs were used to relate species abundance and temporal changes in abundances to forest structure and forest structural change.

Results

Productivity, measured as the site index, was the most important determinant of individual species' abundance. The volume of Picea abies, the density of tree stems and forest age were among the most important forest structural variables. We found that the dwarf shrubVaccinium myrtillus, the narrow-leaved grasses (mainly Avenella flexuosa) and the total field layer cover decreased in boreal Sweden from 1994 through 2010 and that these changes coincided with an increase in forest density and with a reduction in forest age.

Conclusions

Changes in Swedish forests to higher tree layer density and younger age appear to contribute significantly to current changes in forest floor vegetation. The use of more intensive thinning practices to reduce the total density of the forest and to increase the proportion of broad-leaved tree species and Pinus sylvestris would favour the forest floor species in this study. Moreover, increasing forest age (i.e. the length of rotation periods) might favour V. myrtillus in particular, for which the time since disturbance is important for the recovery of pre-disturbance abundance. However, increased thinning intensity and forest age will reduce the potential for wood production, implying a trade-off between production of wood and maintenance of well-developed forest floor vegetation.

The substitution of fossil fuels with biofuels to mitigate climate change has caused increased interest in enhancing forest biomass production through fertilisation. We investigated the effects of different fertilisation frequencies on the diversity of understory vegetation in young stands of Picea abies on five sites distributed in regions in the middle and south of Sweden. The treatments included fertilisation conducted annually, every second year or every third year, as well as an unfertilised control. A lower number of vascular plant species was observed on fertilised plots than on control plots, whereas the number of bryophyte species remained unchanged. Fertilised plots also showed a lower variance in species composition and a lower Shannon’s diversity index than unfertilised plots. Fertilised plots were more similar to each other than unfertilised plots were to each other over the geographical range. The two most intensive fertilisation treatments had similar effects on the vegetation, whereas the effects of fertilisation conducted every third year were not as substantial. However, the treatment in which fertilisation occurred every third year implies a lower stem-wood production, and there is little knowledge of the longterm differences between the treatments. We conclude that fertilisation of young stands will lead to long-term changes in understory vegetation at the stand scale, whereas the effects at the landscape level are still largely unknown.

Forests of northern ecosystems respond slowly to management activities and the possibilities to increase the growth in a short-term perspective and meet swift increases in society's demand for biomass are small. An exception among the silvicultural measures is fertilization which can be applied in combination with present management systems and, almost instantly, enhances forest productivity. There may, however, be both economic and environmental constraints to large-scale applications of fertilizers in forest. Here we review the literature concerning biomass production of forests under different fertilization regimens, environmental constraints and possibilities in northern forests on mineral soils. Further on we discuss the implications of both extensive and more intensive fertilization in relation to the developing bioeconomy, which encompasses the production and conversion of renewable biological resources into food, health and industrial products and energy. Fertilization in Sweden and Finland is currently practiced by extensive fertilization regimens where nitrogen fertilizers are applied once, or up to three times, during a rotation period, mainly in mature forest. This type of fertilization gives, in most cases, a small and transient effect on the environment as well as a high rate of return to the forest owner with low-economic risk. The increase in biomass production, however, is relatively small and consequently the impact on the processing industry and the bioeconomy is limited. More intensive fertilization regimens implying intensive fertilization starting in young forests may, on the other hand, considerably increase the biomass supply and value for the industry. The economic and environmental risks of this type of fertilization may, however, be larger and more research is needed on the effects on the stand level, and especially on the landscape level, including late rotation management of the forest.

Fertilisation with nutrient optimisation has in Sweden resulted in large increases in volume growth in young stands of Norway spruce. There are, however, environmental concerns about repeated fertilisation and one is the risk of nutrient leakage to ground water resources and aquatic ecosystems after clear-cutting of such forests. The present study followed soil-water chemistry in optimised fertilised stands after clear-cutting, as well as effects of harvest of slash on nutrient leakage. Parts of a 30-year-old stand of Norway spruce, which had been subject to a nutrient optimisation experiment for 17 years, were clear-cut. A split-plot design with whole-tree harvesting as the subplot treatment was applied. Lysimeters were installed and soil-water sampled at nine occasions during the following four years. No significant effects of fertilisation on nitrate leaching were found, while harvest of slash affected the concentration of Ca, DOC, DON, K, Mg, ammonium and nitrate, as well as pH in the soil solution. While no effects of fertilisation could be seen on the soil water concentration of N, the results indicate an interaction between fertilisation and harvest of slash on the concentration of nitrate in the soil solution. The results indicate that forest-floor vegetation plays an important role in the retention of N after clear-cutting of fertilised forests.

Forestry practices that aim to increase biomass production may mitigate climate change through increased carbon sequestration and the potential of substituting fossil fuels with renewable biofuels. Fertilising young stands of Norway spruce in Sweden have shown to increase tree growth by more than 200%. Fertilisation, however, also has other effects on forest ecosystems. Here, we studied the response of the species composition of forest-floor vegetation to three different frequencies of fertilisation in young stands of Norway spruce. Fertiliser was applied every year, every second year or every third year. The total amount of N ranged from 425 kg ha⁻¹ to 625 kg ha⁻¹, in combination with P, K, Ca, Mg, S, Mn, Zi, B and Cu. The largest effects of the fertilisation were found among bryophytes and lichens, which lost substantial cover. Unexpectedly, Deschampsia flexuosa, commonly known to be favoured by fertilisation, was negatively affected. Species that increased in frequency were Oxalis acetosella, Brachythecium sp. andPlagiothecium sp. Decreased availability of light, as an indirect effect of fertilisation through increased tree canopy cover, was found to be the most important factor behind the change in species composition of vascular plants. The total cover of bryophytes, however, did not show any significant response to the changes in canopy cover, indicating that the effects seen in this group may be a result of more direct effects of the fertiliser. Few significant differences were found between the two most intensive fertilisation frequencies, although fertilisation every third year was often distinguished from both the control and the other fertilised treatments. Even though the effects at the stand level were substantial, the effects on biodiversity and function of ecosystems on a landscape or regional level need further investigation.

Late-rotation fertilization of Norway spruce stands is a frequently used management tool in Fennoscandia to increase timber yields. Meanwhile, the growing demand for renewable resources has sparked great interest in earlier and repeated fertilizer application but it remains unclear how this affects carbon dioxide (CO₂) fluxes in the understory, especially forest floor respiration (R_ff). This study investigated the effects of forest fertilization on R_ff and net forest floor exchange (NFFE) in young, nitrogen (N) limited Norway spruce stands in southern Sweden. In a short-term dose experiment, R_ff and NFFE were recorded during 2016 after varying doses of N (0,150, 300, or 450 kg ha-1 of N, hereafter N0, N150, N300, N450) were added to circular, 3-m-diameter plots in April. In a second, long-term experiment, two stand-level fertilizer applications with 150 kg ha^-1 of N on each occasion were performed in 2014 and 2016 and R_ff was measured at semi-regular intervals from mid-2013 to the end of 2017. In the dose experiment, fertilization increased R_ff by 23 %, 81 % and 55 % in the N150, N300 and N450 treatments, respectively. Under well-lit conditions, the N300 and N450 treatments significantly enhancedphotosynthetic CO₂ uptake of the forest floor vegetation by 97 % and 66 %, respectively, while the N150 treatment had no significant effect. The results of the long-term experiment indicate an initial stimulation of R_ff, but this effect was transient. Our findings imply that fertilization in young Norway spruce stands, using the N150 dose (the typical dose used in Swedish forestry), may cause a transient burst in R_ff that is far outweighed bynutrient-driven increases in forest floor photosynthesis under favourable light conditions prior to canopy closure.

Climate change mitigation strategies have increased the demand for wood products, resulting in an urgent needto increase wood production. One approach is to fertilize forest land, but this can influence greenhouse gas (GHG) fluxes within the ecosystem. The aim of this study was to examine the effects of forest N fertilization onsoil CH₄ and N₂O fluxes in young Norway spruce (Picea abies (L.) Karst.) stands in southern Sweden. The gasfluxes were measured using flow-through non-steady-state dark chambers. In the first, long-term, experiment,half of the stand was fertilized twice (once in 2014 and once in 2016) with 150 kg ha^-1 of N, and gas fluxmeasurements were taken throughout 2014–2017. In the second, dose, experiment, 0, 150, 300, or 450 kg ha^-1 of N was added to the stand in April 2016, and gas flux measurements were taken during April-December 2016.The dose experiment showed that the sink strength of CH₄ decreased with increasing amounts of N; the long-termexperiment indicated that repeated fertilization decreased the CH₄ sink strength over time. Additionally, thelong-term experiment indicated that, while significantly higher N₂O emissions were recorded in the fertilizationyears, this was not detected in subsequent years, suggesting the effect to be short-lived. In the dose experiment,fertilization tended to increase the N₂O emissions relative to the amount of fertilizer. However, despite thesignificant effects of fertilization on these GHGs, the summed fluxes were a fraction of the net uptake of C at thesites, as recorded in another study. These findings suggest that fertilizing forest land with commercial NP or NPKfertilizers corresponding to 150 kg ha^-1 of N, the level used in operational forestry in Sweden today, can beconducted without changing CH₄ and N₂O fluxes to any great extent.

The purpose of the study was to create a near optimal environment for seedling establishment and growth, without the restrain of water and nutrients but under climate conditions typical for the region. This to give us valuable knowledge about the growth potential of different seedling types in the field. The experimental site was situated in southern Sweden. Six treatment combinations were applied including two site treatments; 1) soil inversion, i.e. the control treatment, and 2) soil inversion, drip irrigation and fertilization combined with plastic cover mulch, i.e. the optimization treatment, and three seedling types of Norway spruce (Picea abies L. Karst.), (a) a 2-year-old Plug+1 seedling, (b) a 1.5-year-old containerized seedling and (c) a 10-week-old mini seedling. Effects on seedling nutrient status and growth were studied during the first three years after planting. Height, diameter and biomass of the seedlings grown in the optimized environment were significantly greater than for seedlings grown in the control. The Plug+1 seedlings grown in the optimization treatment had, after three years, reached a height of 124 cm, while the containerized seedlings were 104 cm and the mini seedlings 45 cm. In practical plantations, this height is usually gained after 5–10 years depending on planting conditions. Biomass partitioning did not differ between optimization treatments, but between seedling types. The mini seedlings allocated less biomass to the roots and more biomass to needles and stem in comparison with the two other seedling types. Mini seedlings also broke bud earlier. Throughout the experimental period, seedling nutrient status for all treatment combinations was followed and a balanced nutrient supply of macro- and micronutrients was given in the optimization treatment. Nutrient concentrations were constantly higher in seedlings grown in the optimization treatment, but the difference decreased over time. Results from this study shows that, by improving site conditions associated with fast establishment, growth check can be avoided.

Adaptation to climate change in forestry has become a growing concern, in part due to the impact of storms and other events that have raised the awareness of such risks amongst forest owners. Sweden is one of Europe's most densely-forested countries, with this sector playing a major role economically. However adaptation has, to a large extent, been limited to the provision of recommendations to forest managers, most of which have only been partially implemented. This paper summarizes research with direct implications for adaptation to climate change within the forestry sector in Sweden. The focus is based in particular on providing examples of adaptations that illustrate the specific Swedish orientation to adaptation, in line with its relatively intensive forest management system. The paper thus illustrates a specific Swedish orientation to adaptation through active management, which can be contrasted with approaches to adaptation in other forestry systems, in particular those with limited management or management based on maintaining natural forests in particular.

Managed forests can play an important role in climate change mitigation due to their capacity to sequester carbon. However, it has proven difficult to harness their full potential for climate change mitigation. Managed forests are often referred to as socio-ecological systems as the human dimension is an integral part of the system. When attempting to change systems that are influenced by factors such as collective knowledge, social organization, understanding of the situation and values represented in society, initial intentions often shift due to the complexity of political, social and scientific interactions. Currently, the scientific literature is dispersed over the different factors related to the socio-ecological system. To examine the level of dispersion and to obtain a holistic view, we review climate change mitigation in the context of Swedish forest research. We introduce a heuristic framework to understand decision-making connected to climate change mitigation. We apply our framework to two themes which span different dimensions in the socio-ecological system: carbon accounting and bioenergy. A key finding in the literature was the perception that current uncertainties regarding the reliability of different methods of carbon accounting inhibits international agreement on the use of forests for climate change mitigation. This feeds into a strategic obstacle affecting the willingness of individual countries to implement forest related carbon emission reduction policies. Decisions on the utilization of forests for bioenergy are impeded by a lack of knowledge regarding the resultant biophysical and social consequences. This interacts negatively with the development of institutional incentives regarding the production of bioenergy using forest products. Normative disagreement about acceptable forest use further affects these scientific discussions and therefore is an over-arching influence on decision-making. With our framework, we capture this complexity and make obstacles to decision-making more transparent to enable their more effective resolution. We have identified the main research areas concerned with the use of managed forest in climate change mitigation and the obstacles that are connected to decision making.

 In Sweden, where forests cover more than 60% of the land area, silviculture and the use of forest products by industry and society play crucial roles in the national carbon balance. A scientific challenge is to understand how different forest management and wood use strategies can best contribute to climate change mitigation benefits. This study uses a set of models to analyze the effects of different forest management and wood use strategies in Sweden on carbon dioxide emissions and removals through 2105. If the present Swedish forest use strategy is continued, the long-term climate change mitigation benefit will correspond to more than 60 million tons of avoided or reduced emissions of carbon dioxide annually, compared to a scenario with similar consumption patterns in society but where non-renewable products are used instead of forest-based products. On average about 470 kg of carbon dioxide emissions are avoided for each cubic meter of biomass harvested, after accounting for carbon stock changes, substitution effects and all emissions related to forest management and industrial processes. Due to Sweden’s large export share of forest-based products, the climate change mitigation effect of Swedish forestry is larger abroad than within the country. The study also shows that silvicultural methods to increase forest biomass production can further reduce net carbon dioxide emissions by an additional 40 million tons of per year. Forestry’s contribution to climate change mitigation could be significantly increased if management of the boreal forest were oriented towards increased biomass production and if more wood were used to substitute fossil fuels and energy-intensive materials.

Continuous-cover forestry (CCF) has been recognized for the production of multiple ecosystem services, and is seen as an alternative to clear-cut forestry (CF). Despite the increasing interest, it is still not well described how CCF would affect the carbon balance and the resulting climate benefit from the forest in relation to CF. This study compares carbon balances of CF and CCF, applied as two alternative land-use strategies for a heterogeneous Norway spruce (Picea abies) stand. We use a set of models to analyze the long-term effects of different forest management and wood use strategies in Sweden on carbon dioxide emissions and carbon stock changes. The results show that biomass growth and yield is more important than the choice of silvicultural system per se. When comparing CF and CCF assuming similar growth, extraction and product use, only minor differences in long-term climate benefit were found between the two principally different silvicultural systems.

Nutrient removal has been one of the key issues since the harvesting of logging residues started in Sweden. This study examined the actual removal of nutrients by measuring the amounts of biomass removed (from a forest products perspective) combined with their respective nutrient concentrations (N, P, Ca, K and Mg), from a clear-felled area when using the dried-stacked and fresh-stacked methods. The most important finding is that the two methods were very similar regarding nutrients remaining at the clear-felled area. Of the nutrients remaining there, most were found to be well distributed between the harvester heaps. Both methods fulfilled the requirements of the Swedish Forest Agency. A sensitivity analysis showed that even if the dried-stacked method left more needles, or the fresh-stacked method extracted more logging residues, there would only be a small impact on the levels of nutrients removed. The sensitivity analysis also showed that the amount of logging residues remaining between the harvester heaps seems to be much more important for nutrients left behind, regardless of extraction method. With this in mind, it is highly probable that improvements to the extraction of logging residues, without increasing nutrient removal, can be made.

This study investigates wood density and anatomy of juvenile silver birch stems in Sweden,grown in mixed conifer stands. Our aim is to investigate if fertilization provides increased growth, aswell as an eventual reduction in stem wood density. Measurements of basic density, ring width, cellwall thickness, and vessels are analyzed for 20 birch trees. Bark to pith radial sections are analyzedusing a light microscope and the freeware ImageJ to compare treatments and ages. The results showthat trees with fertilizer treatment have wider growth rings and thinner cell wall thickness comparedto unfertilized trees. The fertilized trees also have a lower cambium age at the same height and thesame diameter, and a slightly lower stem mean density (420 kg m³) than the unfertilized stems(460 kg m³). Fertilizer is a significant determinant of density and cell wall thickness in nonlinearmodels. The fertilized trees have increased growth and reached a fixed diameter earlier. The agedifference between the trees likely explains some of the differences in cell wall thickness. This studysupports the use of fertilizer as a silvicultural option for increasing the growth rate of silver birch fora relatively small reduction of wood density.

Growth data and modulus of elasticity (MOE) of 11 different pine species and hybrids were examined at six sites in three regions in South Africa. Growth traits and three MOE variables were measured at three years of age in order to evaluate whether other potential pine species were more suitable than the current commercial species. There were strong observed species differences for all three MOE variables both within and across the four sites measured for wood properties, with across-site MOE ranging from 3.03 to 6.40 GPa. Green density varied among species, and an assumed constant green density of 1 000 kg m-3 underestimated MOE for species with a very high green density; similarly, for species with a very low green density, MOE was overestimated. Although survival was poor at several sites, the data shows that there are alternative pine species that exhibit comparable growth rates to the current commercial species. For estimating MOE, it is concluded that assuming a constant green density generally does not affect the species ranking, but if the aim is to find the 'true' MOE, sampling in the field to determine the appropriate green density is needed.

Under climate change, the importance of biomass resources is likely to increase and new approaches are needed to analyze future material and energy use of biomass globally and locally. Using Sweden as an example, we present an approach that combines global and national land-use and forest models to analyze impacts of climate change mitigation ambitions on forest management and harvesting in a specific country. National forest impact analyses in Sweden have traditionally focused on supply potential with little reference to international market developments. In this study, we use the global greenhouse gas concentration scenarios from the Intergovernmental Panel for Climate Change to estimate global biomass demand and assess potential implications on harvesting and biodiversity in Sweden. The results show that the short-term demand for wood is close to the full harvesting potential in Sweden in all scenarios. Under high bioenergy demand, harvest levels are projected to stay high over a longer time and particularly impact the harvest levels of pulpwood. The area of old forest in the managed landscape may decrease. This study highlights the importance of global scenarios when discussing national-level analysis and pinpoints trade-offs that policy making in Sweden may need to tackle in the near future.

Precision forestry allows decision-making on tree level or pixel level, as compared to stand-level data. However, little is known about the importance of precision in thinning decisions and its long-term effects on within-stand variation, stand economy and growth. In this study, silviculture was optimized for Net Present Value (NPV) in 20 conifer-dominated forest stands in hemi-boreal southern Sweden. The precision-thinning approach, Precision Thinning (PT), is compared with a stand-level approach, Stand Level Thinning (SLT) that is optimized for the same criteria but based on stand-level data. The results suggest no substantial long-term benefit or drawback in implementing thinning decisions based on pixel-level data as compared to stand-level data when optimizing stand economy. The result variables NPV and Mean annual increment of living stem volume (MAI(net)) were not higher for PT than for SLT. The within-stand variation in basal area (m(2)/ha(-1)) was lower at the end of the rotation compared to the start of the simulation for both SLT and PT. At the end of the rotation, SLT had higher variation in basal area compared to PT. However, pixel-level information enables adapting the silviculture to the within-stand variation which may favour other forest management goals than strictly financial goals.

This paper examines how forest products can be utilized to contribute tackling climate change. An integrated model-based system analysis approach is applied to estimate forest biomass production and substitution effects of climate change and forest management goals. We estimate net primary production with the use of process based model BIOMASS incorporating climate change effects according to IPCC SRES B2 scenario. BIOMASS considers the processes of radiation absorption, photosynthesis, phenology, allocation of photosynthesis among plant organs, litter-fall, and the stand water balance. The resulting output of net primary production from BIOMASS is input into the empirical model HUGIN to calculate tree growth functions in five scenarios representing different forest management goals. These growth functions determine the total growth and the potential harvestable forest biomass. The harvested products in terms of whole tree biomass and stem wood biomass are then assumed to substitute construction materials and fossil fuels, and the substitution effect is calculated in terms of net CO2 emission reduction. We use the Q-model to estimate soil carbon changes in the forest because of litter fall and soil decomposition processes in different scenarios. The results show that the climate change effect and intensive forestry practice can increase forest production and product harvest by up to 75% and 69% respectively compared to the production in the year 2010. If the harvested biomass is used to substitute fossil fuel and building construction materials a total net carbon emission reduction up to 249 Tg carbon is possible. The carbon stock in standing biomass, forest soils, and wood products all increases. The carbon stock changes are less significant than compared to the substitution benefits. This study can conclude that the climate change effect and improved forest management practices may increase forest biomass significantly, thus will give increased opportunity to reduce carbon emission significantly to contribute to the climate change mitigation.