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As of 2020, the world has an estimated 290 million ha of planted forests and this number is continuously increasing. Of these, 131 million ha are monospecific planted forests under intensive management. Although monospecific planted forests are important in providing timber, they harbor less biodiversity and are potentially more susceptible to disturbances than natural or diverse planted forests. Here, we point out the increasing scientific evidence for increased resilience and ecosystem service provision of functionally and species diverse planted forests (hereafter referred to as diverse planted forests) compared to monospecific ones. Furthermore, we propose five concrete steps to foster the adoption of diverse planted forests: (1) improve awareness of benefits and practical options of diverse planted forests among land‐owners, managers, and investors; (2) incentivize tree species diversity in public funding of afforestation and programs to diversify current maladapted planted forests of low diversity; (3) develop new wood‐based products that can be derived from many different tree species not yet in use; (4) invest in research to assess landscape benefits of diverse planted forests for functional connectivity and resilience to global‐change threats; and (5) improve the evidence base on diverse planted forests, in particular in currently under‐represented regions, where new options could be tested.

Trees have a strong influence on the chemical properties of the soil in which they grow. Establishing plantations with different tree species combinations thus potentially leads to divergence in soil chemistry. To study the degree to which differentiation already occurs during the first years after establishment, we made use of a biodiversity–ecosystem functioning experiment in Belgium, FORBIO. The multi-site experiment replicates tree species richness (1–4 species) and composition in three sites. The sites represent contrasting site contexts, mostly land-use history, soil, and climate. Soil samples (0–10 cm) were taken at the time of planting and approximately 8 years later. We measured the total C and N concentration, the Olsen P, the pH-H₂O, and the concentration of base and Al cations. The change in chemical composition was strongly dependent on site conditions including former land use. Afforestation on former cropland had a positive impact on total C and Olsen P and a negative effect on base cations and pH. On sites reforested after clearcut, soil texture and particular site preparation played an important role. On top of that, we found several significant effects of species composition. Strongest composition effects were detected in the reforested site on loamy soil with little soil disturbance. This study highlights that species choice can already affect soil chemistry in early stages of forest development, but that the nature of the effects may strongly depend on the context in which the plantations are established. Further research is needed to identify the most important contextual factors.

1. There is increasing evidence that species diversity enhances the temporal stability (TS) of community productivity in different ecosystems, although its effect at the population and tree levels seems to be negative or neutral. Asynchrony in species responses to environmental conditions was found to be one of the main drivers of this stabilizing process. However, the effect of species mixing on the stability of productivity, and the relative importance of the associated mechanisms, remain poorly understood in forest communities. 2. We investigated the way mixing species influenced the TS of productivity in Pinus sylvestris L. and Fagus sylvatica L. forests, and attempted to determine the main drivers among overyielding, asynchrony between species annual growth responses to environmental conditions, and temporal shifts in species interactions. We used a network of 93 experimental plots distributed across Europe to compare the TS of basal area growth over a 15-year period (1999-2013) in mixed and monospecific forest stands at different organizational levels, namely the community, population and individual tree levels. 3. Mixed stands showed a higher TS of basal area growth than monospecific stands at the community level, but not at the population or individual tree levels. The TS at the community level was related to asynchrony between species growth in mixtures, but not to overyielding nor to asynchrony between species growth in monospecific stands. Temporal shifts in species interactions were also related to asynchrony and to the mixing effect on the TS. 4. Synthesis. Our findings confirm that species mixing can stabilize productivity at the community level, whereas there is a neutral or negative effect on stability at the population and individual tree levels. The contrasting findings regarding the relationships between the temporal stability and asynchrony in species growth in mixed and monospecific stands suggest that the main driver in the stabilizing process may be the temporal niche complementarity between species rather than differences in species' intrinsic responses to environmental conditions.

Aims In tropical rainforests, soil respiration accounts for the major part of ecosystem respiration, yet a deep understanding of the influence of forest type and species composition is still lacking. We therefore selected patches of the rainforest in the Central Congo basin differing in their species composition, some patches under the Scorodophloeus zenkeri Harms mixed forests (MIF) and others in the Gilbertiodendron dewevrei (De Wild.) J.Léonard monodominant forests (MOF). We measured daily soil respiration over a one-year period. Methods By fitting a simple conceptual model of soil respiration, including fine root biomass, soil organic C stocks and ground climate measurements (soil moisture and temperature), we attempted to distinguish autotrophic and heterotrophic soil respiration, and to better understand the drivers behind total soil respiration. Results On an annual basis, soil respiration was 10% higher under MOF (22.10 Mg C ha −1 y −1 ) compared to MIF (20.01 Mg C ha −1 y −1 ) ( p  < 10 −3 ). While the estimated autotrophic and heterotrophic soil respiration contributed about equally to soil respiration in MOF, autotrophic soil respiration slightly dominated (59%) in MIF. In both forests, the combined contribution of litterfall inputs and fine roots productivity was lower than the heterotrophic flux, with the largest difference observed under MOF (−6.16 Mg C ha −1  year −1 ) compared to MIF (−2.62 Mg C ha −1  year −1 ). The sensitivity analysis of the model showed that the higher heterotrophic soil respiration under MOF was driven by the twofold C accumulation in MOF topsoil compared to MIF. Soil moisture was a major driver of temporal changes in soil respiration, but hardly impacted the differences in annual soil respiration between forests. Conclusion While the difference in SOC accumulation between forests was driven by the low nutrient to C ratios of Gilbertiodendron dewevrei tissues, additional research is needed to identify the causes behind the unbalanced C budget .

Key messageBeech trees growing in biodiverse patches in Belgium have higher radial growth and are less physiologically sensitive to drought than those in monocultures. Forest diversification therefore alleviates the negative effects of drought on beech.Common beech, a widespread and economically important tree species in Europe, is known to be drought sensitive. For ensuring its survival under increasing future drought conditions, we need to advance our understanding on the relationships between drought and its growth performance. Diversifying forests has been proposed as a management strategy to mitigate the effects of drought, because a more complete use of the available water is expected. We made use of a tree species diversity gradient in Belgium to study if beech trees growing in diverse forest patches are more resistant and resilient to drought than beech trees in monocultures. Combining dendrochronological and stable carbon (δ13C) and oxygen isotope (δ18O) data allowed for studying the effect of tree species diversity on the response of beech growth to drought regarding stem radial growth and physiological performance. Up to 62% enhanced stem radial growth strongly increased growth stability, and higher resistance to drought was observed for beech trees in diverse forest patches. Beech performs best in three-species mixtures, particularly those with oak and maple. In drought years, beech growth is more reduced in monocultures than mixtures. During these drought years, δ13C values increased, and the increase was weaker in beech trees of diverse stands compared to monospecific stands, indicating enhanced stomatal conductance and growth continuation in mixtures. δ18O patterns did not indicate a clear effect of diversity or the response of beech trees to drought. Overall, our results indicate that until now, still the positive effects of diversity on beech growth outperform the negative effects induced by drought.

Background Climate change is likely to cause significant modifications in forests. Rising to this challenge may require adaptation of forest management, and therefore should trigger proactive measures by forest managers, but it is unclear to what extent this is already happening. Methods The survey carried out in this research assesses how forest stakeholders in Belgium perceive the role of their forest management in the context of climate change and the impediments that limit their ability to prepare and respond to these changes. Results Respondents indicated strong awareness of the changing climate, with more than two-thirds (71 %) expressing concern about the impacts of climate change on their forests. However, less than one-third of the respondents (32 %) reported modifying their management practices motivated by climate change. Among the major constraints limiting their climate related actions, lack of information was considered the most important for managers of both public and private forests. Conclusions Knowledge transfer is an essential condition for research to lead to innovation. Improving the communication and demonstration of possible solutions for climate change adaptation is therefore likely to be the most effective strategy for increasing their adoption.

Introduction: Scots pine (Pinus sylvestris) is an important tree species that is often regenerated through seedling establishment. The resistance of these seedlings to drought treatments and their recovery are essential to forest regeneration. The association with ectomycorrhizal (EcM) fungi such as Paxillus involutus has the potential to improve drought resistance and possibly recovery. Here, we conducted two experiments to evaluate the impact of P. involutus inoculation on growth and water use of pine seedlings under contrasting drought intensities.Methods: In the first experiment, EcM fungal-inoculated and non-inoculated seedlings were subjected to well-watered (100% field capacity—control treatment) or low-watered (10% field capacity for 1 week—drought treatment) conditions, followed by a recovery phase in a semi-hydroponic cultivation system. In the second experiment, seedlings were initially subjected to well-watered or moderate water conditions (100 and 30% field capacity, respectively), followed by prolonged low-watered conditions (10% field capacity for 1 month), each followed by a recovery phase.Results: In the first experiment, EcM fungal-colonized pine seedlings showed increased aboveground biomass, higher photosynthetic rates, water-use efficiency, and elevated nitrogen and phosphorus contents under low-water conditions. In the second experiment, a prolonged low-water regime reduced root colonization by the EcM fungus. During recovery, P. involutus-inoculated seedlings had an increase in ammonium assimilation under well-watered conditions and appeared to recover more efficiently, in particular from prolonged low-water conditions, with higher chlorophyll production, possibly due to increased nutrient availability and/or fungal demand.Discussion: Our results suggest that the benefits of P. involutus depend on both water regime and drought duration: a short-term low-water regime (10% field capacity for 1 week) enhances EcM benefits, while under a moderate water regime (30% field capacity) the fungal effect was not evident. We conclude that P. involutus can confer significant benefits to pine seedlings, making it a valuable strategy for forest regeneration in temperate climates facing drought episodes.

Forests can foster mental health and well‐being. Yet, the contribution of forest biodiversity remains unclear, and experimental research is needed to unravel pathways of biodiversity–health linkages. Here, we assess the role of tree species richness, both actual and perceived, and how stress reduction and attention restoration can serve as potential mediating pathways to achieve positive mental health and well‐being outcomes. We conducted an experimental, multicentric field study in three peri‐urban forests in Europe, employing a mixed design with 223 participants, that comprised 20‐min stays in forests with either low, medium or high tree species richness or a built control. Participants' short‐term mental health and well‐being and saliva cortisol as a biomarker of stress were measured before and after the intervention. Forest visits for 20 min were found to be beneficial for participants' short‐term mental health, short‐term mental well‐being, subjective stress, subjective directed attention and perceived restorativeness compared with a built environment. No differences were found for the physiological stress indicator saliva cortisol, which decreased in both the forest and the built environments. Increased perceived biodiversity—possibly linked to structural forest attributes—was significantly associated with well‐being outcomes, while no association was found for differences in actual tree species richness. Structural equation modelling indicates that higher levels of perceived biodiversity had an indirect effect on short‐term mental health and well‐being through enhancing perceived restorativeness. While we found no evidence of actual tree species richness effects, perceived biodiversity was associated with positive short‐term mental health and well‐being outcomes. Understanding these biodiversity–health linkages can inform conservation management and help develop effective nature‐based interventions for promoting public health through nature visits. Read the free Plain Language Summary for this article on the Journal blog. Zusammenfassung Wälder wirken sich positiv auf psychische Gesundheit und psychisches Wohlbefinden aus. Der Beitrag der biologischen Vielfalt des Waldes ist jedoch nach wie vor unklar, und es bedarf experimenteller Forschung, um die Zusammenhänge zwischen biologischer Vielfalt und Gesundheit zu entschlüsseln. Die vorliegende Studie zielte darauf ab, die Rolle von tatsächlichem und wahrgenommenem Baumartenreichtum zu beleuchten. Außerdem wurde untersucht, wie und ob Stressreduktion und die Wiederherstellung von Aufmerksamkeitskapazitäten als potenzielle Vermittlungswege zwischen Waldbiodiversität und psychischer Gesundheit und psychischem Wohlbefinden dienen. Hierzu führten wir eine experimentelle, multizentrische Feldstudie in drei urbanen Wäldern in Europa durch. Anhand von Daten von 223 Teilnehmenden, wurden 20‐minütige Aufenthalte in Wäldern mit niedrigem, mittlerem oder hohem Baumartenreichtum oder in einer bebauten Kontrollbedingung verglichen. Gemessen wurde die momentane Ausprägung von psychischer Gesundheit, psychischem Wohlbefinden, sowie Speichelcortisol—einem Biomarker für Stress—jeweils vor und nach der Intervention. Ein 20‐minütiger Waldbesuch wirkte sich im Vergleich zur bebauten Kontrollbedingung positiv auf psychische Gesundheit, psychisches Wohlbefinden, subjektiven Stress, subjektive Aufmerksamkeit und das empfundene Erholungspotenzial der Teilnehmenden aus. Keine Unterschiede wurden für Speichelcortisol gefunden, welches sowohl im Wald als auch in der bebauten Umgebung abnahm. Höhere wahrgenommene biologische Vielfalt—die möglicherweise mit strukturellen Waldmerkmalen zusammenhängt—stand in signifikantem Zusammenhang mit Wohlbefinden, während für Unterschiede in tatsächlichem Baumartenreichtum kein Zusammenhang gefunden wurde. Die Ergebnisse des Strukturgleichungsmodells deuten darauf hin, dass ein höheres Maß an wahrgenommener biologischer Vielfalt eine indirekte Auswirkung auf psychische Gesundheit und psychisches Wohlbefinden hat, indem das wahrgenommene Erholungspotenzial erhöht wird. Während wir keine Hinweise auf Effekte tatsächlichen Baumartenreichtums fanden, konnte wahrgenommene Biodiversität mit positiven kurzfristigen Auswirkungen auf psychische Gesundheit und psychisches Wohlbefinden in Verbindung gebracht werden. Ein Verständnis des Zusammenhangs zwischen biologischer Vielfalt und Gesundheit kann dabei helfen, Naturschutzmanagement gesundheitszentriert auszurichten und gleichzeitig zur Entwicklung wirksamer, naturbasierter Interventionen zur Förderung der Gesundheit durch Naturbesuche beitragen. Résumé Les forêts peuvent améliorer la santé mentale et le bien‐être. Pourtant, la contribution de la biodiversité forestière n'est pas claire et des recherches expérimentales sont nécessaires pour mettre en évidence les liens entre la biodiversité et la santé. Ici, nous évaluons le rôle de la diversité en espèces d'arbres, à la fois réelle et perçue, et comment la réduction du stress et la restauration de l'attention peuvent servir de voies médiatrices potentielles pour obtenir des résultats positifs en matière de santé mentale et de bien‐être. Nous avons mené une étude expérimentale sur le terrain dans trois forêts périurbaines en Europe avec 223 participants, qui comprenait des interventions de 20 min dans des forêts avec une richesse en espèces d'arbres faible, moyenne ou élevée, ou un contrôle urbain. La santé mentale et le bien‐être à court terme des participants, ainsi que le cortisol salivaire en tant que biomarqueur de stress, ont été mesurés avant et après l'intervention. Les visites en forêt pendant 20 min se sont avérées bénéfiques pour la santé mentale à court terme, le bien‐être mental à court terme, le stress subjectif, l'attention subjective dirigée ainsi que la restauration perçue des participants par rapport à un environnement bâti. Aucune différence n'a été trouvée pour l'indicateur physiologique de stress, le cortisol salivaire, qui a diminué à la fois dans la forêt et dans l'environnement bâti. Une plus grande biodiversité perçue—potentiellement liée aux attributs structurels de la forêt—était associée de manière significative aux résultats en matière de bien‐être, alors qu'aucune association n'a été trouvée pour les différences dans la richesse réelle en espèces d'arbres. Nos modélisations indique que des niveaux plus élevés de biodiversité perçue ont un effet indirect sur la santé mentale et le bien‐être à court terme en améliorant la perception de la restaurabilité. Bien que nous n'ayons trouvé aucune preuve des effets de la richesse réelle en espèces d'arbres, la biodiversité perçue a été associée à des résultats positifs à court terme en matière de santé mentale et de bien‐être. La compréhension de ces liens entre biodiversité et santé peut éclairer la gestion de la conservation et aider à développer des interventions efficaces basées sur la nature pour promouvoir la santé publique par le biais de visites dans la nature. Samenvatting Bossen kunnen de geestelijke gezondheid en het welzijn bevorderen. Toch blijft de bijdrage van de biodiversiteit van bossen onduidelijk en experimenteel onderzoek is nodig om de verbanden tussen biodiversiteit en gezondheid te ontrafelen. Hier beoordelen we de rol van de rijkdom aan boomsoorten, zowel reëel als waargenomen, en hoe stressvermindering en aandachtsherstel kunnen dienen als potentiële bemiddelende paden om positieve resultaten te bereiken op het gebied van geestelijke gezondheid en welzijn. We voerden een experimentele veldstudie uit in drie peri‐urbane bossen in Europa met 223 deelnemers, waarbij de deelnemers 20 minuten verbleven in bossen met een lage, gemiddelde of hoge rijkdom aan boomsoorten of een gebouwde controle. De mentale gezondheid en het welzijn van de deelnemers op de korte termijn en het cortisol in het speeksel als biomarker van stress werden voor en na de interventie gemeten. Een bosbezoek van 20 minuten bleek gunstig te zijn op korte termijn voor de mentale gezondheid, het mentaal welbevinden, subjectieve stress, subjectieve gerichte aandacht en de waargenomen herstellende werking van de deelnemers in vergelijking met een bebouwde omgeving. Er werden geen verschillen gevonden voor speekselcortisol, degene zowel in het bos als in de bebouwde omgeving afnam. Verhoogde waargenomen biodiversiteit—mogelijk gekoppeld aan structurele boskenmerken—was significant geassocieerd met welzijnsresultaten, terwijl er geen verband werd gevonden voor verschillen in feitelijke boomsoortenrijkdom. Ons model geeft aan dat hogere niveaus van waargenomen biodiversiteit een indirect effect hadden op de mentale gezondheid en het welzijn op de korte termijn door het verbeteren van de waargenomen herstelbaarheid. Hoewel we geen bewijs vonden voor de effecten van de feitelijke rijkdom aan boomsoorten, werd de waargenomen biodiversiteit geassocieerd met positieve kortetermijnresultaten voor mentale gezondheid en welzijn. Inzicht in deze verbanden tussen biodiversiteit en gezondheid kan helpen bij het beheer van natuurbehoud en bij het ontwikkelen van effectieve natuurinterventies voor het bevorderen van de volksgezondheid door middel van natuurbezoeken. Read the free Plain Language Summary for this article on the Journal blog.

The temporal change of soil chemistry in the forest floor and mineral soil down to a depth of 40 cm was assessed for the 102 permanent plots of the French Network for the Monitoring of Forest Ecosystems (RENECOFOR), over a 15-year period (from 1993–1995 to 2007–2012). In examining the separate and joint evolutions of a large set of parameters, many significant changes were detected reflecting the fact that French forest soils were not in a steady state. A significant increase in soil organic carbon (SOC) stocks was found, mainly in the surface soil (13.0% increase over the forest floor and the 0–10 cm layer). Conversely, the relative increase of the total nitrogen (N tot ) stocks was lower in the surface soil (4.8% increase), and a general and sharp decline of N tot was detected between 10 and 40 cm depth (12.0% decrease). These results led to a substantial raise of C/N ratio over the whole soil profile. Another major finding is the difference in soil acidification recovery depending on the initial trophic level. In highly acidified contexts (top soil pH H 2 O < 4.5), increased soil acidification (pH and base saturation decrease, exchangeable Al increase) over the profile was observed while exchangeable base cation (Ca, Mg, K) pools increased. On the other hand, less acidic soils saw their global buffer capacity enhanced. These observations contrast with what is measured in other European inventories. While a previous study carried out on the same plots and over the same period highlighted SOC as a major driver of soil evolution in the top mineral soil, the possible mechanisms behind the large N decrease in the lower mineral soil remain to be confirmed.

Background Environmental change has resulted in changes in forest growth in Europe during the last century. This has consequences for the products and services delivered by forest. Mixing tree species is often proposed as a strategy to deal with the consequences of climate change. Diversifying forests is believed to result in higher productivity and increased growth stability. Tree species diversity is therefore expected to affect long-term trends in tree radial growth. However, this has not yet been studied. In this paper we study the effect of diversity on the radial growth and its long-term trends for beech and oak trees growing along a gradient of tree species diversity in the loamy region of central Belgium (from monocultures to mixed forests patches up to three species). Results We found that beech trees have a higher radial growth whereas oak trees have a lower one when growing in mixtures. The contrasting diversity-productivity relationship observed for beech and oak is in agreement with their ranking in shade tolerance, where oaks suffer increased competition in mixed oak patches. Overall, in monocultures and mixtures, an increasing radial growth trend of + 2% for the period 1927–2015 and 21% for the period 1899–2015 was found for beech and oak, respectively. Tree species diversity did not alter the shape of this detected long-term radial growth trend. Nevertheless, for oak a lower year-to-year variability in radial growth is found in mixtures indicating a higher resilience. Conclusion We conclude that diversity impacts the average radial growth and its variability (only in the case of oak) but not the shape of the long-term trend in radial growth of beech and oak trees growing in the loamy region of central Belgium.