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Nutrient limitation of forest growth, especially nitrogen (N) deficiency, is widespread in the boreal region. N fertilisation has thus become a common silvicultural practice in Fennoscandian Norway spruce stands, but to what extent phosphorus (P) is co-limiting productivity and how initial basal area affects the growth response to N addition remains unresolved. To address these questions, two experiments were established in mid-rotation Norway spruce stands in southern Sweden where decades of high atmospheric N deposition have reduced the severity of N-limitation. In a P experiment initiated in 2011, we tested P addition alone (two applications of 200 kg P ha−1) and in a second study also starting in 2011 (NP experiment), a single dose of N was administered alone (200 kg ha−1 in thinned and unthinned stands, hereafter called N and N-unthinned treatments) and in combination with P (N+P = one-time 200 kg N ha−1, two applications of 200 kg P ha−1 in thinned stands). P addition alone increased PAI (periodic annual increment) significantly by 21 % during the first, moister assessment period up to 2014 and by 18 % in the drier 2015–2019 period, resulting in a 10 % increase in final stem volume yield. In the NP experiment, significant PAI increases under favourable meteorological conditions up to 2014 occurred in all fertilisation treatments. The strongest effects were seen in the N-unthinned treatment while no significant additive effect resulted from the joint addition of N and P (N: +20 %, N-unthinned: +38 %, N+P: +23 %). In the drier 2015–2019 period, only the N+P treatment caused significantly greater PAI (+29 %). Final stem volume yield in the NP experiment significantly increased by 10 %, 39 % and 16 % in the N, N-unthinned and N+P plots, respectively. In both experiments, foliar P and thus P/N rose drastically in response to P addition alone or in combination with N. Minor increases in leaf area index (LAI) only occurred in P-containing treatments. Our findings indicate that Norway spruce productivity in southern Sweden is constrained to a similar extent by both N and P. Sustainable nutrient management in Norway spruce growing regions with high N loading (like southern Sweden) should prioritise P over N supply. • Norway spruce productivity in southern Sweden is constrained to a similar extent by both N and P. • Growth response to nitrogen stronger in unthinned vs. thinned stands. • P-addition alone or combined with N improves suboptimal foliar P/N ratios. • P-fertilisation causes long-term increase in leaf area index. • P-driven growth stimulation sustained during periods of moisture deficit. [ABSTRACT FROM AUTHOR]

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 2Adj increases with number of predictors from 0.48 to 0.58 to 0.62. The levels of RMSE improve accordingly from 5.02 cm2 year−1 (S) to 4.43 cm2 year−1 (SD) and 4.26 cm2 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. [ABSTRACT FROM AUTHOR]

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 (MAInet) were not higher for PT than for SLT. The within-stand variation in basal area (m2/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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Repeated fertilisation of young Norway spruce supports greater gains than late-rotation fertilisation. • Operationally viable 2-yearly fertilisation more than doubled stem volume yield on nutrient-poor soils. • Plot-level responses remained valid at the stand-level. • High NUE implies small N leaching and gaseous losses despite higher N addition. • Early, repeated fertilisation in northern forests may aid biomass demand and mitigate climate change. Silvicultural techniques aimed at promoting forest biomass production can help meet the growing demand for renewable materials and mitigate climate change. One-time nitrogen (N) addition late in the rotation is a well-established method to stimulate growth in coniferous forests in northern Europe, but the potential gains from earlier and repeated fertiliser application remain uncertain. Here, we tested the impact of repeated fertilisation in juvenile Norway spruce stands across 9 sites covering a wide range of growing conditions over a 700 km stretch from central to southern Sweden. We tested the fertilisation effects using two separate studies: i) an interval trial with a fertilisation frequency of one (F1), two (F2), or three years (F3) performed at plot-level across five sites (2002–2014), and ii) a practice-oriented trial with a two-year fertilisation interval (F2) applied at stand-level and replicated at four sites (2003–2013). The composition of the nutrient mix in each plot was optimised based on foliar nutrient analyses. In the interval trial, all three fertilisation schedules strongly increased periodic annual increment (PAI) (F1: 105 %, F2: 93 %, F3: 79 %) relative to the unfertilised control, resulting in more than a doubling of stem volume yield in the F1 and F2 treatments (110 % and 120 %, respectively) and a significantly smaller but still sizeable yield stimulation of 82 % in the F3 treatment. Nitrogen use efficiency (NUE, stemwood volume increase per unit mass of N added) was similar among fertilisation intervals (on average 130 m3 ha−1 1000 kg N−1), indicating that the extra N provided through yearly fertilisation (F1) is redundant given the similar stemwood yields in the F2 treatment. In the practice-oriented trial, the sole F2 treatment increased PAI by 95 % over the control, translating into a yield stimulation of 114 % and an almost identical NUE to that of the interval trial. NUE greatly exceeded the figures typically observed with traditional late-rotation fertilisation and correlated inversely with baseline site productivity (using site index as a proxy) in the F1 and F2 treatments (the latter pooled across the two trials). Our results clearly indicate that nutrient limitation restricts growth and carbon (C) capture in young Norway spruce plantations in northern Europe to less than half of their potential, highlighting repeated fertilisation at nutrient-poor sites as an effective management tool to support a growing bioeconomy and enhance C sequestration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights: [•] Scots pine stands showed a significant positive fertilization response. [•] No response to fertilization was observed in Norway spruce stands. [•] Fertilization growth effect was greater in non-thinned than thinned Scots pine stands. [•] Fertilization and thinning effects was additive for diameter growth. [•] Fertilization resulted in shifts of the diameter distributions to larger trees. [ABSTRACT FROM AUTHOR]

We used ecosystem model simulations to study the timber and energy biomass potential offered by intensively managed cloned Norway spruce stands. More specifically, we analysed how the use of cloned trees compared with non-cloned trees, together with thinning, nitrogen (N) fertilisation and rotation length (from 60 to 100 years), affects the annual mean production of timber (i.e., saw logs, pulpwood) and energy biomass (i.e., stumps and harvesting residuals in the final felling) and its economic profitability [annual mean of net present value ( NPV) with a 2% interest rate]. Furthermore, we employed a life cycle analysis/emission calculation tool to assess the total net CO2 emissions per unit of energy (kg CO2 MW h−1) produced based on energy biomass. We found that both the annual mean production of timber and the NPV increased substantially, regardless of the management regime, if cloned trees with an annual growth increase of up to 30% compared with non-cloned trees were used in regeneration. In general, the use of a short rotation with N fertilisation clearly increased the annual mean of the NPV. Consequently, the use of cloned trees also clearly increased the annual mean production of energy biomass and decreased the total net CO2 emissions per unit of energy produced based on energy biomass. However, the total annual net CO2 emissions were the lowest if a long rotation was used with N fertilisation. To conclude, the use of cloned trees together with intensive management could potentially be highly beneficial for the cost-efficient and sustainable production of timber and energy biomass in an integrated way. [ABSTRACT FROM AUTHOR]

Abstract: 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 (). 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. [Copyright &y& Elsevier]

Abstract: 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 15years, 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. [Copyright &y& Elsevier]

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 long-term 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. [ABSTRACT FROM AUTHOR]

Abstract: 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 425kgha−1 to 625kgha−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. and Plagiothecium 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. [Copyright &y& Elsevier]

Abstract: 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 (). 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 25Mgha−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. [Copyright &y& Elsevier]

Abstract: In this paper, the potential stem wood production for Norway spruce was estimated for different regions in Sweden. This was done by using basic physiological relationships of intercepted radiation versus biomass production and knowledge of how a water deficit reduces the potential production, derived from results of field experiments on nutrient optimisation. To scale these relationships up to regional and national levels, data of incident radiation and humidity during the growing season for all of Sweden were used. The figures for potential and attainable production indicate that the temperate to boreal climate allows considerably higher production than the current production, if availability of water and nutrients is non-limiting. In northern Sweden, the attainable production is ca. 300% higher than the current production and in southern Sweden, the yield can be increased by ca. 100%. In absolute numbers, as a mean for a whole rotation period, it is possible to achieve an annual stem-wood dry mass production of 7–9Mgha−1 in southern and central and ca. 5–6Mg in northern Sweden. This increased production would mean that rotation periods can be shorter than they are now by 20–30 years in southern Sweden and by ca. 50–60 years in northern Sweden. [Copyright &y& Elsevier]

A process-based model was used to simulate biomass production of Norway spruce under both current climate and climate change scenarios. The model was parameterized for Nordmoen in south-east Norway using real climate data for the period 1987–1989. The model was applied to predict the biomass production responses to three climate change scenarios. The results showed that net primary production (NPP) increased by 7% under an elevated annual mean air temperature of 4°C from the current 10.1 t dry mass ha [sup -1] yr [sup -1]. A doubled current ambient CO[sub 2] concentration significantly increased NPP by 36%. The scenario of both elevated temperature and elevated CO[sub 2] concentration led to an increase in the NPP of 49%, higher than the sum of the two effects acting singly. The results also showed that forest production responses to climate change depend on the conditions of climate used for reference. [ABSTRACT FROM AUTHOR]

We investigated effects of nutrition and soil water availability on sap flux density, transpiration per unit leaf area (EL), and canopy stomatal conductance (GS) of Norway spruce (Picea abies L. (Karst.)) in northern Sweden during the 1996 growing season. Our objectives were to determine (1) if artificially imposed drought (65% rain diversion) reduces soil water sufficiently to cause physiological limitations to whole-tree and plot-scale water transport, and (2) whether increased capacity for water transport resulting from fertilization-induced increases in leaf (> 3-fold) and sapwood areas (> 2.3-fold) deplete soil water sufficiently to cause a negative feedback on GS and EL. We monitored soil water content (θ) and soil water potential (ΨS) in control (C), drought (D), fertilized (F) and irrigated + fertilized (IL) treatment plots, along with site meteorological conditions. Ten trees per plot were monitored for sap flow. Although there were significant treatment differences in mean daily EL (C > D > F; P < 0.01) and GS (C > D > F; P < 0.05), variation in absolute magnitudes was small. Therefore, transpiration differences on a unit ground area basis (EC) were nearly proportional to leaf area differences. Precipitation was well distributed throughout the study period and so ΨS remained high, except during short dry periods in Plot F when it declined rapidly. Thus, although soil water was not limiting to GS, EL or EC when precipitation was uniformly distributed throughout the growing season, we cannot conclude that water availability would not limit GS in fertilized stands if the seasonal distribution of precipitation were altered. [ABSTRACT FROM PUBLISHER]

• Fertilization on mineral soil only causes small short-lived N 2 O emissions. • High levels of N fertilizer do not change mineral forest soils to CH 4 sources. • Soil CH 4 and N 2 O fluxes are minor compared with the net ecosystem uptake of CO 2. Climate change mitigation strategies have increased the demand for wood products, resulting in an urgent need to 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 on soil CH 4 and N 2 O fluxes in young Norway spruce (Picea abies (L.) Karst.) stands in southern Sweden. The gas fluxes 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 flux measurements 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 CH 4 decreased with increasing amounts of N; the long-term experiment indicated that repeated fertilization decreased the CH 4 sink strength over time. Additionally, the long-term experiment indicated that, while significantly higher N 2 O emissions were recorded in the fertilization years, this was not detected in subsequent years, suggesting the effect to be short-lived. In the dose experiment, fertilization tended to increase the N 2 O emissions relative to the amount of fertilizer. However, despite the significant effects of fertilization on these GHGs, the summed fluxes were a fraction of the net uptake of C at the sites, as recorded in another study. These findings suggest that fertilizing forest land with commercial NP or NPK fertilizers corresponding to 150 kg ha−1 of N, the level used in operational forestry in Sweden today, can be conducted without changing CH 4 and N 2 O fluxes to any great extent. [ABSTRACT FROM AUTHOR]

AbstractThe effect of soil thawing and soil temperature on postwinter recovery of photosynthetic capacity was studied, during late spring and early summer, in Norway spruce stands in northern Sweden. Soil temperature was manipulated by means of buried heating cables. The warming treatment was applied to stands with low (natural) and high (fertilized) availability of nutrients. Soil thawing, expressed as water availability, was followed by means of sapflow in stems, and shoot water potentials. The recovery of photosynthetic capacity was assessed by measuring the rate of light-saturated photosynthesis (Amax), and maximum photochemical efficiency of photosystem II in detached shoots, and chlorophyll a fluorescence. Accumulation of starch reserves in the needles was followed as an independent indicator of photosynthetic performance in situ. Snowmelt and soil thawing occurred more than one month earlier in heated than in unheated plots. This was expressed both as sapflow and as differences in shoot water potential between treatments. During May, the rates of Amax were significantly higher on heated than on control plots. The effect of soil warming on Amax was, however, not reflected in chlorophyll fluorescence or needle starch content. The time course of the recovery of photosynthetic capacity was mainly controlled by mean air temperature and by the frequency of severe night frosts, and to a lesser extent by earlier soil thawing and higher soil temperatures. [ABSTRACT FROM AUTHOR]

Examines the low-temperature effects on photosynthetic production in Norway spruce stands growing in northern Sweden. Use of process-based growth model BIOMASS; Simulation of reduction in photosynthetic rates caused by boreal conditions; Gross primary production; Recovery of photosynthetic capacity; Phenology, growth and allocation pattern.

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, as well as an eventual reduction in stem wood density. Measurements of basic density, ring width, cell wall thickness, and vessels are analyzed for 20 birch trees. Bark to pith radial sections are analyzed using a light microscope and the freeware ImageJ to compare treatments and ages. The results show that trees with fertilizer treatment have wider growth rings and thinner cell wall thickness compared to unfertilized trees. The fertilized trees also have a lower cambium age at the same height and the same diameter, and a slightly lower stem mean density (420 kg m−3) than the unfertilized stems (460 kg m−3). Fertilizer is a significant determinant of density and cell wall thickness in nonlinear models. The fertilized trees have increased growth and reached a fixed diameter earlier. The age difference between the trees likely explains some of the differences in cell wall thickness. This study supports the use of fertilizer as a silvicultural option for increasing the growth rate of silver birch for a relatively small reduction of wood density. [ABSTRACT FROM AUTHOR]

• Young forests may return to carbon sinks already eight years after reforestation. • Increased forest productivity shortens the time needed to return to carbon sink. • Harvest of logging residues/stumps for bioenergy may improve the carbon balance. We analyzed ecosystem carbon fluxes from eddy-covariance measurements in five young forests in southern Sweden where the previous stand had been harvested by clear-cutting or wind-felled: three stands with Norway spruce (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 special treatments. These stands returned from positive (sources) to negative (sinks) annual carbon fluxes 8–13 years after disturbance, depending on site productivity and management. This corresponds to approximately 15% of the rotation periods at these sites. Extrapolation in combination with chronosequence data suggests that conventionally regenerated stands reach a neutral carbon balance after approximately 30% of the rotation period. The lowest carbon emissions and shortest recovery time was observed in a stand where the stumps of the trees, in addition to the stems and logging residues, were removed after harvest. This stand not only returned to a carbon sink within this time period but the total carbon gains since disturbance also equaled the total losses after only 11 years. These results stress that production stands in southern Sweden are carbon sources during a relatively small part of the rotation period, and that this part can be considerably shortened by measures that increase productivity or reduce the amount of woody debris left after disturbance. [ABSTRACT FROM AUTHOR]