Dr. Petia Simeonova Nikolova

Die prognostizierte Zunahme von extremen Störungsereignissen wie Borkenkäferbefall und Windwurf im Kontext des Klimawandels stellt die Schweizer Waldbewirtschaftung vor grosse Herausforderungen. Besonders betroffen sind fichtendominierte Gebirgswälder, die grossflächig verbreitet sind und häufig eine zentrale Schutzfunktion gegenüber Naturgefahren erfüllen. Die WSL erarbeitet zusammen mit der Praxis Grundlagen für die Priorisierung der Bewirtschaftung dieser Wälder unter dem Einfluss von extremen Störungen. Dazu werden räumliche Daten zu Waldstruktur, Standort, Störungsanfälligkeit und Naturgefahren auf Landschaftsebene mit waldbaulichen, planerischen und ökonomischen Aspekten verknüpft und in einem interaktiven Dashboard visualisiert. Dieser inter- und transdisziplinäre Ansatz wird hier anhand eines Fallbeispiels im Schilstal (Kanton St. Gallen) vorgestellt.

Management of spruce-dominated mountain forests in the context of extreme disturbances
The predicted increase in extreme disturbance events, such as windthrow and bark beetle infestations in the context of climate change poses major challenges for Swiss forest management. Mountain forests dominated by Norway spruce, which are both widespread and often fulfil an essential protective function against natural hazards, are particularly at risk. In response, the WSL is collaborating with practitioners to develop a framework for prioritising the management of these forests under the influence of extreme disturbances. This involves integrating spatial data on forest structure, site conditions, susceptibility to disturbance and natural hazards at the landscape level with silvicultural, planning and economic aspects. These insights are visualised through an interactive dashboard. This interand transdisciplinary approach is presented here with a case study from the Schilstal in the Canton of St. Gallen.

• The density of young trees has decreased in the past decade as the regeneration areas created by storms Vivian and Lothar have grown over again. Ungulate browsing is also influencing the species composition in many regions and reducing the regeneration density. Both young-growth and adult populations of ash north of the Alps and of sweet chestnut in Ticino have declined.
• Natural regeneration is the dominant form of regeneration in Swiss forests, accounting for 90.9% of the total regeneration area. This reflects the economic and ecological advantages of natural regeneration in close-to-nature forest management.
• Species-rich tree regeneration is necessary to ensure forest’s resilience to disturbances and its ability to adapt to climate change.

• Nell'ultimo decennio la densità dei giovani alberi è diminuita perché le aree di rinnovazione create dalle tempeste Vivian e Lothar sono tornate a essere invase dalla vegetazione. Anche il brucamento della fauna selvatica influenza la mescolanza delle specie in molte regioni e riduce la densità di rinnovazione. Nei popolamenti di frassino al Nord delle Alpi e di castagno in Ticino sono diminuiti sia gli alberi giovani che quelli adulti.
• Nel bosco svizzero domina la rinnovazione naturale, che in termini di superficie rappresenta il 90,9 per cento della rinnovazione complessiva e riflette i vantaggi economici ed ecologici della selvicoltura naturalistica.
• Per garantire la resilienza del bosco alle perturbazioni e la sua capacità di adattarsi al cambiamento climatico, è necessaria una rinnovazione degli alberi ricca di specie.

• La densité des jeunes arbres a diminué durant la dernière décennie, les surfaces de régénération créées par les tempêtes Vivian et Lothar ayant été recolonisées entre-temps. Dans de nombreuses régions, l'abroutissement influence lui aussi la composition en espèces et réduit la densité du rajeunissement. Les peuplements de frênes dans le nord des Alpes et de châtaigniers au Tessin ont diminué tant pour le recrû que pour les arbres adultes.
• Dans les forêts suisses, la régénération naturelle représente 90,9% de la surface totale de régénération. Cette prédominance reflète les avantages économiques et écologiques de sa mise en oeuvre dans la sylviculture proche de la nature.
• Une régénération riche en espèces est nécessaire pour garantir la résilience de la forêt lors de perturbations et sa capacité d’adaptation aux changements climatiques.

• Die Dichte der Jungbäume hat in der letzten Dekade abgenommen, indem die von den Stürmen Vivian und Lothar geschaffenen Verjüngungsflächen wieder zugewachsen sind. Auch der Wildverbiss beeinflusst in vielen Regionen die Artenzusammensetzung und reduziert die Verjüngungsdichte. Die Bestände der Esche nördlich der Alpen und der Kastanie im Tessin haben sowohl beim Jungwuchs als auch bei den adulten Bäumen abgenommen.
• Im Schweizer Wald dominiert die Naturverjüngung mit einem Flächenanteil von 90,9 % an der Gesamtverjüngung. Dies ist Ausdruck der ökonomischen und ökologischen Vorteile der Naturverjüngung im naturnahen Waldbau.
• Um die Resilienz des Waldes bei Störungen sowie die Anpassungsfähigkeit an den Klimawandel zu gewährleisten, ist eine artenreiche Baumverjüngung erforderlich.

Das waldbauliche Potenzial der Baumhasel ist wenig erforscht. Ein Anbauversuch in der Schweiz, der im Rahmen einer Zusammenarbeit mit Partnern aus Deutschland erfolgte, deckt ersten Auswertungen zufolge durchhaus Potenzial für den Schweizer Wald auf.

Le potentiel sylvicole du noisetier de Byzance est peu étudié. Pourtant, dans le cadre d'une collaboration avec des partenaires allemands, un essai de culture réalisé dans le canton d'Argovie a révélé un potentiel certain pour la forêt suisse.

Mixed mountain forests consisting of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), and silver fir (Abies alba Mill.) are among the most productive and stable forest ecosystems in Europe. Their southeasternmost geographical distribution range is located in the Western Rhodopes, where they have high economic, recreational, and ecological value. In the past, shelterwood cuttings dominated forest management practices in these forests and were mainly aimed at maintaining and reproducing conifers. During the past two decades, single-tree and group-tree selection systems have been promoted as alternative management approaches to support the conversion of spruce-dominated stands to close-to-nature mixed forests of fir, beech, and spruce. However, the natural regeneration dynamics in these stands are barely known, and their dependence on microsite and management effects needs to be better understood.
The objective of this study was to investigate ecological factors under management regimes of different intensity ("single-tree"-selection and "group-tree"-selection) that influence the regeneration processes in mixed mountain forests in the Bulgarian Rhodopes. Data on regeneration and microsite conditions were collected on 105 systematically distributed plots (25 m²/100 m²) in four 100–120 years old stands located in the regional forest district of Smoljan, Bulgaria (1580–1650 m a.s.l.). We relied on generalizeds linear mixed models to analyse for each species the (1) size-dependent regeneration density and (2) height increment in dependence on management practices, competing vegetation, as well as soil and light conditions.
Our study revealed an overall high potential for recruitment in the Western Rhodopes. Regeneration density was highest in fir (median 12800 N ha-1), followed by spruce (median 1600 N ha-1) and beech (median 1200 N ha-1). Fir benefited most from "single-tree" selection cuttings, while "group-tree" selection cutting tended to promote beech and fir but also spruce. Competing ground vegetation was detrimental for seedling density of all species. Annual height increment increased with plant size, was lowest in spruce, and similar in fir and beech. Sapling increment was driven by light, whereas seedlings did not react to increased radiation. Browsing was species-specific and was highest in beech (15–30 %), followed by fir (5–10 %) and spruce (<1 %). It was not a crucial factor in impeding tree recruitment. We conclude that frequent harvest activities of low intensity which consider advanced regeneration are a promising approach to successfully convert the formerly spruce-fir-dominated forests to climate-adapted fir-beech-(spruce)-mixed stands.

Die in Südosteuropa und Asien heimische und in Deutschland vor allem im Stadtgrün verbreitete Baumhasel zeichnet sich durch Mischungsfähigkeit, verlässliches Jugendwachstum und Wertholzerwartung aus. Sie gilt als dürreresistent sowie frost- und stresstolerant. Ihr möglicher forstlicher Anbau in mitteleuropäischen Wäldern ist jedoch bisher kaum erforscht. Neuere Versuche widmen sich deshalb dem Überleben, dem Wuchs und der Qualität sowie den Waldschutzrisiken.

The biggest challenge for forest management in the near future will be the silvicultural adaptation of forests on sites where climate change will increase the risk for drought induced tree mortality and for a decrease of the growth performance. One management option is the diversification by promoting climate change adapted non-native tree species. Currently, we have in depth experience with only a few non-native tree species in Central Europe. Other tree species that may complement the species pool in Central Europe need to be carefully selected by a range of criteria. Before establishing larger silvicultural experiments at stand-level, the suitability of species to grow well with a low risk of failure under expected future climatic conditions as well as a good performance already today should be tested, amongst other aspects. We present results of a multi-site common garden experiment where growth and survival of five non-native tree species (Abies bornmuelleriana, Cedrus libani, Fagus orientalis, Tilia tomentosa, and Tsuga heterophylla) and one climate adapted local native species per site are tested in five field trials in Austria, Germany and Switzerland along a temperature (7.9–10.4 °C mean annual Temperature) and precipitation (490–1147 mm annual precipitation) gradient. Each species was replicated on three plots per site using a block-wise design. Lower survival and growth of the non-native species as the native reference on most sites indicate that the studied non-native species are today not as well adapted to current climatic conditions as native species. However, the performance of the non-native species was sufficiently high to justify further trials. Survival and growth rates were mostly equally high on the two sites at the opposing ends of the temperature and humidity gradient and lower on all other sites. Height growth and survival were only marginally affected by the strong summer drought in 2018 in all except one site, presumably due to the low water storage capacity of the sandy soils on this site. This indicates that the tested non-native tree species grow well in the wide range of climatically changing weather conditions from our transnational study sites across Central Europe. So far, the tested tree species did not reach their climatic growth limitation. However, edaphic site conditions may have influenced the performance of the tested non-native species as well and must be considered when discussing the suitability of these tree species.

Bei der Waldbewirtschaftung geht es darum, gewünschte Leistungen mit möglichst wirksamen und kostengünstigen Eingriffen bereitzustellen. Von besonderer Bedeutung sind dabei Eingriffe, die über den Erfolg der Waldverjüngung bestimmen. Entsprechend wird die Verjüngung bei der Planung und Kontrolle von waldbaulichen Eingriffen, in der Waldplanung wie auch in der Jagdplanung regelmässig beurteilt. Allerdings sind Erfassung und Beurteilung der Verjüngung mit einigen Herausforderungen verbunden. In diesem Beitrag stellen wir fünf Indikatoren für die Beurteilung der Verjüngungssituation vor: die Verjüngungsdichte, der Verjüngungsdeckungsgrad, die Distanz zwischen Verjüngungsansätzen, der Flächenanteil ohne Verjüngung sowie die Verjüngungsfläche. Wir diskutieren ihre Eigenschaften und zeigen ihren Einsatzbereich in der Praxis auf. Die verschiedenen Indikatoren unterscheiden sich insbesondere darin, ob sie mithilfe von Messungen oder von Schätzungen erfasst werden, was Folgen für den Erhebungsaufwand und die Reproduzierbarkeit hat. Bei der Planung und Kontrolle von waldbaulichen Eingriffen ist in vielen Fällen eine gutachtliche Beurteilung der Verjüngung durch okulare Schätzung ausreichend. Wo die Verjüngungssituation jedoch unklar ist oder kontrovers beurteilt wird, werden gemessene Daten benötigt. Für den auf Messwerten beruhenden Indikator Verjüngungsdichte sind die Zielvorstellungen (Sollwerte, Bandbreiten) erst noch zu konsolidieren.

Der Klimawandel verändert die Wuchsbedingungen der Waldbäume, weshalb langfristig markante Verschiebungen in der Baumartenzusammensetzung des Schweizer Waldes zu erwarten sind. Die TreeApp ist ein Werkzeug für die Forstpraxis zur Ermittlung von Baumarten, die für das zukünftige Klima wahrscheinlich geeignet sind. In dieser Arbeit vergleichen wir Empfehlungen der TreeApp mit der im 4. Landesforstinventar (LFI4) beobachteten Artenzusammensetzung der Bäume (≥12 cm Brusthöhendurchmesser [BHD]) und der Verjüngung (<12 cm BHD), wobei wir auch die Präsenz von potenziellen Samenbäumen berücksichtigten. Wir stellen ein einfaches Bewertungsschema vor, mit dem wir die Stichprobenflächen des LFI in drei Stufen der Zukunftsfähigkeit (Grad der Anpassung an das Klima am Ende dieses Jahrhunderts) einteilen konnten. Schweizweit wurde der Wald auf 57% der Stichprobenflächen als zukunftsfähig, auf 23% als bedingt zukunftsfähig und auf 17% als nicht zukunftsfähig beurteilt. Dabei zeigte sich eine Häufung von nicht zukunftsfähigen Flächen in Fichtenwäldern in den Kantonen Graubünden und Wallis. Im Schutzwald der Voralpen, der Alpen und der Alpensüdseite wurde die Verjüngung auf 25% der Stichprobenflächen als nicht zukunftsfähig eingeschätzt, und auf 22% wurde keine Verjüngung beobachtet. Mit steigendem Fichtenanteil nahm der Anteil des Baumbestands ohne Zukunftsfähigkeit zu. Im Mittelland und im Jura wurde die Verjüngung auf einem grösseren Flächenanteil als zukunftsfähig eingeschätzt als der Baumbestand. Wenn Samenbäume berücksichtigt wurden, war der Stichprobenflächenanteil mit zukunftsfähiger Bestockung um bis zu 16 Prozentpunkte (Voralpen) höher, als wenn nur die Verjüngung betrachtet wurde. Diese Resultate unterstreichen die Notwendigkeit der Förderung einer zukunftsfähigen Verjüngung im Gebirgswald, bieten nützliche Grundlagen für die strategische Waldplanung und tragen so zur Anpassung des Waldes an den Klimawandel bei.

Widely observed inertia of forest communities contrasts with climate change projections that suggest dramatic alterations of forest composition for the coming decades. Disturbances might be a key process to catalyse changes in tree species composition under environmental change by creating opportunities for 'new' species to establish. To test this assumption, we compared two assessments (1993-1995, 2009-2017) from the Swiss National Forest Inventory to evaluate which forests were opened by natural canopy disturbance (that is, wind, insect outbreaks, fire and drought) and if these disturbances altered tree species composition both in terms of species-specific basal area and recruitment densities. Natural disturbances affected 14% of the Swiss forests within 25 years, with wind and insect outbreaks being the most frequent (75%) and fire and drought being rare (< 1.5%). Disturbances led to a shift from conifer to broadleaf tree species at low elevation, in particular in dense Picea abies stands, but no change was observed at higher elevations. The composition of undisturbed sites persisted during the same period. Our results demonstrate that undisturbed forests widely resist changes in tree species composition as an effect of direct ingrowth by stand-forming species. Disturbance events seem necessary to create opportunities for climatically 'better suited and site-adapted' species to (re-)establish and therefore potentially catalyse tree species turnover under environmental changes. We detected a reduction of tree species that were - centuries ago - cultivated outside their primary natural range, in particular P. abies, or depended on traditional management practices (Pinus sylvestris, Castanea sativa), which may inform us on how the projected increase in disturbance frequency and severity might filter tree species composition and hereby alter forest structure.

In den vergangenen 30 Jahren wurden Schweizer Gebirgsnadelwälder stellenweise mit Kahlhieben verjüngt und nicht mit kleinen Lücken, wie dies bei der Gebirgsplenterung üblich ist. Wie sich die Entwicklung in den entstandenen Bestandeslücken nach 13 bis 29 Jahren präsentiert, wurde in zehn Kahlhieben in der Südostschweiz und im Tessin mit Luftbildanalysen und Stichprobeninventuren untersucht. Historische Luftbilder dienten zur Erfassung der Bestandesgeschichte und der Kronendachöffnung vor dem Schlag. Mit Stichprobeninventuren wurden die aktuelle Baumverjüngung und die Kleinstandorte inklusive Bodenbedeckung erhoben. Die Dichte der Verjüngung zwischen 10 cm Höhe und 12 cm Brusthöhendurchmesser (BHD) lag im Mittel bei 5450 St./ha, wovon 71% Fichten waren. Der hohe Mittelwert kam durch Probeflächen mit sehr dichter Verjüngung zustande. Die Fichtenverjüngung stellte sich zum Grossteil nach dem Holzschlag ein; Vorverjüngung war im Nachwuchs (130 cm Höhe bis 12 cm BHD) zu 30% vorhanden. Die Vorverjüngung war dichter in Probeflächen, in denen der Vorbestand schon aufgelichtet gewesen war. Die höchsten Dichten der Nachverjüngung traten bei mittlerer bis hoher Kronendeckung vor dem Holzschlag auf. Verdämmende Konkurrenzvegetation reduzierte die Dichte der Vor- und der Nachverjüngung. Die Präsenz von Moderholz und Baumstümpfen war mit einer höheren Verjüngungsdichte verbunden, Wildhuftierverbiss dagegen mit einer geringeren. Bei wenig Konkurrenzvegetation tritt unmittelbar nach dem Schlagen einer grossen Lücke eine Phase mit hoher Verjüngungsgunst auf, in der die Verjüngung oft gelingt. Auf Standorten hingegen, die zur Bildung einer dichten Konkurrenzvegetation neigen, stellt sich die Verjüngung nur zögerlich ein, und sie ist auf Moderholz und Baumstümpfe angewiesen.

Over the past 30 years, in a few places, Swiss coniferous mountain forests have been regenerated with clear-felling and not with small openings, as it is common with group selection management in mountain forests. The development of such openings after 13 to 29 years was examined in ten clear-cuts in south-eastern Switzerland and Ticino with aerial photo analysis and sampling inventories. Historical aerial photographs were used to assess stand history and crown cover before the harvest. The density of regeneration and the local site conditions including the cover of ground vegetation were recorded using sampling inventories. The density of the regeneration (trees between 10 cm tall and 12 cm diameter at breast height [dbh]) averaged 5450 stems/ha, of which 71% were Norway spruce. This high mean value came about through plots with very dense regeneration. Most of the spruce regeneration originated from post-harvest establishment, but 30% of the regeneration in the size class 130 cm to 12 cm dbh had established before harvesting. The pre-harvest regeneration was denser in plots in which the pre-existing stand had been thinned already. The highest densities of regeneration occurred in stands with medium to high crown cover before the harvest. Competing vegetation reduced the density of pre- and post-harvest regeneration. The presence of rotting wood and tree stumps was associated with a higher regeneration density, while browsing by ungulates was associated with a lower one. If there is little competing vegetation, the creation of a large opening is followed by a phase of favourable regeneration conditions during which regeneration is often successful. In contrast, on sites that tend to develop dense competing vegetation, regeneration establishes only slowly, and is dependent on rotting wood and tree stumps.
 

Silvicultural interventions such as strip cuttings can change the resource availability of the edge trees. This may alter tree allometry, as light regime, water, and nutrient availability can change at the forest edge. Increased root growth may optimize resource uptake and/or enhance tree anchorage to withstand the altered wind regime. However, little is known about the patterns of the root–shoot allometric responses to strip cuttings. In three alpine stands differing in climate, site productivity, and stand characteristics, we selected 71 Norway spruce trees and took increment cores from stems, root collars, and main roots. This enabled us to study changes in the long-term root-stem allometry for 46 years and short-term allometric responses to intervention. The effects of cutting were compared between edge trees and trees from the stand interior in 10 years before and after the intervention. The long-term allocation to roots increased with stem diameter, with the strongest effects on the regularly managed stand with the tallest and largest trees. These results support the allometric biomass partitioning theory, which postulates resource allocation patterns between different plant organs to depend on plant size. Strip cutting on north-facing slopes boosted edge-tree growth in all plant compartments and enhanced allocation to roots. This change in allometry started 2 years after cutting but disappeared 7-8 years later. In the post-cutting period, the highest root–shoot increase was observed in the small trees independent of the site. This indicates the change in growing conditions to have the strongest effects in formerly suppressed trees. Thus, the effect of such acclimation on the wind firmness of subdominant spruce trees is a question with high importance for optimizing cutting layouts in lowering post-cutting vulnerability to disturbance. The results from this case study contribute to a better understanding of the structural acclimation of spruce trees from high-elevation forests to new forest edges. However, for a more mechanistic understanding of environmental drivers, further analyses of tree-ring stable isotopes are recommended.

Trees adjust multiple structural and functional organ-specific characteristics, "traits", to cope with diverse soil conditions. Studies on traits are widely used to uncover ecological species adaptability to varying environments. However, fine-root traits are rarely studied for methodological reasons. We analyzed the adaptability of the fine-root systems of European beech and Norway spruce to extreme drought within species-specific tree groups at Kranzberger Forst (Germany), focusing on the seasonality of morphological, physiological, and biochemical key traits in view of carbon (C) and nitrogen dynamics. We hypothesized that fine roots of both species adjust to seasonal drought: with beech representing a "fast" (i.e. with fast C turnover), and spruce a "slow" (i.e. with long-term C retention) ecological strategy. We identified three functional fine-root categories, based on root function (absorptive or transport fine roots), and mycorrhizal status of the absorptive fine-roots (mycorrhizal or non-mycorrhizal). Solely the non-mycorrhizal absorptive roots adjusted in a species-specific manner supporting fine-root ecological strategy hypothesis. During drought, beech produced thin ephemeral (absorptive non-mycorrhizal) fine roots with high specific fine-root area and high respiratory activity, representing fast C turnover and enabling effective resource exploitation. These adjustments reflect a "fast" ecological strategy. Conversely, spruce absorptive fine roots did not respond to the soil moisture deficit by growth but instead increased root suberization. Drastically lowered respiratory activity of this functional category facilitated C retention and structural persistence during drought, indicating a "slow" ecological strategy in spruce. Absorptive mycorrhizal fine roots maintained respiration throughout the drought event in both tree species, but in spruce this was the only fine-root category with high respiration. This suggests, that spruce relies heavily on mycorrhizal associations as a method of drought resistance. Accumulation of non-structural carbohydrates and high C concentrations were observed in the transport fine roots of both species, indicating drought-induced osmotic protection of these roots. Thus, functional classification enabled us to determine that fine-root branches of each species are not tied to one sole ecological strategy. The suggested approach helps to better understand the complex interplay between structure and function belowground.

Climate projections indicate large increases in temperature, requiring tree species to adapt or migrate faster than observed historic rates. In Central Europe, discussion in forest management has increasingly focused on how humans can assist these adaptation/migration processes, especially when important forest ecosystem services are at risk. One option is to introduce non-native tree species to help forest ecosystems adapt to climate change. We established a long-term experiment in autumn 2012 on five study sites in climatically different regions of Central Europe. The performance of five non-native tree species from warmer climates (Abies bornmuelleriana Mattf., Cedrus libani A. Rich., Fagus orientalis Lipsky, Tilia tomentosa Moench and Tsuga heterophylla Sarg.) was compared to that of one local species native to each site. We monitored sapling survival during the first 4 years after planting. Small differences in timing of inventory campaigns were overcome by fitting a Weibull function to survival curves and analysing time-harmonized estimates for equal biannual periods. Sapling mortality decreased over time, but early after planting, it differed between species. The native and the two non-native broadleaved species showed higher survival than the non-native coniferous species. Moreover, the site conditions of the open areas seemed to initially hamper the growth of shade-tolerant species. All species performed reasonably well during the relatively short observation period. Any differences between hazard rates almost vanished 4 years after planting, which suggests that all species tested are likely to thrive under current climate conditions. Long-term observations are, however, needed to corroborate these results.