Ulrich Wasem

Qua o là sta amo ün tal eroe plain fideltà sün seis post, cun büst sfrattamà, e stenda il bratsch our da spelm e grava, sco üna sgür pronta pel cumbat, o ch'el sta ingramüschà sco ün vegliet cha la burasca da la vita ha fat sgobar sün sias crotschas.

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.

Nach einer Störung von Wäldern in rutschanfälligen Hängen stellen sich oft die Fragen: «Wie schnell erholt sich die Schutzwirkung?» und «Wie soll die Gefahr am besten vermindert werden?» Anhand zweier Fallstudien in subalpinen und hochmontanen Fichtenbeständen wurde die Dynamik der Wurzelverstärkung quantifiziert und die Kosten-Effekte von Aufforstungen und Naturverjüngung diskutiert.

Die Sommertrockenheit 2018 führte in verschiedenen Regionen der Schweiz zum vorzeitigen Laubfall bei Buchen (Fagus sylvatica L.). Es stellte sich die Frage, ob dieser Laubfall als Resistenzreaktion aufgefasst werden soll, die den Buchen einen physiologischen Vorteil verschafft, oder als ausgeprägtes Schwächesymptom. Zur Beantwortung dieser Frage führten wir im Spätsommer 2018 Zustandserhebungen bei 781 ausgewachsenen Buchen mit frühem und 138 mit normalem Laubfall (insgesamt 919 Bäume) in den Regionen Aargau/Zürich (AG/ZH), Baselland (BL) und Schaffhausen (SH) durch. Erneute Begutachtungen und Probenahmen verschiedener Parameter im Frühling und Sommer 2019 zeigten, dass von den Buchen mit frühem Laubfall regional zwischen 1.4% und 4.7% abstarben. Die mittlere Kronenmortalität im Sommer 2019 betrug für Bäume mit frühem Laubfall in BL 35.6%, in SH 30.0% und in AG/ZH 17.4%. Bei Bäumen mit normalem Laubfall in der Nachbarschaft lagen die Werte nur bei 13.0% (BL) und 5.3% (AG/ZH). Die mittleren Mortalitätsraten stiegen 2019 vom Frühling bis zum Sommer an. Bäume mit frühem Laubfall trugen generell mehr Früchte (Mast-Effekt). In BL traten bei Bäumen mit frühem Laubfall signifikant mehr Rindenschäden und Nekrosen auf als bei Bäumen mit normalem Laubfall. Bäume mit frühem Laubfall wurden dort häufiger von Buchenwollschildläusen befallen, und deren Blätter im Kronendach wurden stärker durch Insekten und Pilze beeinträchtigt. In AG/ZH wiesen die Bäume mit frühem Laubfall deutlich häufiger Schleimfluss auf, und sie standen öfter am Waldrand. Aufgrund unserer Resultate stellt der verfrühte Laubfall bei Buchen, wie er sich während der ausgeprägten Sommertrockenheit 2018 ereignete, ein Schwächesymptom dar, das durch die Beobachtung gestützt wird, dass viele Zweige und Äste im Jahr nach der Trockenheit nicht mehr ausgetrieben haben bzw. abstarben.

Le cerf élaphe (Cervus elaphus) se propage sur le Plateau suisse. Les animaux nouvellement arrivés montrent une préférence marquée pour l’écorce des ifs (Taxus baccata).

Ein Kaltlufteinbruch Ende April sowie die regional anhaltende Trockenheit und die Hitze im Spätsommer hinterliessen 2016 ihre Spuren im Wald. Im Frühling erlitt das frisch austreibende Buchenlaub vielerorts Spätfrostschäden und verfärbte sich braun. Die regional ausgeprägte Trockenheit im Sommer führte wiederum zu Blattverfärbungen am Laubholz und dürfte für das erneute, lokal auffällige Absterben von Föhrenbeständen im Wallis verantwortlich sein. [...]

In anticipation of more severe summer droughts, forestry in temperate Europe is searching for drought-resistant ecotypes of native tree species that might maintain ecosystem services in the future. We investigated how spring precipitation and soil conditions interact with summer drought and affect the establishment of conifer seedlings from different climatic origin. Emergence, establishment and subsequent performance of seedlings originating from autochthonous, Central Alpine, continental Eastern European, and Mediterranean Pinus sylvestris and Picea abies populations were studied in the dry Alpine Rhine valley, Switzerland, at three sites with differing soil water holding capacities and in 3 years with contrasting weather conditions. In addition to this natural inter-annual variation, precipitation was manipulated within sites with throughfall reduction roofs. Seedling establishment and growth were principally affected by the spring weather in the year of emergence. In years with average to positive spring water balance, seedlings grown at the site with the highest water holding capacity had 2–5 times more aboveground biomass than seedlings grown at sites with less favourable soils. Effects of seed origin were marginal and only detectable at the drier sites: contrary to our expectations, seedlings from the Central Alpine Rhone valley, where the climatic spring water deficit is large, outperformed those from the Mediterranean. Consequently, plantation of non-native populations from dryer origin will mitigate the effects of increased summer drought at driest sites only, while the inter-annual variability of spring precipitation will continue to enable temperate conifers to regenerate on a wide range of forest soils independent of seed origin.

Why is the annual fruit production in forest tree species not constant, and which factors cause massive fruit production (seed mast year)? These and other related questions were already posed more than 100 years ago when tree breeding was economically beneficial. The questions have not been fully answered, yet. Rather, the same questions are studied again today as the climate is changing and the uncertainty about the continuation of forests at their current locations is growing. A 25 year long observation series on the variation of fruit production in Switzerland revealed a mean frequency of three years for mast seeding (full and medium mast) at low elevation on the Central Plateau in European beech (Fagus sylvatica), oak (Quercus petraea, Q. robur) and silver fir (Abies alba). In contrast, mast seed years of Norway spruce (Picea abies) occurred, on average, every sixth year. In 1992 and 2011, all four species synchronously showed mast seeding. The results are discussed in the light of different theories and new research findings. From the state of the current know ledge, we derive the need for long-term and fine-scale baseline data and present the new reporting and information webpage «mast web.ch». Here, volunteers can report observations on the fruit production of main tree species following a few simple criteria (citizen science approach). With this data, distribution maps on mass fructification levels will be made available and will serve for spatio-temporal fine-scale studies on mast seeding phenomena.

After windthrow, questions arise about the appropriate silvicultural management. Answers can be derived from long-term studies on 19 storm-damaged areas caused by Vivian (1990) and Lothar (1999), which encompass cleared, cleared and planted as well as uncleared subareas. Forest succession on these areas was studied using repeated regeneration inventories. Ten resp. 20 years after the storms, the resulting young forests were 3–12 m tall and had a stem density of 500 to 31,400 per ha. Many tree species grow in the storm areas, with climax species like European beech (in the lowlands) and Norway spruce (in high-altitude forests) being most frequent. Advance regeneration has only a small share of the young stands, since seedlings were scarce in the pre-storm stands. Regeneration is slightly more dense on cleared than on uncleared storm areas. The yearly increase in seedling density ranged from 25 to 4,000 trees per ha, with low values occurring mainly if dense vegetation of tall forbs, bramble or bracken covered the ground. The increase in density has fallen since the storms, and in thickets with high stem numbers, the regeneration density has even started to decrease. Pionieer trees as well as sycamore maple and ash grow fastest, and climax species like Norway spruce and silver fir slowest. For spruce, planting results in an advance of 1.0 to 2.4 m after 20 years in high montane storm areas; moreover, gaps, which are widespread in storm areas even 10 or 20 years after the storm event, can be avoided. On areas with total damage, cluster planting should be considered, in particular in protection forests and in cases with scarce advance regeneration, missing seed trees and dense ground vegetation.

Silver fir is a highly desirable species in European mountain forests from a management and conservation point of view, but it is also highly palatable for ungulates. We planted Abies alba saplings along natural light gradients to explore under natural browsing: (1) the light conditions under which saplings grow best in the absence of browsing; (2) when and by which ungulate species which saplings are browsed; and (3) under which conditions saplings react most vigorously to browsing. The experiment was carried out in a Swiss forest using four fenced plots each containing about 30 saplings planted in 2001, which were between 17 and 100 cm tall in 2009. For half of the saplings, browsing was allowed and filmed from April 2009 to January 2010. Saplings grew better with more light (13 % rather than 10 % canopy openness). Browsing was not a single event per sapling but a continuous process during the winter months. Almost all saplings exposed to browsing suffered heavy browsing by chamois and red deer, but no roe deer browsing was observed. The remainder of the browsed shoots were longer and contained more buds under 13 % canopy openness. The browsing-induced height differences between browsed and unbrowsed trees increased over time under all light conditions, and almost no diameter growth was measured in the years after browsing. In cases with severe browsing, management aimed at letting in more light does not lead to a substantially better tolerance of browsing. We thus recommend integrating the "feeding strength" in the assessment of browsing.

Rising temperatures will lead to extended periods of summer drought, which may challenge the persistence of Scots pine and Norway spruce in dry alpine valleys where these species play an important role in the protection against natural hazards. We tested whether the natural regeneration of the two species in the Rhine valley near Chur, Switzerland, might be limited under future climatic conditions and we compared the performance of autochthonous provenances with that of seedlings originating from regions with already drier summer climate such as the Rhone valley, continental Eastern Europe or the Mediterranean basin. Seeds of Scots pine and Norway spruce were sown repeatedly in forest clearings at three south-exposed sites in the Rhine valley near Chur, Switzerland. Soil moisture was manipulated to a minor extent with throughfall reduction roofs. In both species, regeneration success was primarily driven by the weather conditions during the three months following seed sowing: the seedlings having emerged in the rainy spring of 2013 had a higher survival rate and accumulated up to five times more aboveground biomass than the seedlings emerging in the dry spring of 2011. In years with an average or even positive water balance, Scots pine seedlings were able to establish at all sites. In Norway spruce, by contrast, establishment rate exceeded 10% of viable seeds only at the site with the highest water retention capacity. In years with a positive water balance during spring, the seedlings from the Rhine and Rhone valleys outperformed those from most Mediterranean and Eastern European provenances, while no differences between provenances were found in the dry spring of 2011. We suggest that periodical regeneration of Scots pine will be likely in the Rhine valley even under future climatic conditions, whereas the establishment of Norway spruce may remain an exceptional event in dry, south-exposed clearings.

Le rajeunissement du sapin blanc atteint son optimum de croissance sous ombrage léger. C’est là aussi que sa mortalité est minimale. S’ils sont fortement abroutis, les jeunes sapins ne parviennent plus à compenser la perte de croissance en hauteur. Dans l’évaluation de l’abroutissement, les auteurs conseillent donc de prendre en compte le degré d’abroutissement des plants concernés.

Ungulate browsing is one of the many factors that affect structure and species composition of forests. Silver fir (Abies alba) is a highly desirable tree species in many European mountain regions, but at the same time one of the most preferred tree species by ungulates. Failure of natural fir regeneration is thus often attributed to browsing.
We employ natural light gradients with planted fir saplings but natural browsing to analyze: (i) under which light conditions fir saplings grow and survive best; (ii) where and which ungulate species browse saplings; and (iii) which saplings react and survive best after browsing. The experiment was carried out in a Swiss forest in two fenced plots where parts extended from closed stands into gaps and an unfenced plot under closed canopy. On the three plots 803 fir saplings had been planted in 2008. On half of the total area, browsing had been allowed in at least the winter of 2009/2010.
The more light available, the better was the growth of the saplings before and after browsing. For canopy openness >= 11%, some fir saplings had large height increments irrespective of the exact light level. Video surveillance demonstrated that chamois was the only browsing ungulate species. Mortality was positively correlated with light availability, probably due to site preparation, i.e. mainly where ground vegetation was abundant, and with mouse browsing in one plot. Mortality due to chamois or mice was 2.8-16.4% in plots unfenced for one winter and 52% in the plot unfenced for three winters. A small fraction (10%) of browsed saplings that survived used flagging of a twig to form a new leader, and fully compensated for browsing-related height loss. The other saplings showed a partial compensation in the 2nd year after browsing with equally long shoots, but they still remained shorter than unbrowsed saplings. Saplings under a very dense canopy often did not react (7%).
The strong impacts of ungulates on fir recruitment are thus more due to the strong preference of the ungulates for this tree species rather than to a low tolerance of fir to browsing. We do not recommend planting firs in forest gaps due to higher mortality rates, since: (1) competition with ground vegetation is higher in gaps, (2) they risk becoming desiccated and (3) mouse browsing is more likely. Thus we provide evidence that A. alba regeneration benefits from moderately shaded conditions.

Die Tannenverjüngung erreicht ihr Wachstumsoptimum unter leichter Beschattung, wo auch die Mortalität am niedrigsten ist. Stark verbissene Tannen können den verbissbedingten Höhenzuwachsverlust nicht kompensieren; wir empfehlen bei der Beurteilung des Verbisses auch die Verbissstärke zu berücksichtigen.