To increase the resilience of forests to drought and other hazards, foresters are increasingly planting mixed stands. This requires knowledge about the drought response of tree species in pure- and mixed-culture neighborhoods. In addition, drought frequently interacts with continued atmospheric nitrogen (N) deposition. To disentangle these factors for European beech, Norway spruce and Douglas fir, we conducted a replicated three-factorial sapling growth experiment with three moisture levels, (high, medium and low), two N levels (high and ambient) and pure and mixed-culture neighborhoods. We measured biomass, stomatal conductance (G_S), shoot water potential (at predawn: Ψ_PD, midday, and turgor loss point: Ψ_TLP), branch xylem embolism resistance (Ψ₅₀), and minimum epidermal conductance (G_min). The three species differed most with respect to G_min (10-fold higher in beech than in the conifers), hydroscape area (larger in beech), and the time elapsed to reach stomatal closure (T_ΨGS90) and Ψ_TLP (T_TLP; shorter in beech), while Ψ₅₀ and Ψ_TLP were remarkably similar. Neighborhood (pure vs. mixed-culture) influenced biomass production, water status and hydraulic traits, notably G_S (higher in Douglas fir, but lower in spruce and beech, in mixtures than pure culture), hydraulic safety margin (smaller for beech in mixtures), and T_ΨGS90 and T_TLP (shorter for spruce in mixture). High N generally increased G_S, but no consistent N effects on leaf water status and hydraulic traits were detected, suggesting that neighbor identity had a larger effect on plant water relations than N availability. We conclude that both tree neighborhood and N availability modulate the drought response of beech, spruce and Douglas fir. Species mixing can alleviate the drought stress of some species, but often by disadvantaging other species. Thus, our study suggests stabilizing and building resilience of production forests against a drier and warmer climate may depend primarily on the right species choice; species mixing can support the agenda.

In addition to the compilation of biodiversity inventories, checklists, especially if combined with abundance data, are important tools to understand species distribution, habitat use, and community composition over time. Their importance is even higher when ecological indicator taxa are considered, as in the case of moths. In this work, we investigated macro-moth diversity in a forest area (30 ha) in the Western Italian Alps, recently subjected to intense management activities. Indeed, an ecological corridor, which includes 10 clearings, has been shaped thanks to forest compensation related to the construction site of the Turin–Lyon High-Speed Railway. Here, we identified 17 patches (9 clearings and 8 forests), and we conducted moth surveys using UV–LED light traps. A total of 15,614 individuals belonging to 442 species were collected in 2020 and 2021. Two and fifteen species are new records for Piedmont and for Susa Valley, respectively. In addition to the faunistic interest of the data, this study—using a standardized method—provides geo-referenced occurrences, species-richness, and abundance values useful to compile a baseline dataset for future comparisons. Indeed, the replicable and easy shareable method allows us to make comparisons with other research and thus assess the impact of environmental changes.

Summary: Ectomycorrhizal (ECM) fungi distribute tree‐derived carbon (C) via belowground hyphal networks in forest ecosystems. Here, we asked the following: (1) Is C transferred belowground to a neighboring tree retained in fungal structures or transported within the recipient tree? (2) Is the overlap of ectomycorrhizal fungi in mycorrhizal networks related to the amount of belowground C transfer? We used potted sapling pairs of European beech (Fagus sylvatica) and North‐American Douglas‐fir (Pseudotsuga menziesii) for ¹³CO₂ pulse‐labeling. We compared ¹³C transfer from beech (donor) to either beech or Douglas‐fir (recipient) and identified the ECM species. We measured the ¹³C enrichment in soil, plant tissues, and ECM fractions of fungal‐containing parts and plant transport tissues. In recipients, only fungal‐containing tissue of ectomycorrhizas was significantly enriched in ¹³C and not the plant tissue. Douglas‐fir recipients shared on average one ECM species with donors and had a lower ¹³C enrichment than beech recipients, which shared on average three species with donors. Our results support that recently assimilated C transferred belowground is shared among fungi colonizing tree roots but not among trees. In mixed forests with beech and Douglas‐fir, the links for C movement might be hampered due to low mycorrhizal overlap with consequences for soil C cycling.

Key message: In mixed combinations, the negative impact of water reduction and N addition is mitigated for F. sylvatica, but not for conifers. Insight into the responses of trees in mixed and monospecific cultivation to water scarcity and nitrogen (N) excess is necessary to recommend suitable tree mixtures for future European forests. Our aim was to investigate the impact of water reduction, N addition or water reduction + N addition in comparison with control (well-watered, no excess N) on mycorrhizal roots and biomass of three temperate forest species (Fagus sylvatica Fs, Picea abies Pa, and Pseudotsuga menziesii Pm) grown in monospecific (FsFs, PaPa, PmPm) and mixed combinations (PaFs, PmFs). We predicted the probability of the responses with a Bayesian approach. Mycorrhizal colonization declined marginally for P. menziesii in PmFs under water reduction. Under water reduction and N addition combined, we found a decline in root tip vitality of F. sylvatica in PaFs and of P. abies in PaPa. F. sylvatica shoot biomass declined under water reduction + N addition in the monospecific combination. P. abies and P. menziesii had lower root biomass in mixture with F. sylvatica under water reduction + N addition and under water reduction as single factor, respectively. With water reduction and N addition, P. menziesii performed better than P. abies in terms of biomass production. F. sylvatica had a growth advantage in mixture with conifers instead of conspecifics. These findings suggest that young trees show rapid and interspecific responses to species mixing and water + N availability. The selection of suitable tree species should, therefore, consider their interactive responses to changing abiotic factors.

Isohydric species reduce water potential fluctuations through more stringent stomatal regulation. It is unclear whether isohydric behavior leads to smaller reversible diurnal stem shrinkage (i.e. greater nocturnal stem rehydration) during drought and a lower drought sensitivity of radial growth compared to anisohydric behavior. With synchronous high-resolution sap flow and dendrometer measurements in mature anisohydric European beech and isohydric Douglas fir trees in pure and mixed stands, we quantified declines in stem water content (SWC), stem rehydration, sap flow and radial growth during soil dry-down and determined the critical soil moisture levels (expressed as Relative Extractable Water, REW) and elapsed desiccation time until 30-90% reductions in these traits. Sap flow and growth started to decline in both species at REW ~0.6, preceding declines in stem rehydration. Water-spending Douglas fir approached 50% drops in SWC, sap flow, stem rehydration and growth during soil dry-down faster than beech, indicating higher drought sensitivity. In mixture, both species reached these reduction levels later than in monoculture, suggesting positive mixing effects on the species drought resistance. Our findings demonstrate that SWC, sap flow and radial growth decrease earlier than nocturnal stem rehydration, with isohydric and anisohydric species exhibiting different timelines of physiological downregulation during soil dry-down.

Understanding how tree species interact within forest stands and how these interactions influence forest functioning—particularly biotic interactions with soil microbes—is crucial for informing forest management strategies under a rapidly changing climate.Altering tree species composition can shift soil fungal community structure and affect tree performance and competitive outcomes. For instance, enriching monospecific beech forests with conifers (mainly spruce or the non-native Douglas fir) can improve beech growth and stress resistance, especially under drought conditions, and it has more recently been linked to higher fungal diversity. Although several mechanisms have been proposed to explain these positive effects on beech growth, site conditions likely play a major role, and potentially also feedback with fungi, such as mycorrhizal symbionts.Here, we investigated whether belowground fungal communities may act as mediators of tree species interactions in mixed beech–conifer forests. Specifically, we hypothesized that variation in fungal community composition is associated with variation in the intensity of tree species interactions, focusing on European beech. To show how beech growth differs under interspecific competition in beech-spruce and beech-Douglas fir forests, we calculated the relative interaction index (RII). We further hypothesize that the effects of site conditions on beech RII are indirect and mediated by fungal communities.We calculated diameter increment of beech trees between 2017 and 2024 in pure beech stands and in mixed beech–spruce and beech–Douglas fir stands. Tree growth was estimated using allometric equations to derive annual aboveground biomass increment, which was used as the performance metric for calculating beech RII in mixed stands with either spruce or Douglas fir as competitors.Soil- and root-associated fungal communities were characterized using DNA metabarcoding. Fungal community composition was analysed separately for soil and root samples using principal coordinates analysis (PCoA), and it was used to predict RII while accounting for the effects of site and stand covariates (e.g., stand age, stand density, and soil properties). To disentangle the relationships among soil environment, fungal community composition, and beech RII, we applied a stepwise regression framework reflecting a hypothesized causal pathway. We further examined associations between beech RII and differentially abundant fungal taxa putatively involved in mediating tree species interactions.We found that beech RII was associated with fungal community composition but only in beech–spruce forests, indicating a strong neighbour identity effect. In beech–spruce stands, the influence of site conditions on beech RII was mediated by both soil and root fungal communities. Additionally, ectomycorrhizal fungal taxa which significantly differed in relative abundance between beech–spruce and pure beech forests were negatively correlated with beech RII, making them candidates involved in or responding to shifts in tree species interactions.Overall, our results demonstrate that fungal communities are tightly coupled to tree species interactions in beech–spruce forests but not in beech–Douglas fir forests, where alternative mechanisms beyond soil conditions may predominantly regulate tree interactions.