Dr. Barbara Moser

Climate change is expected to significantly alter forest ecosystems, reducing the suitability of the key economic tree species Norway spruce (Picea abies) and European beech (Fagus sylvatica) in low- and mid-elevation forests of Central Europe. As these species face increasing pressures from drought, storms, and pests, it is crucial to identify alternative tree species that are economically viable and capable of maintaining primary ecosystem services. This study investigated the potential of Douglas fir (Pseudotsuga menziesii), a non-native conifer, to establish from seed and compete with native broadleaf and conifer species during the early regeneration stage under differing resource availabilities. We assessed the growth performance and phenotypic plasticity of Douglas fir seedlings over three years in a controlled common-garden experiment. Seedlings of Douglas fir, along with seven native species — Norway spruce, silver fir (Abies alba), Scots pine (Pinus sylvestris), European beech, pedunculate oak (Quercus robur), sessile oak (Q. petraea), and sycamore (Acer pseudoplatanus) — were grown for three years under factorial combinations of high and low availabilities of light, nutrients, and water. Seedling height, biomass allocation to shoots and roots and phenotypic plasticity of these traits were measured to evaluate the competitive ability of individual species and their potential to adapt to changing environmental conditions. While Douglas fir seedlings exhibited strong growth performance compared to the conifers Norway spruce and silver fir, their biomass production and height growth was considerably lower than that of the broadleaved sycamore and beech. However, Douglas fir’s height growth rate in the third year exceeded all species except sycamore. This was particularly pronounced under dry and/or nutrient-poor conditions, indicating a potential competitive advantage under expected future climatic conditions. In agreement with field studies, our results indicate that non-native Douglas fir may sustainably establish in dry, nutrient poor European lowland forests due to its superior early growth performance under these conditions and the high phenotypic plasticity, of its root system. This holds especially in situations where the species competes with other conifers, while its ability to successfully compete with broadleaves appears to be largely restricted to nutrient-poor sites.

• 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.

La végétation du sous-bois est une composante majeure de la biodiversité des écosystèmes forestiers. Sa composition floristique est recensée depuis le milieu des années 1990 à intervalles de deux à dix ans dans le cadre du programme de recherches à long terme sur les écosystèmes forestiers LWF. Le nombre d’espèces vasculaires et le recouvrement de la strate herbacée ont diminué significativement depuis le début des inventaires, vraisemblablement en raison d’une fermeture des peuplements due à l’absence d’interventions sylvicoles. La variation temporelle des valeurs indicatrices de moyennes suggère une faible diminution de la disponibilité en nutriments (valeur N) lorsque les bryophytes sont incluses dans le calcul des moyennes par site, et une faible augmentation de la température (valeur T) lorsqu’elles ne le sont pas.

Anthropogenic nitrogen (N) input is known to alter the soil microbiome, but how N enrichment influences the abundance, alpha-diversity and community structure of N-cycling functional microbial communities in grasslands remains poorly understood. Here, we collected soils from plant communities subjected to up to 9 years of annual N-addition (10 g N m^-2 per year using urea as a N-source) and from unfertilized plots (control) in 30 grasslands worldwide spanning a large range of climatic and soil conditions. We focused on three key microbial groups responsible for two essential processes of the global N cycle: N₂ fixation (soil diazotrophs) and nitrification (AOA: ammonia-oxidizing archaea and AOB: ammonia-oxidizing bacteria). We targeted soil diazotrophs, AOA and AOB using Illumina MiSeq sequencing and measured the abundance (gene copy numbers) using quantitative PCR. N-addition shifted the structure of the diazotrophic communities, although their alpha-diversity and abundance were not affected. AOA and AOB responded differently to N-addition. The abundance and alpha-diversity of AOB increased, and their community structure shifted with N-addition. In contrast, AOA were not affected by N-addition. AOA abundance outnumbered AOB in control plots under conditions of low N availability, whereas N-addition favoured copiotrophic AOB. Overall, N-addition showed a low impact on soil diazotrophs and AOA while effects for AOB communities were considerable. These results reveal that long-term N-addition has important ecological implications for key microbial groups involved in two critical soil N-cycling processes. Increased AOB abundance and community shifts following N-addition may change soil N-cycling, as larger population sizes may promote higher rates of ammonia oxidation and subsequently increase N loss via gaseous and soil N-leaching. These findings bring us a step closer to predicting the responses and feedbacks of microbial-mediated N-cycling processes to long-term anthropogenic N-addition in grasslands.

Sur les surfaces d’incendies des forêts valaisannes de Loèche (2003; exposition sud, substrat calcaire) et de Viège (2011; exposition nord, substrat majoritairement composé de Flysch), des relevés ont été effectués entre 850 et 1530 mètres d’altitude sur le rajeunissement forestier deux, dix/onze et dix-neuf ans après. Les densités des tiges (≥25 cm) étaient à Viège (en moyenne 11 380 tiges par hectare), dix-sept fois plus élevées qu’à Loèche (en moyenne 667 tiges par hectare). Dans les deux régions, les essences pionnières étaient les plus nombreuses. La densité des conifères était significativement plus élevée à Viège qu’à Loèche, tandis que les feuillus semblaient se régénérer plus fréquemment à Loèche.

Während Störungsfrequenz und -intensität die Struktur eines Bestandes und damit seine Eignung für verschiedene Baumarten prägen, beeinflusst die vorherrschende Baumartenzusammensetzung die Empfindlichkeit des Bestandes gegenüber Störungen. Wir haben folgende Aspekte dieser Huhn-oder-Ei-Frage analysiert: (1) Die Interaktion zwischen der Naturnähe eines Bestandes und seiner Störungsanfälligkeit, (2) den kurz- und langfristigen Einfluss unterschiedlicher Störungsereignisse und -intensitäten auf die Baumartenzusammensetzung, und (3) die Betrachtung von Störungen als Risiko für prioritäre Waldfunktionen sowie als Chance für eine nachhaltige Waldbewirtschaftung und Anpassung an den Klimawandel.

Tree species composition is linked to the local disturbance regime, shaping stand structure and suitability. This study analyzes the interaction between stand naturalness and vulnerability to disturbances, short- and long-term effects of various disturbances on tree species composition, and considers disturbances as risks for priority forest functions and opportunities for sustainable forest management and climate change adaptation. Frequent disturbances lead to younger, less dense stands with more pioneer species and fewer mature trees. Repeated disturbances affect tree species composition on the long term, increasing resilience to events like wildfires, droughts, or avalanches. Similarly, tree species’ characteristics determine vulnerability to disturbances, such as pest infestations, fire probability, intensity, and drought damage, which all are directly related to stand composition and local conditions. The article examines the impact of tree species composition on stand vulnerability to disturbances and the influence of key disturbance events on forest structure.

Werden Störungen in Wäldern auf der Alpennordseite in den nächsten Jahrzehnten zunehmen? Auf der Suche nach Antworten liefern quantitative Angaben über Waldschäden wie Sturmholz, Käferholz, Bruchholz und Waldbrandflächen vom Beginn des 20. Jahrhunderts bis heute konkrete Anhaltspunkte. Trotz Unvollständigkeit der Datenreihen zeigen die meisten Trends eine starke Zunahme der Waldschäden infolge von Störungen seit den 1980er-Jahren an. Gründe dafür sind der verschärfte Klimawandel, steigender Holzvorrat, invasive Schadorganismen und verschiedene Interaktionen. Wir gehen davon aus, dass diese Trends sich in gleicher Richtung fortsetzen werden.

Will disturbances in forests on the north of the Alps increase in the coming decades? In the search for answers, quantitative data provide concrete clues on forest damage such as storm wood, beetle wood, broken wood and forest fire areas from the beginning of the 20th century. Despite the incompleteness of the data series, most trends indicate a strong increase in forest damage caused by disturbances since the 1980s. The reasons for this increase are exacerbated by climate change, increasing stock of wood, invasive pests, and various interactions. We expect these trends to continue in the same direction.

Douglas fir (Pseudotsuga menziesii) has regenerated naturally in European forests since the middle of the 20^th century. Some cases of an invasive character of the species have been reported under specific site conditions, but systematic data on the extent of natural regeneration and spread of Douglas fir across different forest communities are largely lacking. Due to its potential tolerance to increasing summer droughts, Douglas fir has been suggested as a sustainable future tree species for Central European forests. In this study, we investigated natural regeneration of Douglas fir in comparison to native tree species in 39 forest stands in Switzerland belonging to different forest communities. We analyzed the regeneration success of Douglas fir, Norway spruce (Picea abies), silver fir (Abies alba), and European beech (Fagus sylvatica) with respect to ecological site conditions. The proportion of Douglas fir seedlings (<130 cm height) was <5% in the majority of stands; but in four stands, Douglas fir was the most abundant species among seedlings. In most other stands, Norway spruce (occurring in 37 stands/dominant in 10 stands), silver fir (38/2 stands) and beech seedlings (35/10 stands) were more abundant than Douglas fir seedlings. Saplings (≥130 cm height but <12 cm diameter at breast height) of Douglas fir were observed in five stands with proportions between 10% and 23% and in eight stands with proportions of <10%, in particular in stands with a high proportion of Douglas fir seed trees. Beech saplings occurred in 28 stands and were most abundant in eleven stands. Saplings of silver fir (24/3 stands) and Norway spruce (19/6 stands) were less frequent. The abundance of Douglas fir seedlings correlated positively with the proximity to seed trees and light transmission of the canopy, but negatively with understory vegetation cover and litter abundance. Ungulate browsing did not significantly affect the regeneration of any tree species in the study stands. On the Central Plateau, Douglas fir was mainly planted in productive beech forest communities where it is strongly limited due to its low competitiveness compared to beech and other fast-growing deciduous tree species. On dry and less productive sites, where the canopy is not closed, Douglas fir is able to establish successfully. On such sites, a close monitoring of Douglas fir regeneration and the potential implementation of control measures is recommended.

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.

Global warming is pushing populations outside their range of physiological tolerance. According to the environmental envelope framework, the most vulnerable populations occur near the climatic edge of their species' distributions. In contrast, populations from the climatic center of the species range should be relatively buffered against climate warming. We tested this latter prediction using a combination of linear mixed effects and machine learning algorithms on an extensive, citizen-scientist generated dataset on the fruitbody productivity of the Burgundy (aka summer) truffle (Tuber aestivum Vittad.), a keystone, ectomycorrhizal tree-symbiont occurring on a wide range of temperate climates. T. aestivum's fruitbody productivity was monitored at 3-week resolution over up to 8 continuous years at 20 sites distributed in the climatic center of its European distribution in southwest Germany and Switzerland. We found that T. aestivum fruitbody production is more sensitive to summer drought than would be expected from the breadth of its species' climatic niche. The monitored populations occurring nearly 5°C colder than the edge of their species' climatic distribution. However, interannual fruitbody productivity (truffle mass year⁻¹) fell by a median loss of 22% for every 1°C increase in summer temperature over a site's 30-year mean. Among the most productive monitored populations, the temperature sensitivity was even higher, with single summer temperature anomalies of 3°C sufficient to stop fruitbody production altogether. Interannual truffle productivity was also related to the phenology of host trees, with ~22 g less truffle mass for each 1-day reduction in the length of the tree growing season. Increasing summer drought extremes are therefore likely to reduce fruiting among summer truffle populations throughout Central Europe. Our results suggest that European T. aestivum may be a mosaic of vulnerable populations, sensitive to climate-driven declines at lower thresholds than implied by its species distribution model.

1. Despite worldwide reports of high tree mortality, growing evidence indicates that many tree species are well adapted to survive repeated dry spells. The drought resilience of trees is related to their phenotypic plasticity and ecotypic differentiation. Whether these two mechanisms act at the same organisational level of a tree and involve similar plant traits is still unknown.
2. We assessed phenotypic plasticity and ecotypic differentiation across four populations of Pinus sylvestris and Pinus nigra seedlings grown for three years under a recurrent summer drought treatment or well-watered control conditions in a common garden. We measured the response to the summer drought treatment of a total of 26 traits including shoot and needle morphology, needle anatomy, and foliar macronutrients, and related the traits to the growing season water deficit (GSWD) at the location of the seed origin.
3. Foliar phenotypic plasticity in response to recurrent summer drought was surprisingly low, with the needle length and the fraction of mesophyll and phloem tissue adjusting to some extent. In comparison, shoot morphological traits were much more plastic in both species with predominant responses to the summer drought stress including shorter and less numerous apical and lateral shoots. These three traits were also correlated with GSWD at the seed origin, indicating local adaptation. In contrast, between-population variation of foliar morphological and anatomical traits, and macronutrients were mostly unrelated to the GSWD at the seed origin.
4. Consequently, phenotypic plasticity and ecotypic differentiation occurred at the same level of organisation and in the same plant traits, i.e. shoot morphology. This combination of plasticity and ecotypic differentiation allowed P. sylvestris and P. nigra seedlings to rapidly acclimate to recurrent and long-lasting dry-spells.
5. Synthesis: P. sylvestris and P. nigra seedlings showed considerable ecotypic differentiation and phenotypic plasticity of shoot morphological traits, and not foliar traits. Acclimation to recurrent severe summer drought was achieved by reducing shoot growth.

Die Douglasie verjüngt sich in Europa seit Mitte des 20. Jahrhunderts auf natürliche Weise. Würde sie einheimische Arten verdrängen, müsste sie als invasiv bezeichnet und entsprechend behandelt werden. Da empirische Daten über das effektive Ausmass der natürlichen Verjüngung und Ausbreitung der Douglasie in der Schweiz noch weitgehend fehlen, wurde die Baumartenverjüngung in 39 Douglasienaltbeständen untersucht. Aus den Daten geht hervor, dass der Anteil der Douglasie im Jungwuchs (Höhe <130 cm) in der Mehrzahl der Bestände weniger als 5% ausmachte. In dieser Höhenklasse dominierte die Douglasie in 4 der 39 Bestände mit einem Anteil von mehr als 40% . Der Douglasienjungwuchs korrelierte positiv mit der Nähe zu Samenbäumen und dem Lichtdurchlass, aber negativ mit dem Deckungsgrad der Krautvegetation, der Mächtigkeit der Streuauflage und dem mittleren Temperatur-Zeigerwert der Krautvegetation. In der Dickung (Höhe ≥ 130 cm) wurden Douglasien mit Anteilen zwischen 10% und 23% in fünf Beständen (13%) festgestellt, insbesondere dort, wo ein hoher Anteil Douglasienaltbäume vorhanden war. In fast der Hälfte aller untersuchten Bestände (19 von 39; 49%) dominierte die schattentolerante Buche die Dickung. Die Douglasie wurde im Schweizer Mittelland vorwiegend an produktiven Buchenwaldstandorten angepflanzt. An solchen Standorten ist die Nadelbaumart aufgrund ihrer geringen Konkurrenzkraft in der Jungwuchs- und Dickungsphase gegenüber Buchen und anderen Laubbäumen stark eingeschränkt und würde ohne Pflegemassnahmen verdrängt. An trockenen und mageren Standorten hingegen kann die Douglasie in Waldöffnungen anwachsen und sich stellenweise etablieren. Bei diesen Standorten empfiehlt sich, die Ausbreitung der Douglasie zu beobachten und gegebenenfalls zu kontrollieren.

In der Vergangenheit wurde die Douglasie (Pseudotsuga menziesii) in der Schweiz vorzugsweise auf mässig feuchten, produktiven Waldstandorten angebaut. Im Hinblick auf den Klimawandel dürfte sie als Zukunftsbaumart jedoch vermehrt auch auf trockeneren Standorten gepflanzt werden. Um abschätzen zu können, unter welchen Bedingungen die gebietsfremde Baumart in der Verjüngungsphase gegenüber einheimischen Baumarten konkurrenzfähig ist, wurden acht Baumarten (Douglasie, Bergahorn, Buche, Fichte, Weisstanne, Stiel- und Traubeneiche, Waldföhre) während drei Jahren im Versuchsgarten gezogen und deren Wachstum unter verschiedenen Umweltbedingungen (viel/wenig Licht, viel/wenig Wasser, viel/wenig Nährstoffe) gemessen und verglichen. Bei ausreichender Wasser- und Nährstoffverfügbarkeit wuchs die Douglasie zwar höher als die Weisstanne und ebenso hoch wie die Fichte, aber deutlich weniger hoch als die Waldföhre und die vier Laubbaumarten. Unter wasser- und/oder nährstofflimitierten Bedingungen hingegen überwuchs sie nicht nur die anderen Nadelbäume, sondern auch die Traubeneiche. Bei gleichzeitig eingeschränktem Lichtdurchlass übertraf ihr relativer Höhenzuwachs alle Baumarten ausser den Bergahorn. Diese Resultate stützen Erfahrungen aus der Praxis und zeigen, dass sich die Douglasie auf mässig trockenen Eichenstandorten natürlich etablieren und dort die einheimischen Baumarten möglicherweise konkurrenzieren kann. Auf tiefgründigen Buchen-, Fichten- und Weisstannenstandorten hingegen wird sie als mässig schattentolerante Art auch in Zukunft ohne Pflegemassnahmen nicht aufkommen.

• Microclimatic effects (light, temperature) are often neglected in phenological studies and little is known about the impact of resource availability (nutrient and water) on tree’s phenological cycles.
• Here we experimentally studied spring and autumn phenology in four temperate trees in response to changes in bud albedo (white- vs. black-painted buds), light conditions (non-shaded vs. c. 70% shaded), water availability (irrigated, control and reduced precipitation) and nutrients (low vs. high availability).
• We found that higher bud albedo or shade delayed budburst (up to +12 days), indicating that temperature is sensed locally within each bud. Leaf senescence was delayed by high nutrient availability (up to +7 days) and shade conditions (up to +39 days) in all species, except oak. Autumn phenological responses to summer droughts depended on species, with a delay for cherry (+ 7 days) and an advance for beech (-7 days).
• The strong phenological effects of bud albedo and light exposure reveal an important role of microclimatic variation on phenology. In addition to the temperature and photoperiod effects, our results suggest a tight interplay between source and sink processes in regulating the end of the seasonal vegetation cycle, which can be largely influenced by resource availability (light, water and nutrients).

In der Diskussion um die Aufrechterhaltung der Ökosystemleistungen von Wäldern im Klimawandel stehen dem Nutzen von gebietsfremden Baumarten ernsthafte Bedenken des Naturschutzes gegenüber. Die Frage, ob eine Baumart invasiv ist oder nicht, bringt die beiden Positionen auf den Punkt. Der vorliegende Artikel überprüft den Invasivitätsstatus für die Gewöhnliche Douglasie (Pseudotsuga menziesii [Mirb.] Franco), die als Zukunftsbaumart gilt. Es wird dabei von der in der Schweiz gültigen Definition ausgegangen, wonach sich ausbreitende gebietsfremde Arten invasiv sind, wenn sie die Biodiversität und Ökosystemleistungen beeinträchtigen oder Mensch und Umwelt gefährden. Angesichts fehlender Hinweise auf eine selbstständige Ausbreitung der Baumart kann die Douglasie in der Schweiz nicht als invasiv bezeichnet werden. Vergleichsstudien in Mitteleuropa belegen, dass der Anbau der Douglasie nicht nachteilig auf die Bodeneigenschaften wirkt. Im Vergleich zu Reinund Mischbeständen aus Eiche, Buche und Fichte bieten jedoch Douglasienreinbestände ungünstigere Lebensräume für Tier- und Pilzarten, was sich in geringeren Artenzahlen ausdrückt. Für Gefässpflanzen und Moose sind die Unterschiede stark von der Bestandesstruktur abhängig und daher steuerbar. Wir folgern daher, dass die Douglasie in naturnah bestockten Wäldern beigemischt werden kann, wobei mögliche Auswirkungen auf die Biodiversität kontinuierlich beobachtet werden müssen.

Long generation times have been suggested to hamper rapid genetic adaptation of organisms to changing environmental conditions. We examined if environmental memory of the parental Scots pines (Pinus sylvestris L.) drive offspring survival and growth. We used seeds from trees growing under naturally dry conditions (control), irrigated trees (irrigated from 2003 to 2016), and formerly irrigated trees ("irrigation stop"; irrigated from 2003-2013; control condition since 2014). We performed two experiments, one under controlled greenhouse conditions and one at the experimental field site. In the greenhouse, the offspring from control trees exposed regularly to drought were more tolerant to hot-drought conditions than the offspring from irrigated trees and showed lower mortality even though there was no genetic difference. However, under optimal conditions (high water supply and full sunlight), these offspring showed lower growth and were outperformed by the offspring of the irrigated trees. This different offspring growth, with the offspring of the "irrigation‐stop" trees showing intermediate responses, points to the important role of transgenerational memory for the long-term acclimation of trees. Such memory effects, however, may be overridden by climatic extremes during germination and early growth stages such as the European 2018 mega‐drought that impacted our field experiment.

Temperature manipulation experiments are an effective way for testing plant responses to future climate conditions, especially for predicting shifts in plant phenological events. While passive warming techniques are widely used to elevate temperature in low stature plant communities, active warming has been applied less frequently due to the associated resource requirements. In forest ecosystems, however, active warming is crucial to simulate projected air temperature rises of 3–5 K, especially at the warm (i.e., southern and low elevation) range edges of tree species. Moreover, the warming treatment should be applied to the complete height of the experimental plants, e.g., regenerating trees in the understory. Here, we combined open top chambers (OTCs) with active heat sources, an electric heater (OTC-EH) and warming cables (OTC-WC), and tested the effectiveness of these set-ups to maintain constant temperature differences compared to ambient temperature across 18 m² plots. This chamber size is needed to grow tree saplings in mixture in forest gaps for 3 to 10 years. With passive warming only, an average temperature increase of approx. 0.4 K as compared to ambient conditions was achieved depending on time of the day and weather conditions. In the actively warmed chambers, average warming exceeded ambient temperatures by 2.5 to 2.8 K and was less variable over time. However, active warming also reduced air humidity by about 15%. These results underline the need to complement passive warming with active warming in order to achieve constant temperature differences appropriate for climate change simulations under all weather conditions in large OTCs. Since we observed considerable horizontal and vertical temperature variation within OTCs with temperature differences of up to 16.9 K, it is essential to measure and report within-plot temperature distribution as well as temporal temperature variation. If temperature distributions within large OTCs are well characterized, they may be incorporated in the experimental design helping to identify non-linear or threshold responses to warming.