ABSTRACT Mass production of Larix decidua × L. kaempferi seed in first-generation hybridization orchards (F1s) is hindered by wide variation in the proportion of hybrid seeds. Second-generation hybridization orchards (F2s) have therefore been established in Belgium and in France. Seed crops from F2 orchards produced 13-18% less viable seed/kg than F1s but with similar seed quality. Across 6 progeny trials, survival of F2s at age 9–10 was generally better than that of F1s as well as European (EL) and Japanese larch (JL) controls. The volume growth of the F2 hybrids was 7-36% lower than that of F1s across sites and orchards but exceeded that of EL and JL. Stem straightness varied between orchards, reflecting differences in the level of selection of parental clones: the French F2 orchard achieved much better straightness than F1s, EL, and JL, whereas the Belgian F2 orchard had a systematically lower proportion of straight stems than F1s (although its stem straightness generally matched or exceeded that of EL and JL). Second-generation hybridization orchards were thus shown to be a promising alternative to 1st-generation orchards for mass-producing hybrid seed. Further research is needed to identify optimal strategies for advanced hybridization.

ABSTRACT The growth process shapes the structure of trees in forest, and the rate the stems thicken over time is linked with wood properties such as ring width and wood density. This study investigated the potential of bi-temporal terrestrial laser scanning (TLS) for measuring the secondary growth of coniferous trees and assessing wood properties derived from core samples using X-ray microdensitometry over a nine-year period in boreal forests. TLS-derived DBH measurements demonstrated high agreement with reference data (r > 0.96). Volumetric secondary growth ( Δ v ) slightly outperformed basal area increment ( Δ g ) in capturing secondary growth dynamics as moderate correlations were found between TLS-derived estimates and mean ring width (RWm; r = 0.60–0.67). In contrast, correlations with ring basal-area-weighted wood density (WDg) were weak but statistically significant (r = –0.16 to –0.18) for both TLS and core-sampled measurements. The findings suggest that TLS can be used to measure secondary growth, but the ability to predict wood density of coniferous species via secondary growth measurements – neither with TLS nor core-sampling – is limited due to internal anatomical factors not captured by external measurements. Overall, these findings support the integration of TLS into forest monitoring frameworks, as it provides equally reliable yet more versatile data than callipers for measuring secondary growth.

ABSTRACT Tree stumps are a potentially significant source of renewable energy, but their extraction may result in significant adverse impacts on ecosystems. To assess mid-term effects (up to 8 years after harvesting) of stump removal on nutrient status and young stand development in mesotrophic Latvian hemiboreal forests, we evaluated soil and soil solution chemistry, subsequent young stand growth, and foliar micro- and macroelement status in five sites where above-ground whole-tree harvesting (WTH) was compared to above-ground WTH with additional stump biomass removal (WTH + SB). Post-harvest soil organic carbon stocks and C/N ratio declined more after the WTH + SB than after WTH while soil nitrogen stocks increased. No additional acidification of the soil solution caused by stump removal was observed, and the inconsistent pattern of nutrient concentration change in soil solution (partial increase of phosphates, calcium and magnesium in WTH + SB relative to WTH, no clear response of nitrates) reflected site-specific conditions. N concentration in planted Norway spruce needles did not differ consistently between the treatments, and foliar nutrient concentrations of the regenerated stands were mostly within the typical range of similar forest site types in Latvia. Mean tree height was significantly greater after WTH compared to WTH + SB in sites with monodominant regeneration.

ABSTRACT Weevils are notorious pests in conifer regeneration, yet their impact on broadleaved species such as silver birch (Betula pendula) remains poorly understood. This study investigated the incidence and severity of weevil damage on newly planted birch saplings in southern Sweden and tested the effectiveness of Hylonox, a mechanical coating, against weevil attack. A field experiment was established across fresh and one-year-old clear-cuts and damage assessments were conducted over two growing seasons. Strophosoma melanogrammum was the most frequently observed weevil species, followed by Otiorhynchus carinatopunctatus, Hylobius abietis and Hylobius pinastri. Overall, 62% of the birch saplings showed bark damage by weevils, with mortality reaching 16% among the damaged individuals. Mortality was significantly higher in untreated saplings (30%) than in treated ones (21%). Weevil-associated leaf feeding affected 87% of saplings in 2024 and 49% in 2025, but severity was predominantly minor (<20% leaf area affected). Initial sapling height significantly affected both the incidence of bark damage and mortality, with shorter saplings experiencing higher mortality. Ungulate browsing was frequent (45% of saplings), often affecting terminal shoots. The results of this study suggest that weevils can damage birch during early establishment, although mortality is modest relative to that in conifers.

ABSTRACT The debarking of tree saplings by ungulates is a critical problem for conifer regeneration. However, how coarse woody debris (CWD), specifically fallen logs that act as nurse logs by providing microhabitats on their upper surface, influences the debarking of conifer saplings by ungulates remains poorly understood. We compared the frequency of debarking of Sakhalin fir saplings by sika deer between saplings growing on CWD and those growing from the ground in a subboreal mixed forest. The saplings growing from the ground were debarked more frequently than those growing on CWD, even though the saplings on CWD were more accessible to deer during the snowy period, when food resources for the deer are generally limited. More frequent occurrence of debarking on the saplings growing from the ground indicates the deer likely accessed the saplings on the ground during the non-snowy period. Furthermore, saplings with wider stems were debarked more frequently than those with smaller stems. This might have occurred because trees with wider stems with sufficient anchorage and stability are suitable for debarking. Less frequent occurrence of debarking on the saplings growing on CWD highlights the importance of the upper surface of CWD for conifer regeneration in the face of an increasing sika deer population.

ABSTRACT While forests are generally regarded as beneficial for water quality, forest management can significantly impact water quality, even if sustainable forest management practices are applied. Fellings alter water balance and nutrient cycling, while forest roads with impermeable surfaces fragment ecosystems and enhance runoff. Road construction often requires extensive landscape modification and material relocation, which increases substance leaching and erosion. In the present study, water quality parameters and nutrient export were monitored over eight years (2016–2023) in a forested catchment (2212 ha) and its sub-catchments. During this period, 15.8 km of forest roads were constructed, and fellings occurred across 18.8% of the catchment. Fellings increased nitrogen compound concentrations and export, with effects persisting for four-five years. Runoff patterns strongly controlled the export of total nitrogen, calcium, and dissolved organic carbon. The onset of fellings and higher felling intensities raised total suspended solids levels, though high variability masked statistical significance. Calcium and potassium concentrations also increased during periods of more intense felling, though their export patterns closely followed runoff dynamics. Road construction increased calcium and total suspended solids concentrations.

ABSTRACT Herbivory is a source of damage for Fennoscandia commercial stands. Silver birch (Betula pendula Roth) plantations are of increased interest to address the climate and biodiversity crises, but there are concerns about browsing risk. We inventoried 92 planted silver birch stands from a survey focused on browsing variables (e.g. symptom, severity, cause, and position), and one on tree measurements. We estimated ungulate density from national surveys and pellets observation. We assessed landscape characteristics (neighbouring land use/land cover classes and distance to roads). We observed considerable browsing risk for silver birch plantations, with 38 ± 35% and 23 ± 19% of planted birches browsed in younger and older stands, respectively, with mean height <5 and >5 m. Moose were considered responsible for almost half of browsing (44%), while other species accounted for a small amount (4.1%), the majority by unidentified cervids. Especially, crown damage in younger stands developed into vertical branches in older stands, leading to low stem external quality. In a model predicting the number of browsed stems at stand level, damage occurrences were dependent only on stand characteristics and abundance of browsing animals. Our analysis could not provide site selection recommendations for lower browsing risk sites. However, avoiding early damage seems the most crucial step.

ABSTRACT Mixed forests are expected to be highly productive carbon (C) sequestering ecosystems. The study reports the effect of stand age on net ecosystem production (NEP) in a hemiboreal mixed forest chronosequence composed of silver birch and Norway spruce. The biometric C budgeting method was applied in seven chronosequence mixed stands aged between 20 and 70 years. Average NEP across the studied development classes ranged from 3.8 to 5.3 tC ha−1 yr−1. The highest value of NEP, 7 tC ha−1 yr−1, was estimated for a 30-year-old middle-aged stand. The main driver shaping the annual NEP of the stands along the successional gradient was the net primary production (NPP) of trees, while the contribution of the ground vegetation was minor. Annual cumulative soil heterotrophic respiration (Rh) varied between 4.1 and 5.5 tC ha−1, irrespective of stand age. Birches produced the major share (61%) of the annual aboveground litter input to the soil while the belowground C input through fine root production was largely formed of spruces (63%). The studied birch-spruce mixed stands on fertile soils acted as strong C sinks. Still compared to the literature data for corresponding pure stands, no overyielding effect of mixed stands was revealed.

ABSTRACT Vegetation Optical Depth (VOD) data, derived from satellite microwave observations, are sensitive to above-ground biomass (AGB) and vegetation water content. While VOD is extensively used for global AGB estimation, its relationship with forest structural attributes in boreal forests remains inadequately understood, particularly during the growing season. This study explores the dependencies between forest structural attributes, including height, basal area, volume, Leaf area index, and AGB, and L- and X-band VOD during the growing season in boreal forests in Finland and Sweden. The analysis revealed that L-VOD values increased with increasing values of height, basal area, volume, and LAI (R = 0.37–0.55); conversely, X-VOD values did not exhibit a significant relationship within our study area. Our study revealed a weak to moderate relationship between forest structure and VOD within managed boreal forests. Additionally, the sensitivity of the relationship between VOD and forest structural attributes varied between summer months, with the strongest correlations observed in June for L- and X-VOD. Both VOD bands showed variability beyond the explanatory capacity of forest structural attributes, including AGB. This consideration should be taken into account when developing AGB products for boreal forests and using L- and X-band VOD datasets.

ABSTRACT Climate change is shifting the suitable habitats for many tree species northwards. Large parts of southern Sweden are projected to become unsuitable for Norway spruce by the end of this century. With the majority of forests in Sweden under management, spruce plays an important role, both economically and in natural ecosystems. Despite its significance, there is limited understanding of whether current trends in spruce prevalence align with the projected changes in habitat suitability. In this study, we addressed this gap by analysing projected range shifts for Norway spruce between 2005 and 2095 under two climatic scenarios (RCPs) and compared them with long-term forest inventory data. Our results show that: (1) In southern Sweden (Götaland), large parts (59.1% for RCP 4.5 and 85.7% for RCP 8.5) of current spruce stands (as by 2005), are projected to fall into areas unsuitable for spruce by 2095. (2) Observed trends in growing stock between 2005 and 2020 are often opposite to those projected for changes in suitability, though trends varied between counties. (3) The growing stock of young trees is increasing across all regions. Combined, these results show a clear mismatch between projected spruce habitat suitability under climate change and current management practices.