Forestlands suitable for timber production also have a significant role supporting biodiversity. Here, we synthesized the current state of knowledge on voluntary biodiversity credits as a market-based mechanism designed to incentivize conservation and restoration efforts in production forests. We compared these mechanisms with features common to carbon credits and assessed their consistency and alignment with existing European Union policies. Attributes particular to production forests, including well-defined tenure rights and the common use of forest management plans, poise them for the implementation of biodiversity credits. Among EU policies, the Taxonomy Regulation for Sustainable Activities and the Directive on Corporate Sustainability Reporting lay a framework that supports emerging biodiversity credit markets, and the EU Restoration Law provides a pathway for adoption across European production forests. Converging ecological opportunities, and policy and financial frameworks, may unlock voluntary biodiversity credits in production forests as a scalable pathway for private capital to support biodiversity conservation and restoration in Europe.

Boreal forests are globally important carbon (C) sinks, but strategies for maximising their climate benefit remain under debate. Major uncertainties in this discussion arise from contrasting sink-source estimates, which largely emanate from inherent limitations of standard measurement techniques to distinct spatio-temporal scales. Here, we use a spatially-nested measurement framework that integrates bottom-up (forest-plot inventory and chamber-based fluxes) and top-down (eddy-covariance; atmospheric observations and atmospheric transport modelling) approaches to reconcile the C balance of actively managed boreal forests in Northern Sweden across plot-, ecosystem-, landscape-, and regional scales during 2016–2018. We found that 3-year mean estimates of the net ecosystem production (NEP) across plot-, landscape-, and regional scales did not differ significantly, converging into a mean (± 95 % confidence interval) C sink of 118 ± 27 g C m-2 yr-1. We also noted a convergence across these scales for the 3-means of the NEP components, i.e., gross primary production (908 ± 48 g C m-2 yr-1) and ecosystem respiration (790 ± 40 g C m-2 yr-1). However, estimates of the inter-annual variations in NEP and its components were inconsistent among most scales and measurement approaches. Furthermore, our results indicate a scale-dependency in the NEP response to the 2018 European summer drought, with a greater reduction of NEP observed in bottom-up compared to top-down estimates. Thus, this study consolidates the C sink-strength of managed boreal forests and advocates the need for cross-scale assessments to constrain forest C cycle-climate feedbacks.

Forest management can impact forest carbon stocks, above- and belowground. The even-aged management practice removes the aboveground carbon stock at harvest, which is thereafter restored as the new forest stand establishes. The effects of even-aged management on forest soils in earlier unmanaged stands are however less well understood, and it has been suggested that large carbon losses may occur. In this study we use a unique paired sampling approach of stands in north inland Sweden. Half of the sampled stands had been clear cut within the previous 54 years, and half were left unmanaged. Our results show that clear-cut harvesting and subsequent transformation of unmanaged stands into even-aged management resulted in lower aboveground carbon stock in the living trees. For the soil there was weak evidence for a loss of c. 15 % of the carbon stock in the organic layer. No evidence of an effect in the more stabilized soil organic carbon within the mineral soil layers was found.

The future role of boreal forests in the global carbon cycle is uncertain given the rapid climate change in high latitudes. At the landscape scale, heterogeneity in stand age and land cover, contributions from terrestrial and aquatic fluxes, and harvest export may create complex carbon cycle-climate interactions. However, the integrated response of the net landscape carbon balance (NLCB) to inter-annual variations (IAVs) in environmental conditions is poorly understood. Here, we used tall-tower eddy covariance and stream monitoring to integrate terrestrial and aquatic carbon fluxes with harvest export for a 68 km2 boreal catchment in Sweden during 2016-2020. This actively managed forest landscape acted as a net carbon sink with a 5-year mean (+ standard deviation) NLCB of 128+55 g C m-2 yr-1. The NLCB IAV included a reduced sink (36 g C m-2 yr-1) during the cool/ cloudy year 2017. In the other four years, featuring a drought summer (2018) and an exceptionally warm/wet winter (2020), the landscape acted as a significant sink (127-180 g C m-2 yr-1). The NLCB IAV corresponded primarily to variations in landscape respiration, followed by GPP and harvest export, with negligible contributions from landscape CH4 and aquatic carbon fluxes. The NLCB IAV was not correlated to any single environmental factor. However, daily NLCB contrastingly responded to key environmental factors as a function of forest aboveground biomass and mire contributions. Overall, our study indicates that the annual carbon sink-strength of the managed boreal forest landscape may be resilient to a wide range of IAVs in environmental conditions.

There is growing interest in continuous cover forestry in Sweden. The historical domination of even-aged forest management means there is a need to focus on methods for transforming even-aged to uneven-aged stands. Practical conversion management today is highly adaptive, and the possibility of failures, such as persistent lack of tree recruitment, must be allowed for. We used simulations to evaluate various scenarios in which conversion management is introduced at different development stages in even-aged stands and then abandoned in favour of clear-felling. A shift towards an inverse J-shaped diameter distribution at the end of the simulations was most evident in northern Sweden and with an early introduction of conversion forestry. The largest losses were incurred in a scenario where the conversion management started with removing larger trees at pre-commercial thinning: it reduced the volume of production by up to 25% and, if clear-felled, resulted in a lower land expectation value than conventional management. Earlier decision to abandon conversion management reduced the losses, but the effect of timing was minor. In summary, the results indicate that conversion management could be started and abandoned without any major economic loss during the timeframe of a normal rotation.

Silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh.) are the two most common broadleaf species in Sweden, together making up approximately 12% of the standing timber volume. The two birch species are usually not distinguished in practice, although they tend to differ in terms of volume production, timber quality, and site preferences. To map the proportion of the two birch species in Sweden's young forests, we used survey data from 123 stands 6-7 years after clearfelling, and data from Sweden's meteorological and hydrological institute. We also examined Sweden's young forests in terms of area, volume, and stem density between 1983 and 2021, using national forest inventory data. Proportions of the two birch species varied significantly across Sweden, and the average temperature sum over the first five years after clearfelling explained 72% of the variation. There was no significant change in area of forest classified as young forest in Sweden, over the last four decades, although there was a significant increase in volume and stem density in Sweden's young forests, with birch making up most of the increase in stem numbers.

The boreal forest is an important global carbon sink, but its response to drought remains uncertain. Here, we compiled biometric- and chamber-based flux data from 50 boreal forest stands to assess the impact of the 2018 European summer drought on net ecosystem production (NEP) across a 68 km 2 managed landscape in northern Sweden. Our results reveal a non-uniform reduction in NEP (on average by 80 ± 16 g C m −2 yr − 1 or 57 ± 13%) across the landscape, which was greatest in young stands of 20–50 years (95 ± 39 g C m − 2 yr − 1 ), but gradually decreased towards older stands (54 ± 57 g C m − 2 yr − 1 ). This pattern was attributed to the higher sensitivity of forest-floor understorey to drought and its decreasing contribution to production relative to trees during stand development. This suggests that an age-dependent shift in understorey–tree composition with increasing stand age drives the drought response of the boreal forest NEP. Thus, our study advocates the need for partitioning ecosystem responses to improve empirical and modelling assessments of carbon cycle–climate feedbacks in boreal forests. It further implies that the forest age structure may strongly determine the carbon sink response to the projected increase in drought events across the managed boreal landscape.

In the EU Forest Strategy for 2030 continuous cover forestry (CCF) is promoted, while rotation forestry (RF) is recommended only if required by forest health or environmental reasons. RF is the current practice on about two thirds of the EU forests. The envisioned shift toward CCF will have significant implications for Europe's forests and the industries. Using Sweden as a case, we simulated two scenarios: RF as a continued practice and a rapid transition to CCF. The results show that RF would ensure an even wood flow and slightly increasing harvest levels over the 50 years period. In contrast, the annual harvest in CCF exhibited significant variation, ranging from 50% to 108% of the RF harvest in the same year. This variation in harvest outcomes for CCF can be largely attributed to both economic and legal constraints during the transition from RF to CCF. As a result, the growing stock in CCF increased, allowing the forest to serve as a significant carbon sink. During the transition period, the ratio of pulpwood to sawlogs fluctuated for CCF, while it remained stable for RF. Consequently, the volume of sawlogs harvested under CCF was 86% of that harvested under RF.

An essential metric for describing carbon dynamics in managed forest landscapes is the recovery time of the carbon balance after clear-cutting. Here, we demonstrate how the age-dependent mathematical trajectory is affected by both the selected model and data availability, leading to considerable uncertainty in the modelling of the net ecosystem production (NEP) over stand age. We further show that the initial carbon loss estimates associated with the timing of the source-sink transition (SST) are significant, but may have a limited effect on the total carbon sequestration at the end of the standard (RP, 120 years) or optimal (OCS) rotation periods.

There is an increasing interest in implementing Continuous Cover Forestry (CCF) as a tool to mitigate negative effects of the traditional rotation forestry system on biodiversity. However, the effects of CCF on forest growth and yield and on biodiversity is still poorly known. In this qualitative review, we compare biodiversity and longterm yield between the selection system, which is a type of CCF practiced in full-storied forests, and the traditional rotation forestry system. We specifically focus on forests dominated by Picea abies, which is a tree species of high economic relevance. Our literature search resulted in 17 publications on stand growth and yield and 21 publications on biodiversity. A majority of simulation studies found a higher long-term yield in the rotation forestry system, but it is challenging to conclude which system is the most productive. The magnitude of the difference in yield between systems, and how it varies across different environmental conditions, remains to be determined. For biodiversity, comparisons of species assemblage and individual species were only made to certain phases of the rotation cycle (recent clearcuts and middle-aged stands). Nevertheless, two aspects can be highlighted: i) the species assemblage in clearcuts differ substantially from stands managed with the selection system. Some of these effects may however be short lasting as examplified by studies on beetle assamblages showing that middle-aged rotation forestry stands become more similar to stands managed with the selection system, ii) the selection system maintains a similar species assemblage as the uncut control during the first years after cutting. In conclusion, management with the selection system may come with a loss in long-term stand yield, but much of the species assemblage is maintained after logging. We recommend future studies to specifically focus on long-term effects on biodiversity - in particular on species of conservation concern. There is also a need to establish a long-term research infrastructure to further develop the field.

Letter to the Editor

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 wellestablished 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.

<p>The boreal biome exchanges large amounts of carbon (C) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape typically consists of various sinks and sources of carbon dioxide (CO<sub>2</sub>), methane (CH<sub>4</sub>), and dissolved organic and inorganic carbon (DOC and DIC) across forests with different stand ages, mires, lakes, and streams. Due to the spatial heterogeneity, a full understanding of the landscape-scale C balance requires capturing all C fluxes. Here, we investigate the five-year interannual variability in the net landscape carbon balance (NLCB) by compiling terrestrial and aquatic fluxes of CO<sub>2</sub>, CH<sub>4</sub>, DOC, DIC, and harvested C obtained from 2016 to 2020. For that purpose, we applied tall-tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks (i.e. harvested C via clearcutting) to estimate the landscape-scale C fluxes across the land-water-atmosphere continuum for an entire boreal catchment (~68 km<sup>2</sup>) in Sweden. Our results show that this managed boreal forest landscape was a net C sink during 2016-2020 (123 ± 63 g C m<sup>-2</sup> yr<sup>-1</sup>) with the lowest and highest sink-strength occurring during a wet year 2017 (16 g C m<sup>-2</sup> yr<sup>-1</sup>) and a drought year 2019 (182 g C m<sup>-2</sup> yr<sup>-1</sup>), respectively. The net landscape-atmosphere CO<sub>2</sub>exchange was the dominant component of NLCB, followed by the C export via harvest and streams. We further found that global radiation and vapor pressure deficit regulated the inter-annual variations of NLCB, whereas forest biomass and source area contribution of mires determined its spatial variability. Overall, our multi-year NLCB investigations provide a holistic understanding of the inter-annual variations in NLCB of managed boreal forest landscapes to better evaluate their potential for mitigating climate change.</p>

Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses. We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance. N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20×). Only fine-root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m-2 yr-1 . Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon-use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m-2 yr-1 ; P < 0.01) that eddy covariance did not (19 (62) g C m-2 yr-1 ; ns). The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.

In sustainable forest resource management, establishing forest conservation areas is important to maintain forest biodiversity. However, assessing the conservation value of forests is challenging because the target areas are often both large and remote. We explored using dense airborne laser scanning (ALS) data to estimate conservation values. Field data were collected at sites in Sweden for standing deadwood (S trees), laying deadwood (L trees), and trees considered particularly important for conserving biodiversity (N trees), and forest conservation values were determined using a commonly employed method in the field. A template matching method was then used to detect L, S and N trees from ALS data. L trees were identified from linear features in the point cloud above the ground, with a 52 % detection error rate. S and N trees were identified from unusually small or large crown diameters, with 71 % and 83 % error rates, respectively. We also tested the relationships between the three types of indicator tree, their summed values and the field inventory-assessed conservation values. Regressions between the assessed conservation values and ALS indicators were most robust comparing the three test sites when using the summed number of L, S, and N trees. A wall-to-wall map covering a 3 km × 4 km area was generated using Kernel density estimation of the summed number of ALS-derived indicators, to represent relative conservation values. The map was validated using 10 1-ha plots, and yielded an R2 value of 0.6 for predicted conservation values at the plot level. We conclude that ALS data can be used to map forest conservation values and inform decisions about which forests should be used for timber production and which should be set aside as conservation areas. The maps could also be used as a data source for habitat analysis.

Applying arginine-phosphate (AP) to tree seedlings at planting is a novel silvicultural practice in Northern Europe to improve the success of forest regeneration. We present three case-studies of the potential advantages of adding AP at planting on the establishment and damage susceptibility of seedlings in pure and mixed plantings of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst. ) and silver birch (Betula pendula Roth) over two years in the field. Location of study sites were in southern (S), northeastern (NE) and northwestern (NW) Sweden. The main agents of damage were pine weevil (Hylobius abietis L.) on conifers at the south site, browsing of birch at all sites and browsing/other top damage to conifers at the north sites. The effect of adding AP varied between the sites. It was positive for survival of pine at site S, despite considerable damage by pine weevil. However, at the S site more of the surviving spruce and birch were browsed when treated with AP. At the NE site AP-treatment had positive effects on conifer growth. At the NW site adding AP positively affected survival and growth of all three species, and AP-treated seedlings of all species were less browsed than untreated seedlings. AP treatment presents a potential tool to improve the success of forest regeneration, especially when establishing pine stands in south Sweden.

We tested three soil scarification approaches of varying intensity (intensive preparation exposing bare mineral soil, medium intensity with a mixture of organic material and mineral soil, and control without site preparation) on six clear-felled sites in two localities in northern and central Sweden between 2018 and 2021. The effect of soil scarification intensity and soil moisture on the occurrence of naturally regenerated birch seedlings was tested one, two and three years after soil scarification, and the density of direct-seeded birch seedlings one year after seeding. In addition, we tested the effect of the annual seed rain, and differences between the two birch species, on the density of direct-seeded seedlings. Soil scarification and its interaction with soil moisture had a significant positive effect on both the occurrence of naturally regenerated birch seedlings and the density of direct-seeded birch seedlings. There was no significant effect of neither annual natural seed rain nor species choice on direct-seeded seedling density. Time since soil scarification had a significant effect on the occurrence of naturally regenerated birch seedlings. In moist soils with high volumetric water content (≥ 28%), birch seeds germinate at high rates and seedlings survive without soil scarification. In mesic soils, birch seeds germinate with higher rates after soil scarification. In dry soils, birch seeds rarely germinate regardless of any disturbance of the humus layer.

The boreal forest is an important global carbon (C) sink. Since low soil nitrogen (N) availability is commonly a key constraint on forest productivity, the prevalent view is that increased N input enhances its C sink-strength. This understanding however relies primarily on observations of increased aboveground tree biomass and soil C stock following N fertilization, whereas empirical data evaluating the effects on the whole ecosystem-scale C balance are lacking. Here we use a unique long-term experiment consisting of paired forest stands with eddy covariance measurements to explore the effect of ecosystem-scale N fertilization on the C balance of a managed boreal pine forest. We find that the annual C uptake (i.e. net ecosystem production, NEP) at the fertilized stand was 16 +/- 2% greater relative to the control stand by the end of the first decade of N addition. Subsequently, the ratio of NEP between the fertilized and control stand remained at a stable level during the following five years with an average NEP to N response of 7 & PLUSMN; 1 g C per g N. Our study reveals that this non-linear response of NEP to long-term N fertilization was the result of a cross-seasonal feedback between the N-induced increases in both growing-season C uptake and subsequent winter C emission. We further find that one decade of N addition altered the sensitivity of ecosystem C fluxes to key environmental drivers resulting in divergent responses to weather patterns. Thus, our study highlights the need to account for ecosystem-scale responses to perturbations to improve our understanding of nitrogen-carbon-climate feedbacks in boreal forests.

Nutrient enrichment can alleviate productivity limitations and thus substantially increase carbon (C) uptake in northern coniferous forests. Yet, factors controlling stand-to-stand variation of forest ecosystem responses to nutrient enrichment remain unclear. We used five long-term (13 years) nutrient-enrichment experiments across Sweden, where nitrogen (N), phosphorus, and potassium were applied annually to young Norway spruce forests that varied in their baseline ecosystem properties. We measured tree biomass and soil C and N stocks, litterfall C inputs, soil CO2 efflux, and shifts in composition and biomass of soil microbial communities to understand the links between above and belowground responses to nutrient enrichment. We found that the strongest responses in tree biomass occurred when baseline site productivity was lowest. High increases in tree biomass C stocks were generally balanced by weaker responses in organic soil C stocks. The average ecosystem C-N response rate was 35 kg C kg-1 N added, with a nearly five-fold greater response rate in tree biomass than in soil. The positive nutrient enrichment effects on ecosystem C sinks were driven by a 95% increase in tree biomass C stocks, 150% increase in litter production, 67% increase in organic layer C stocks, and a 46% reduction in soil CO2 efflux accompanied by compositional changes in soil microbial communities. Our results show that ecosystem C uptake in spruce forests in northern Europe can be substantially enhanced by nutrient enrichment; however, the strength of the responses and whether the enhancement occurs mainly in tree biomass or soils are dependent on baseline forest productivity.