Dr. Marco Conedera

Waldbrände sind ein sporadisches und unregelmässig auftretendes weltweites Phänomen. Obwohl die Schweiz international als Land mit relativ geringem bis mittlerem Waldbrandrisiko gilt, erfordern die sich stetig ändernde Landnutzung und klimatischen Bedingungen eine kontinuierliche Anpassung der bestehenden Waldbrandmanagementstrategien. Hauptziel der Waldbrandprävention ist es nicht, jeden potenziellen Waldbrand zu verhindern, sondern vielmehr, die durch grosse Waldbrände gefährdete Schutzfunktion des Waldes zu bewahren, sowie die Bewohnbarkeit der Berggebiete zu sichern.

In the complex Alpine topography, canyon-like terrain morphology combined with steep slopes may cause sudden changes in fire behavior, for instance, turning fast-spreading surface fires into eruptive fires. This phenomenon was observed during the vegetation dormancy in mountain beech (Fagus sylvatica L.) stands, a forest type usually spared from wildfires. The aim of this study is to understand the impact of eruptive fire behavior on the post-fire trajectories of beech trees. To this end, over the course of 3 years, we monitored the post-fire dynamics of 100 beech trees subjected to different fire intensities in February 2020, including a small-scale eruptive fire behavior associated with a steep slope (~41.4° on average) and canyon-like terrain features. Dendrometric and sylvicultural parameters and fire-related damages such as the presence of bark blackening, scars, foliage loss, crashes of branches or whole trees, and parasitic attacks were assessed. The observed medium to high fire severity was highly dependent on the position of the tree within the eruptive fire sector, whereas the relationship with dendrometric and sylvicultural tree characteristics, such as the crown position or the stem diameter, was less clear. Due to the lethal temperatures generated by the heat convection within the eruptive fire sector, the exposed bud and cambium tissues of the beech trees partially or totally failed to develop in the first post-fire season. Beech crown and stem tissue killed by exposure to convective heat fluxes rapidly dries out, loses its elasticity, and becomes brittle. Starting from the second post-fire season, this greatly increases the risk of the crashing of these crown parts and/or entire trees. A detailed understanding of the factors leading to an eruptive fire behavior would highly benefit the fire risk assessment and the detailed planning of the firefighting tactics in mountain areas, eventually increasing the safety of firefighters and preventing post-fire natural hazards, such as rockfall, erosion, surface runoff, and debris flow.

• La culture ciblée d'essences non indigènes reste marginale dans l'économie forestière suisse. Durant la dernière décennie, la surface dominée par ces essences est passée de 0,2 à 0,7%.
• Dans certaines conditions environnementales, des espèces ligneuses non indigènes peuvent se propager de manière invasive, surtout à basse altitude. La culture d'essences non indigènes non envahissantes en tant que stratégie d'adaptation aux changements climatiques gagne en importance.
• Les risques et les chances que comporte l'utilisation de ces essences doivent être soigneusement évalués. La gestion des espèces envahissantes est à prendre en compte dans la planification forestière.

• Der gezielte Anbau nicht einheimischer Baumarten spielt in der Schweizer Waldwirtschaft nach wie vor eine kleine Rolle. In der letzten Dekade hat sich die Fläche, auf der solche Arten dominieren, um 0,2 % auf 0,7 % erhöht.
• Unter besonderen Umweltbedingungen können sich nicht einheimische Gehölzarten invasiv ausbreiten, vor allem in tieferen Lagen. Als Strategie zur Anpassung an den Klimawandel gewinnt der Anbau nicht invasiver gebietsfremder Baumarten an Bedeutung.
• Beim Einsatz nicht einheimischer Baumarten müssen die Risiken und Chancen sorgfältig abgewogen werden. Der Umgang mit invasiven Arten muss in die waldbauliche Planung einbezogen werden.

• Targeted cultivation of non-native tree species still plays a minor role in Swiss forestry. The area dominated by such species has increased by 0.2% to 0.7% over the past decade.
• Under particular environmental conditions, non-native woody species may spread invasively, especially at lower altitudes. The cultivation of non-invasive, non-native tree species is also becoming increasingly important as a climate change adaptation strategy.
• The risks and opportunities of using non-native tree species must be carefully weighed up, and invasive species management must be included in silvicultural planning.

• La coltivazione mirata di specie arboree non autoctone continua a svolgere un ruolo minore nella selvicoltura
svizzera. Nell'ultimo decennio la superficie su cui dominano queste specie è cresciuta dallo 0,2 allo 0,7 per cento.
• In particolari condizioni ambientali, le specie legnose non autoctone possono diffondersi in modo invasivo, soprattutto alle basse altitudini. La coltivazione di specie arboree esotiche non invasive sta diventando sempre più importante come strategia di adattamento al cambiamento climatico.
• Occorre ponderarne attentamente i rischi e i benefici così come il loro impiego deve essere integrato nella
pianificazione selvicolturale.

Abies alba Mill. is a prominent European tree species predominantly inhabiting cool and humid montane environments. However, paleoecological evidence reveals that during the Eemian and mid-Holocene, A. alba thrived in much warmer and drier climates. This capacity is nowadays reflected in cryptic meso- and sub-Mediterranean lowland populations. To link A. alba populations across diverse climates spanning from the Mediterranean lowlands to the Alpine timberline, we genotyped 421 specimens from Italy and Switzerland at 190 single-nucleotide polymorphisms (SNPs). Population genetic structure analyses indicate that isolated meso- and sub-Mediterranean lowland populations in Tuscany and Ticino align genetically with higher elevation populations in each region, suggesting that their capacity to thrive in warmer, drier conditions cannot be attributed to plantations with planting stock originating from different climates or to the occurrence of a single warm-adapted lineage showing a disjunct distribution, unless migration between Tuscany and Ticino stands occurred. Genotype–environment associations reveal that temperature seasonality, precipitation during critical seasons, and relative humidity are important for explaining the species' genetic variation. With genotype–environment and genotype–phenotype associations, we identified candidate adaptive genes potentially linked to climatic conditions and drought response. While certain adaptive alleles may have spread from Tuscany and Southern Italy or could be explained by a shared ancestry of Ticino and Tuscan populations, local adaptation may have occurred at specific loci. These findings underscore the importance of considering the hitherto overlooked lowland Mediterranean populations of A. alba to better understand the species' climatic niche and its potential for forest conservation and management under global warming.

Abies alba is an important European tree species currently mostly found at cool and humid sites in the montane zone. In the past, it grew under markedly warmer and drier climates during the Eemian and mid-Holocene, and cryptic Mediterranean populations confirm the species' capacity to grow under warm, summer-dry conditions. However, it is unknown if warm-loving Mediterranean occurrences are related to specific genetic properties (e.g., subspecies or ecotypes). Investigating the genetics of cryptic warm-loving populations is crucial for a better understanding of past and future population dynamics of A. alba. We genotyped 478 A. alba samples at 174 single-nucleotide polymorphisms (SNP), covering a broad latitudinal range from Southern Italy to Switzerland while accounting for local altitudinal gradients, and combined these newly introduced genotypes with those of other European Abies populations from the literature. Population genetic structure analyses grouped the warm-loving meso- and sub-Mediterranean populations into the same genetic cluster as the mountain populations of each region. The occurrence of three genetic clusters from Northern to Southern Italy is in line with the glacial refugia history. The inferred evolutionary and demographic history suggests a northward expansion of A. alba after glaciation, as well as a trans-Adriatic gene flow between Balkan and Southern Italian populations. Collectively, the combined genotypic data from individuals across the species' range demonstrate that cryptic Mediterranean populations of A. alba align with the local and large-scale genetic structure of populations from its main range, suggesting that the species is able to thrive in a warmer and drier environmental range than hitherto anticipated. This finding implies that it is unneeded to postulate extinct subspecies or ecotypes to explain the occurrence of meso- and sub-Mediterranean Eemian or mid-Holocene silver fir forests, with important implications for future A. alba population dynamics.

Rising temperatures and an expected increase in the frequency of droughts and heat waves have raised concerns about forest fires in cool-temperate and humid-continental regions. This concern is accompanied by a lack of detailed knowledge of fire behaviour and fire ecology in many of these regions. A special issue 'Forest fires in cool-temperate and humid-continental forests' was launched in early 2022 in Forestry to collect studies that address existing knowledge gaps. The featured research papers cover several topics related to remotely-sensed and field-based fuel assessment, flammability of forest fuels, fire behaviour, and fire effects. In this article, we provide an overview of the research papers and their main findings. Based on the results of the special issue and a review of recent literature, we identified major directions for future research that address fire behaviour, fuel characteristics and post-fire forest management.

Sustainable regeneration of silver fir (Abies alba Mill.) is important for maintaining and improving the future resilience and protective capacity of mountain forests in the southern Swiss Alps. However, increasing densities of wild ungulates, such as red and roe deer, can hamper the regeneration success of silver fir. A comprehensive understanding of forest–wildlife interactions is essential to plan concrete measures for improving the adaptation potential of mountain forests. Here, we monitored the dynamics of natural silver fir regeneration in mountain protective forests. This study encompassed 30 unfenced permanent plots across two sites, Lostallo and Soazza, located in Valle Mesolcina in the southern Swiss Alps. These sites were selected for their comparable ecological characteristics related to silver fir regeneration, yet contrasting densities of mature, seed-bearing silver fir trees. We recorded the number of seedlings (<10 cm height) and saplings (10–200 cm height), and repeatedly assessed height growth, frequency, and intensity of ungulate browsing on saplings in the summers 2016, 2019, and 2022. In 2022, we additionally recorded the presence of browsing on other tree species in the unfenced plots and the height growth increments of silver fir saplings in pre-existing fences. On average, 509 silver fir seedlings/ha were present in the study area, but there was a lower density of saplings (382 saplings/ha). Between 2016 and 2022, the browsing frequency on silver fir saplings increased from 33 per cent to nearly 100 per cent, accompanied by a significant rise in browsing intensity. Furthermore, sapling height growth from 2019 to 2022 was significantly reduced or even negative in the unfenced plots as compared to the growth of saplings in the fenced plots. The observed seedling density indicated a good potential for natural regeneration of silver fir, irrespective of the differences in the density of seed-bearing trees between the two sites. However, the increased ungulate browsing impaired successful regeneration in the considered mountain forests in the southern Swiss Alps. Thus, without an effective control of ungulate browsing, silver fir may not be able to substantially contribute to the transition to more resilient and future-proof mountain forests.

Understanding the ecological needs of forest tree species at the warm edge of their range can provide valuable information for management strategies in a warming climate. Here, we carried out a dendroecological study focusing on submediterranean and mesomediterranean low elevation (i.e., 30–800 m asl) marginal stands of Abies alba Mill. (silver fir) under warm mean July-August temperatures of 23–25 °C. Such marginal stands are compared to an Apennine core stand at 1450 m asl in central Italy amid the climatic niche with optimal growing conditions. Additionally, we used understory vegetation surveys to assess local growing conditions. During periods of low growth rates, the growth of low-elevation stands is significantly influenced by both short-term current-year climatic signals (monthly to seasonal; immediate effect on growth) and long-term (pluriannual) cumulative signals, whereas the strength of the climatic influence progressively increases from the monthly to the multi-annual scale. Trees are adversely affected by high summer and early autumn temperatures and drought and benefit from sufficient rainfall in late summer, autumn, and early winter, in addition to spring rainfall. Moreover, when temperature and moisture are high, low-elevation stands likely benefit from a prolonged late growing season. These findings illustrate that the growth of A. alba at low elevations is rather limited by moisture availability than high temperatures. In contrast, silver fir in the cooler and moister uppermost reference stand was less affected by current year drought and cumulative climatic effects. Our results confirm the potential of A. alba to thrive under warm climates, as previously evidenced by palaeoecological data and dynamic modelling. However, in Mediterranean ecosystems, this capacity is modulated by local growing conditions, highlighting the need for site-oriented management strategies.

Wann, wie oft, warum und wie intensiv Wälder in einem Gebiet brennen, hängt von vielen Faktoren ab. Kurzfristig wird die Waldbrandgefahr vor allem durch extreme Trockenheit beeinflusst, die oft mit hohen Temperaturen oder starkem Wind einhergeht und brennbares Material leicht entzündlich macht. Langfristig sind neben dem Klimawandel die Landnutzung und der Bevölkerungsdruck entscheidend sowie die allgemeine Einstellung der Menschen zur Umwelt. Die in den letzten Jahren häufiger auftretenden längeren Trockenperioden und die Zunahme der Waldfläche durch die Nutzungsaufgabe landwirtschaftlicher Grenzertragsflächen im Alpenraum weisen auf eine mögliche Verschärfung des Waldbrandphänomens hin. Ziel eines modernen Waldbrandmanagements ist es, das Waldbrandgeschehen so zu beeinflussen, dass es zu keinen gravierenden Auswirkungen auf die Umwelt und die Waldfunktionen – und damit auch auf Menschen und Infrastruktur – kommt. Dies erfordert eine enge Zusammenarbeit zwischen Behörden, Administration und Forschung. Wo steht die Schweiz?

Forest fires in a changing climate: is Switzerland prepared?
When, how often, why and how intensively forests burn in an area depends on many aspects. In the short term, forest fires danger is mainly driven by drought, which is often accompanied by high temperatures or strong winds and makes combustible material highly flammable. In the long term, in addition to climate change, land use and population pressure are decisive, as are people’s general attitudes towards the environment and dealing with fire. The longer dry periods that have occurred more frequently in recent years and the increase in forest area point to a possible sharpening of the forest fire phenomenon.
Modern forest fire management should minimize serious effects on the environment and forest functions - and thus also on people and infrastructure. The forest fire strategies and concepts developed so far in the most affected cantons have proven their worth, and the regions north of the Alps can build on this experience to effectively address the challenges of the future. It will be particularly important to strengthen the necessary coordination between the various actors (forestry service, fire brigade, police, local, cantonal and national authorities, research).

• L'aumento delle temperature e dei periodi di siccità comportano un maggior rischio di incendi boschivi. L'effetto combinato di questi fattori abiotici e dei parassiti autoctoni e alloctoni comporta eventi dannosi nuovi sia per tipologia che per dimensioni.
• Spesso gli alberi dei boschi sono indeboliti e in aree isolate o localizzate deperiscono a causa di cambiamenti
del clima, incendi, grandinate, complesse fitopatologie o diffusione di specie invasive.
• Affinché il bosco possa continuare a fornire le sue prestazioni sono essenziali specie arboree più resistenti
al cambiamento climatico e agli organismi nocivi anche per la loro provenienza.

• La hausse des températures et la multiplication des périodes de sécheresse accroissent le danger d'incendie de forêt. Les effets combinés de ce type de
facteurs abiotiques et d'organismes nuisibles indigènes et exotiques causent des dégâts d'une nature et d'une ampleur inédites.
• En bien des endroits, les arbres sont affaiblis et ont subi des pertes isolées ou étendues résultant des changements climatiques, d'incendies de forêt, de la grêle, de maladies complexes et de la pression d'espèces envahissantes.
• Les essences et provenances plus résistantes aux changements climatiques et aux organismes nuisibles sont indispensables pour que la forêt puisse continuer de fournir ses prestations.

• Higher temperatures and more frequent droughts are increasing the risk of forest fires. The combined effects of such abiotic factors and of native and non-native harmful organisms are generating new types and dimensions of damaging events.
• Forest trees are weakened in many places, and single trees – or in some cases large areas of forest – have died as a result of climate change, forest fires, hail, complex tree diseases and the pressure of invasive species.
• Tree species and provenances that are more resistant
to climate change and harmful organisms are essential to ensure that forests can continue to provide their services.

• Erhöhte Temperaturen sowie die Häufung von Dürreperioden haben eine Zunahme der Waldbrandgefahr zur Folge. Kombinierte Effekte von solchen abiotischen Faktoren und einheimischen und gebietsfremden Schadorganismen führen zu in Art und Dimension neuartigen Schadereignissen.
• Die Waldbäume sind vielerorts geschwächt und infolge von Klimawandel, Waldbrand, Hagel, komplexen Baumkrankheiten und dem Druck invasiver Arten vereinzelt oder lokal flächig abgestorben.
• Baumarten und Herkünfte, die gegen Klimawandel und Schadorganismen resistenter sind, sind unerlässlich, damit der Wald weiterhin seine Leistungen erbringen kann.

Das Monitoring-Netzwerk TreeNet untersucht mit automatischen Sensoren an Baumstämmen (Punktdendrometern) den Wasserhaushalt und das Wachstum von Waldbäumen an rund 70 Standorten in der Schweiz (inkl. 13 LWF-Standorten). Alle zehn Minuten werden an über 500 Bäumen die Stammradien in Mikrometergenauigkeit gemessen sowie Daten in Atmosphäre und Boden gesammelt. Hier zeigen wir charakteristische physiologische Durchschnittswerte von neun Baumarten basierend auf Einzelbaummessungen (1998–2023). Unterschiede in den Art- (Wachstumsraten, Baumwasserdefizit) und Standorteigenschaften (Niederschlag und Temperatur während der Wachstumsphase) bestimmen das Gedeihen der Bäume. Das durchschnittliche Jahreswachstum wurde hauptsächlich durch die Anzahl der Wachstumsstunden pro Jahr und die mittlere Wachstumsrate pro Stunde erklärt. Die Douglasie und die Tanne zeigten die grössten Zuwächse. Die Fichte hatte die höchste Wachstumsrate pro Stunde, erreichte aber aufgrund der wenigen Wachstumsstunden einen nur durchschnittlichen Jahreszuwachs. Die Wasserspeicherkapazität eines Baumes – hergeleitet aus der maximalen, täglichen Schrumpfung des Stammes – und das Baumwasserdefizit sind weitere Faktoren, die das Wachstum beeinflussen. Innerhalb derselben Art deutet eine grössere Wasserspeicherkapazität auf mehr Wachstum hin. Über alle neun untersuchten Arten hinweg wachsen jedoch Arten mit einer generell kleinen Wasserspeicherkapazität am besten (Douglasie, Tanne, Buche). Wir zeigen, wo die TreeNet-Bäume am besten wachsen (Schweizer Mittelland) und wo sie unter dem grössten Trockenstress leiden (Wallis und Nordschweiz).

Forests are exposed to human-induced and natural disturbances generating opportunities for new species to establish, including non-native woody species. Understanding how disturbance characteristics interact with the site context and invasive species traits to drive invasion processes is crucial for designing sustainable forest management. Here, we use the case study of the deciduous broadleaved forest belt of Canton Ticino (Switzerland) to identify the main factors determining the presence of non-native invasive woody species in recently disturbed forests. We identified and selected 851 forest gaps caused by natural, silvicultural or other anthropogenic disturbances between 2008 and 2015. For each gap, the presence of non-native invasive woody species as well as the characteristics of the corresponding landscape, site, and the gap itself were determined. For the six most frequent non-native invasive woody species, we identified the parameters that influenced their presence by means of modelling approach. The resulting variable coefficients were then used to create maps of colonisation risk. Among the six target species (i.e., Ailanthus altissima, Buddleja davidii, Robinia pseudoacacia, Paulownia tomentosa, Prunus laurocerasus, and Trachycarpus fortunei), the factors influencing their presence were either shared (e.g., distance to the next colonisation hotspot), partially shared (e.g., distance to buildings, altitude, aspect), or completely species-dependent (e.g., topography, slope, distance to powerlines). Like in other mountainous areas, non-native invasive woody species concentrate at low altitudes and near densely populated areas where hotspots of seed-bearing individuals can be found. Moreover, different life-history groups (i.e., wind-dispersed pioneer species vs. animals-dispersed evergreens) shared specific invasion patterns characteristics. In addition to general management strategies, we argue that more species-specific strategies should be adopted to prevent the colonisation by non-native invasive woody species. In this context, the consideration of seed-bearing individual hotspots should be given highest priority, followed by the type and the characteristics of the disturbance.