Dr. Thibault Lachat

Purpose of review: We are amid a historical momentum encouraging forest restoration, yet the translation of ambitious targets into reality is hindered by poor documentation and understanding of the success and failure of past restoration efforts. This review aims to evaluate the ecological, social, political and economic characteristics of forest restoration across Europe, their development over time and key lessons learned to guide future restoration initiatives. The analysis is based on the synthesis of expert assessments from 18 European countries.
Recent findings: Early restoration initiatives in central and southern Europe were largely reactive to natural disasters or timber shortages, and while effective in erosion control and timber production, their ecological benefits were often limited due to monoculture plantations and short-rotation systems. Geopolitical crises intensified timber production, with nationalized and centrally managed forests in several countries, but often at the cost of biodiversity. Since the 1990s, a shift toward multifunctionality has emerged driven by the convergence of environmental, social, political and economic events.
Summary: Forest restoration in Europe has transitioned from disaster reduction and production-driven efforts to a more multifunctional approach that promotes biodiversity. Changes have been driven by a combination of environmental (e.g., catastrophic consequences following natural disasters), political (e.g., wars, forest nationalization and management centralization), legal (e.g., strict and ambitious national and international policies), social (e.g., rural abandonment and changes in societal values) and economic (e.g., new funding mechanisms or market fluctuations) events. Despite the development, conflicting goals, insufficient funding, climate change and short-term thinking persist as key barriers.

Tree-related Microhabitats (TreMs) are of prime concern for biodiversity since they host thousands of taxa. TreMs are discrete habitat patches borne by trees and cover a wide range of lifespans, from days to decades to centuries. The taxa associated with such discrete and sometimes highly ephemeral habitat patches are likely to be sensitive to spatial distribution because they need to search for new habitats after the occupied one disappears. Although many studies have recently been dedicated to TreMs, only very few have investigated their spatial distribution. Focusing on European beech-dominated forests, we used a European TreM database with 12,362 trees and 296 plots (ranging from local (0.1–1 ha) to landscape scale (10,000 ha)) to assess TreM spatial distribution patterns in long unmanaged forests. Then, with a TreM sub-database with 6828 trees and 21 plots, we assessed whether and if so, how harvesting changes spatial patterns at the plot scale. In long unmanaged forests, most TreMs showed a regular pattern at the plot scale and an aggregated pattern at larger scales. Tree diameter was the most influential factor for spatial patterns in TreMs. Spatial patterns at the plot scale in harvested forests differed slightly from those observed in unmanaged forests. To favor TreM-dwelling taxa in harvested stands, our results suggest retaining habitat trees in a regular spatial pattern to mimic the natural pattern. However, some TreMs should be conserved in an aggregate pattern; we specify the spatial scale at which this should be done.

Balancing increasing demand for wood products while also maintaining forest biodiversity is a paramount challenge. Europe's Biodiversity and Forest Strategies for 2030 attempt to address this challenge. Together, they call for strict protection of 10% of land area, including all primary and old growth forests, increasing use of ecological forestry, and less reliance on monocultural plantations. Using data on country wide silvicultural practices and a new database on strict forest reserves across Europe, we assess how triad forest zoning could help meet these goals. Our analysis reveals that zoning in Europe is overwhelmingly focused on wood production, while there has been little concomitant protection of forests in strict reserves. Moreover, most strict forest reserves are < 50 ha in size, likely too small to capture the minimum dynamic area necessary to sustain many taxa. We outline research priorities to meet future demands for timber while minimizing the impact on native biodiversity.

The umbrella species concept is a popular conservation planning tool which postulates that conservation schemes targeting a specific species will indirectly benefit many other sympatric species. In Scandinavia and Central Europe, the White-backed Woodpecker (Dendrocopos leucotos) is considered an umbrella species for woodland birds and cryptogam species of conservation concern. Whether this also applies to saproxylic beetles, a group of high conservation concern, remains open. Therefore, we tested that umbrella function in Central European beech forests that are currently recolonized by this woodpecker. Relying on radiotracking data, we compared saproxylic beetle communities within the breeding home ranges of White-backed Woodpeckers (high and low activity of the bird) against forests with ascertained absence of the bird (control). Bayesian inference for linear regressions identified that species richness of threatened saproxylic beetles was 1.51 (lower and upper 5 % PPCrI=[1.09; 2.01]) times higher in sites with high White-backed Woodpecker activity compared to the control. Community composition analyses on threatened saproxylic beetles showed a reduced β-diversity at low and high White-backed Woodpecker sites compared to the control. Finally, an indicator species analysis showed that 17 saproxylic beetle species, including 4 threatened species, were positively associated with White-backed Woodpecker’s breeding home ranges, while only 3 species, but no threatened species, were associated with the control sites. Overall, our results suggest that the White-backed Woodpecker plays the role of an umbrella species for threatened saproxylic beetle communities, opening new opportunities for conservation planning in European beech forests.

1. Human intervention often alters the availability of habitat for biodiversity. The conservation of biodiversity therefore requires an optimized habitat management. In forests, dead wood represents one of the most important habitats and in boreal and temperate regions around 25% of forest species depend on it (= saproxylic species). Increasing the amount of dead wood in managed forests has thus become a policy objective, but there is no consensus on how to best distribute dead wood in space.
2. In a landscape experiment, we exposed freshly cut beech branches in bundles of different sizes (one, three, six and 12 branches) in the forest, representing newly created habitat patches to be colonized by saproxylic beetles. We investigated how species richness in a 'single large' branch bundle compares to that in 'several small' bundles (SLOSS debate). We further tested the effects of dead wood availability (amount and isolation) in the surrounding landscape (20–200 m) and environmental factors (temperature and light availability) on species richness, abundance and community composition.
3. The species richness of the pooled small bundles (1 + 3 + 6 = 10 branches) was as high as that of the large bundle (12 branches), despite having a smaller total surface, demonstrating the benefit of spatially dispersed habitat patches for total diversity. Also community composition differed and every bundle size yielded some unique species. Dead wood availability in the surrounding landscape had a minor effect in comparison. Our results further highlight the importance of microsite heterogeneity: species richness was related to light availability, and abundance and community composition were related to temperature.
4. Synthesis and applications: Larger amounts of dead wood harbour more saproxylic beetle species and the distribution of dead wood in patches of different sizes within the forest can promote the development of variable species communities. Combined, this results in a higher species diversity. In managed forests, where retained dead wood is often homogeneous in terms of size or tree species, increasing heterogeneity by distributing dead wood in the forest could foster higher diversity of saproxylic species.

Saproxylic beetles and fungi are regularly used as indicators of naturalness in natural forest reserves. In order to assess the initial state of these two groups of organisms, an inventory was carried out in 2022 and 2023 on 70 plots distributed within the perimeter of the Jorat Nature Park, i.e. in its protected area as a natural forest reserve and in its transition zone. In total, 393 species of saproxylic beetles and 376 species of saproxylic fungi were inventoried. For now, the two areas of the NaturePark show very similar beetle and fungal communities. A repetition of this inventory in the future will highlight the evolution of the forest and the effects of the abandonment of logging on the diversity of saproxylic species.

Les coléoptères et champignons saproxyliques sont régulièrement utilisés comme indicateurs de la naturalité dans les réserves forestières naturelles. Afin d'évaluer l'état initial de ces deux groupes d'organismes, un inventaire a été conduit en 2022 et 2023 sur 70 placettes distribuées dans le périmètre du Parc naturel du Jorat, soit dans son aire protégée également réserve forestière naturelle et dans sa zone de transition. Au total, 393 espèces de coléoptères saproxyliques et 376 espèces de champignons saproxyliques ont été inventoriées. Pour l'heure, les deux zones du Parc naturel montrent des communautés de coléoptères et de champignons très similaires.
Une répétition de cet inventaire dans le futur permettra de mettre en évidence l'évolution de la
forêt et les effets de l'abandon de l'exploitation du bois sur la diversité des espèces saproxyliques
 

The European biodiversity and forest strategies rely on forest sustainable management (SFM) to conserve forest biodiversity. However, current sustainability assessments hardly account for direct biodiversity indicators. We focused on forest multi-taxon biodiversity to: i) gather and map the existing information; ii) identify knowledge and research gaps; iii) discuss its research potential. We established a research network to fit data on species, standing trees, lying deadwood and sampling unit description from 34 local datasets across 3591 sampling units. A total of 8724 species were represented, with the share of common and rare species varying across taxonomic classes: some included many species with several rare ones (e.g., Insecta); others (e.g., Bryopsida) were represented by few common species. Tree-related structural attributes were sampled in a subset of sampling units (2889; 2356; 2309 and 1388 respectively for diameter, height, deadwood and microhabitats). Overall, multi-taxon studies are biased towards mature forests and may underrepresent the species related to other developmental phases. European forest compositional categories were all represented, but beech forests were over-represented as compared to thermophilous and boreal forests. Most sampling units (94%) were referred to a habitat type of conservation concern. Existing information may support European conservation and SFM strategies in: (i) methodological harmonization and coordinated monitoring; (ii) definition and testing of SFM indicators and thresholds; (iii) data-driven assessment of the effects of environmental and management drivers on multi-taxon forest biological and functional diversity, (iv) multi-scale forest monitoring integrating in-situ and remotely sensed information.

Un arbre hàbitat o portador de microhàbitats es defineix com un arbre que té almenys un dendromicrohàbitat (Fig. 1). Un dendromicrohàbitat (DMH) és una característica morfològica present en un arbre, utilitzada per determinades espècies durant almenys una part del seu cicle de vida. Aquestes singularitats poden servir com a refugis, llocs de reproducció o llocs crucials d'hibernació o alimentació per a milers d'espècies. Les pertorbacions, biòtiques o abiòtiques, poden crear dendromicrohàbitats: per exemple, la caiguda d'una roca podria malmetre l'escorça, un llamp podria caure sobre un arbre i trencar la fusta, o un picot podria crear una cavitat per a la cria en el tronc. Per a alguns dendromicrohàbitats, com els nius de vertebrats o les escombres de bruixa, l'arbre és simplement un suport físic. Només les singularitats que se sap que tenen un vincle directe amb una o més espècies associades es classifiquen com a dendromicrohàbitats (Larrieu et al. 2018). [...]

Een habitatboom wordt gedefinieerd als een boom met ten minste één boommicrohabitat (Fig. 1). Een boommicrohabitat (afgekort TreM, voor 'Tree Related Microhabitat') is een bijzondere morfologische eigenschap van een boom die gebruikt wordt door soms zeer gespecialiseerde soorten tijdens ten minste een deel van hun levenscyclus. Deze elementen kunnen schuilplaatsen, broedplaatsen of cruciale overwinterings- of voedingsplaatsen vormen voor duizenden soorten. Verschillende biotische of abiotische oorzaken kunnen boommicrohabitats creëren. Een vallend rotsblok kan bijvoorbeeld de schors verwonden, de bliksem kan op een boom inslaan en het hout openbreken, of een specht kan een broedholte uithakken in de stam. Voor sommige TreMs, zoals nesten van gewervelden, is de boom slechts een fysieke ondersteuning. Alleen morfologische kenmerken waarvan bekend is dat ze een direct verband hebben met één of meer geassocieerde soorten worden geclassificeerd als TreMs (Larrieu et al. 2018). [...]

Several adults and larvae of Pytho abieticola were discovered in 2021 and 2022 at three different localities in two regions of Switzerland, the Jura mountains and the Swiss National Park in the Eastern Alps. This very rare saproxylic beetle has not been detected in Switzerland so far. Considered a relict of primeval forests, it is in strong decline or has already disappeared from large areas of Central Europe. The isolated Swiss populations are the westernmost in its distributional range. Information on the ecology and distribution of P. abieticola in Switzerland and Europe is provided and criteria to distinguish P. abieticola from the congeneric and syntopic P. depressus are defined.

Das Einrichten von Naturwaldreservaten ist eine wichtige Massnahme zur Erhaltung und Förderung der Biodiversität im Wald - das gilt besonders für Arten, die auf späte Waldentwicklungsstadien und Totholz angewiesen sind. Im Rahmen des Projekts «Biodiversität und Habitatstrukturen im Wildnispark Zürich Sihlwald» untersuchten wir zwischen 2016 und 2020 vier Organismengruppen (Moose, Flechten, xylobionte Käfer und totholzbewohnende Pilze) auf 69 Probeflächen der kantonalen Stichprobeninventur. Um zu verstehen, wie ihr Vorkommen mit verschiedenen Waldparametern zusammenhängt, verglichen wir Modelle, die auf Inventurdaten basieren, mit solchen, die nur Fernerkundungsdaten verwenden. Während ein etwas stärkerer Zusammenhang der Artenvielfalt totholzbewohnender Pilze mit Inventurdaten (z.B. Totholzvolumen) gefunden wurde, waren es bei den xylobionten Käfern eher Fernerkundungsdaten (z.B. Vegetationshöhe). Bemerkenswert ist, dass der Sihlwald nach knapp 20 Jahren ohne Bewirtschaftung einige Artenhighlights beherbergt, etwa die Zitronengelbe Tramete (Antrodiella citrinella) - der Pilz gilt als sogenannte Urwaldart. Unsere Modelle liefern wertvolle Hinweise darauf, wie zukünftige Biodiversitätsmonitorings mithilfe von Fernerkundung unterstützt werden können. Zudem bilden die erhobenen Daten die Grundlage für ein Langzeitmonitoring der Biodiversität im grössten Naturwaldreservat der Schweiz in der Buchenwaldstufe und ermöglichen in Zukunft Aussagen über die Entwicklung des Waldes zu einem Refugium für Arten, die in bewirtschafteten Wäldern kaum überleben können.

Protecting structural features, such as tree-related microhabitats (TreMs), is a cost-effective tool crucial for biodiversity conservation applicable to large forested landscapes. Although the development of TreMs is influenced by tree diameter, species, and vitality, the relationships between tree age and TreM profile remain poorly understood. Using a tree-ring-based approach and a large data set of 8038 trees, we modeled the effects of tree age, diameter, and site characteristics on TreM richness and occurrence across some of the most intact primary temperate forests in Europe, including mixed beech and spruce forests. We observed an overall increase in TreM richness on old and large trees in both forest types. The occurrence of specific TreM groups was variably related to tree age and diameter, but some TreM groups (e.g., epiphytes) had a stronger positive relationship with tree species and elevation. Although many TreM groups were positively associated with tree age and diameter, only two TreM groups in spruce stands reacted exclusively to tree age (insect galleries and exposed sapwood) without responding to diameter. Thus, the retention of trees for conservation purposes based on tree diameter appears to be a generally feasible approach with a rather low risk of underrepresentation of TreMs. Because greater tree age and diameter positively affected TreM development, placing a greater emphasis on conserving large trees and allowing them to reach older ages, for example, through the establishment of conservation reserves, would better maintain the continuity of TreM resource and associated biodiversity. However, this approach may be difficult due to the widespread intensification of forest management and global climate change.

The species-energy hypothesis predicts increasing biodiversity with increasing energy in ecosystems. Proxies for energy availability are often grouped into ambient energy (i.e., solar radiation) and substrate energy (i.e., non-structural carbohydrates or nutritional content). The relative importance of substrate energy is thought to decrease with increasing trophic level from primary consumers to predators, with reciprocal effects of ambient energy. Yet, empirical tests are lacking. We compiled data on 332,557 deadwood-inhabiting beetles of 901 species reared from wood of 49 tree species across Europe. Using host-phylogeny-controlled models, we show that the relative importance of substrate energy versus ambient energy decreases with increasing trophic levels: the diversity of zoophagous and mycetophagous beetles was determined by ambient energy, while non-structural carbohydrate content in woody tissues determined that of xylophagous beetles. Our study thus overall supports the species-energy hypothesis and specifies that the relative importance of ambient temperature increases with increasing trophic level with opposite effects for substrate energy.

Tree-related microhabitats (TreMs) provide a quantitative indicator of habitat heterogeneity in forests, including beech (Fagus) forests. However, systematic analyses of the factors driving TreM diversity and composition in Fagus sylvatica and F. orientalis forests are lacking. In this study, the TreMs of beech forests on 203 plots of 22 forest sites (production and old-growth forest) across the full longitudinal range of both species were assessed following a standardized TreM protocol. A unified diversity and ordination framework based on Hill numbers was applied to account for unobserved TreM types and to extend the sensitivity of our findings focusing from rare to dominant TreMs. The composition of TreM assemblages was mostly determined by Fagus species and elevation, a surrogate for climate, and with focus on dominant TreMs by DBH, whereas old-growth versus production forest had no effect. The coverage of detected TreMs per plot increased with the number of trees assessed and DBH, but was lower in old-growth than in production forests. When standardized for sampling coverage, the diversity of rare and dominant TreM types was higher in old-growth than in production forests, but increased with elevation only focusing on dominant TreMs. These findings corroborate regional studies showing a higher TreM diversity in old-growth forests. Moreover, they demonstrate the importance of focusing conservation efforts on forests of both Fagus species and at different elevations, covering the full range of TreM assemblages. Future studies comparing TreM diversity in different forests should standardize diversity by sample coverage, as currently done in many biodiversity studies.