Summary: Trees are known to be atmospheric methane (CH₄) emitters. Little is known about seasonal dynamics of tree CH₄ fluxes and relationships to environmental conditions. That prevents the correct estimation of net annual tree and forest CH₄ exchange. We aimed to explore the contribution of stem emissions to forest CH₄ exchange. We determined seasonal CH₄ fluxes of mature European beech (Fagus sylvatica) stems and adjacent soil in a typical temperate beech forest of the White Carpathians with high spatial heterogeneity in soil moisture. The beech stems were net annual CH₄ sources, whereas the soil was a net CH₄ sink. High CH₄ emitters showed clear seasonality in their stem CH₄ emissions that followed stem CO₂ efflux. Elevated CH₄ fluxes were detected during the vegetation season. Observed high spatial variability in stem CH₄ emissions was neither explicably by soil CH₄ exchange nor by CH₄ concentrations, water content, or temperature studied in soil profiles near each measured tree. The stem CH₄ emissions offset the soil CH₄ uptake by up to 46.5% and on average by 13% on stand level. In Central Europe, widely grown beech contributes markedly to seasonal dynamics of ecosystem CH₄ exchange. Its contribution should be included into forest greenhouse gas flux inventories.

The compaction of forest soils can deteriorate soil aeration, leading to decreased CH₄ uptake and increased N₂O efflux. Black alder (Alnus glutinosa) may accelerate soil structure regeneration as it can grow roots under anaerobic soil conditions. However, symbiotic nitrogen fixation by alder can have undesirable side-effects on greenhouse gas (GHG) fluxes. In this study, we evaluated the possible trade-off between alder-mediated structure recovery and GHG emissions. We compared two directly adjacent 15-year old beech (Fagus sylvatica) and alder stands (loamy texture, pH 5–6), including old planted skid trails. The last soil trafficking on the skid trails took place in 1999. GHG fluxes were measured over one year. Undisturbed plots with beech had a moderately higher total porosity and were lower in soil moisture and soil organic carbon than undisturbed alder plots. No differences in mineral nitrogen were found. N₂O emissions in the undisturbed beech stand were 0.4 kg ha⁻¹ y⁻¹ and 3.1 kg ha⁻¹ y⁻¹ in the undisturbed alder stand. CH₄ uptake was 4.0 kg ha⁻¹ y⁻¹ and 1.5 kg ha⁻¹ y⁻¹ under beech and alder, respectively. On the beech planted skid trail, topsoil compaction was still evident by reduced macro porosity and soil aeration; on the alder planted skid trail, soil structure of the uppermost soil layer was completely recovered. Skid trail N₂O fluxes under beech were five times higher and CH₄ oxidation was 0.6 times lower compared to the adjacent undisturbed beech stand. Under alder, no skid-trail-effects on GHG fluxes were evident. Multiple regression modelling revealed that N₂O and CH₄ emissions were mainly governed by soil aeration and soil temperature. Compared to beech, alder considerably increased net fluxes of GHG on undisturbed plots. However, for skid trails we suggest that black alder improves soil structure without deterioration of the stand’s greenhouse gas balance, when planted only on the compacted areas.

Production and consumption of methane (CH₄) and nitrous oxide (N₂O) in soils have a strong influence on global greenhouse gases (GHG) budgets. Therefore, it is crucial to precisely measure GHG fluxes at the soil–atmosphere interface. In upland soils, CH₄ and N₂O can be consumed by microbiological processes, and the respective concentrations can be lower than in the atmosphere, demanding highly sensitive gas analysing systems. Traditionally, soil air is sampled in vials and analysed in the laboratory by gas chromatography (GC). During the last decade, different technologies have been developed that allowed to build portable gas analysers that are able to measure sub-ambient gas concentration directly in the field. Here, we compared sub-ambient to ambient CH₄ and N₂O concentration values from four portable devices using different measurement technologies (a portable GHG analyser based on laser absorption spectroscopy [LAS], two portable Fourier transform infrared spectroscopy [FTIR] devices and a field gas analyser using photoacoustic spectroscopy [PAS]) to traditional GC analysis in the laboratory (a GC system equipped with a flame ionisation detector [GC-FID] and an electron capture detector [GC-ECD]). The accuracy and precision of photoacoustic spectroscopy measurements are strongly influenced by the water vapour content and non-target gases in the sampling air. We used an advanced set-up for a widely used PAS analyser enabling N₂O measurements at sub-ambient concentrations with similar precision and accuracy as the GC-ECD system. Measurements of CH₄ and N₂O by FTIR and LAS devices were in good agreement with the GC systems. We conclude that the portable devices are suitable for studies of GHG fluxes in the field. Thanks to their universal and portable character, LAS, PAS and FTIR devices represent useful alternatives to currently used technologies for field studies.

Biotic disturbances caused by insects and pathogens have a major impact on forests in the Northern Hemisphere. Knowledge of the effects of increasing temperature on the performance of these forest pests will thus be crucial for predicting future disturbance risks. Here, we systematically review the direct effects of increasing temperatures on four functional subgroups of forest pests, including leaf chewers, sap suckers, bark and wood borers, and pathogens. We considered 118 studies (2003–2022) representing 72 pest species feeding on 33 host genera from sub‐tropical, temperate and boreal forests in Asia, Europe and North America. Based on a subset of 89 studies reporting the required data, we conducted a meta‐analysis 1) distinguishing the functional subgroups, and 2) considering the main life‐history traits, i.e. development, fitness and survival. A temperature corresponding to the expected mean temperature during the growing season in the years 2081–2100 had a significant positive effect on the overall performance of leaf chewers (+18%) and bark and wood borers (+10%), while sap suckers and pathogens remained unaffected. In contrast, performance was not or even significantly negatively affected when a more pronounced temperature increase, i.e. corresponding to the temperature maxima in 2081–2100, was considered. This finding reflects the non‐linear temperature–performance relationship beyond currently evolved temperature optima in insects and pathogens. Furthermore, we showed that differential responses of life‐history traits to increased temperatures may counterbalance each other (e.g. development versus survival), highlighting the importance of a multi‐trait approach to assessing the impact of global warming on pest performance. By quantifying the effects of increasing temperatures on forest pest performance, our results facilitate the prediction of biotic disturbance impacts in a future climate. For instance, they could provide a valuable contribution to the parameterization of large‐scale ecosystem models, which often do not explicitly consider the response of biotic disturbances to climate change.

Forest harvesting removes and redistributes nutrients through felling and forwarding. Substantial quantities of nutrients can accumulate in brash mats on permanent skid trails, but their availability and uptake after multiple thinnings on soils susceptible to leaching are unknown. In this study, we modeled the deposition of base cations and phosphorus on a permanent skid trail after five thinnings of a Picea abies (L.) Karst. stand, and measured the resulting nutrient stocks in both the forest floor and mineral soil. An estimated 35%, 44%, 41%, and 61% of harvested Ca, K, Mg, and P, respectively, were redistributed to the skid trail. Of those deposited stocks, 32–65% of nutrients remained in decomposed brash material on the skid trail. Mineral soil stocks for Ca, K, and P were significantly higher in the skid trail than in the stand, which included minor increases in bioavailable pools. Skid trail root densities were not lower than the stand while bulk densities were only partially higher. Both would not limit nutrient uptake. There were no significant relations between needle nutrient concentrations and distance to the skid trail. Altogether, these results indicate that nutrient uptake from the skid trail was minimal despite their accumulation, chemical availability, and physical accessibility. This suggests that other factors such as liming and frequent thinning disturbances can repress uptake of available nutrients on skid trails.

Forest soil compaction caused by heavy machines can cause ecosystem degradation, reduced site productivity and increased greenhouse gas (GHG) emissions. Recent studies investigating the plant-mediated alleviation of soil compaction with black alder showed promising results (Alnus glutinosa). This study aimed to measure soil recovery and GHG fluxes on machine tracks with and without black alders in North-East Switzerland. In 2008, two machine tracks were created under controlled conditions in a European beech (Fagus sylvatica) stand with a sandy loam texture. Directly after compaction, soil physical parameters were measured on one track while the other track was planted with alders. Initial topsoil bulk density and porosity on the track without alders were 1.52 g cm⁻³ and 43%, respectively. Ten years later, a decrease in bulk density to 1.23 g cm⁻³ and an increase in porosity to 57% indicated partial structure recovery. Compared with the untreated machine track, alder had no beneficial impact on soil physical parameters. Elevated cumulative N₂O emission (+30%) under alder compared with the untreated track could result from symbiotic nitrogen fixation by alder. Overall, CH₄ fluxes were sensitive to the effects of soil trafficking. We conclude that black alder did not promote the recovery of a compacted sandy loam while it had the potential to deteriorate the GHG balance of the investigated forest stand.