Forests play a major role in wood production and other ecosystem services, such as carbon (C) sequestration and filling reservoirs in drinking water quality. However, it is still under discussion to what extent environmental changes, such as elevated nitrogen (N) deposition and related eutrophication, may affect such services.Our study aimed to assess long-term changes in N and C storage in Swiss forest soils along a gradient in N deposition (about 10 to 30 kg N/ha/y). At five long-term forest ecosystem research plots in Switzerland, which are part of the ICP Forests Level II network, nutrient fluxes (atmospheric deposition, litterfall, soil solution) have continuously been measured since the 1990s. Soil samples were taken from fixed depth layers in the course of soil inventories in the 1990s and 2022.The observed flux patterns indicated that the forests had reached nitrogen saturation on some sites, resulting in nitrogen leaching. At sites with a higher carbon-to-nitrogen ratio (C/N), we found comparatively lower levels of N leaching. The comparison of the two soil inventories showed that the N concentration in soils has actually increased (and the C/N ratio decreased) on the sites with high C/N ratio and high N deposition. We will discuss the observed accumulation and transformations of organic C and N in these soils and the potential impacts on selected ecosystem services.

Das Programm «Langfristige Waldökosystem-Forschung» in der Schweiz liefert detaillierte Ergebnisse zu den Auswirkungen von Luftverschmutzung und Klimaänderung auf die Waldböden und Baumernährung. Seit den 1980er-Jahren sind die Emissionen von Schwefeldioxid und Stickoxiden in Mitteleuropa signifikant zurückge- gangen. Diese Trends können auch für die Immissionen in der Schweiz bestätigt werden. Die Monitoringdaten zeigen, dass die Auswaschung von Sulfat aus dem Bodenprofil zurückgegangen ist. Auch die Stickstoffauswa- schung nahm grösstenteils ab, jedoch nicht an Standorten mit weiterhin hohen Stickstoffeinträgen, wo sogar ansteigende Trends beobachtet wurden. Daher bleibt die Stickstoffbelastung trotz der insgesamt positiven Ent- wicklungen ein Problem. Ein weiterer kritischer Befund ist die fortschreitende Bodenversauerung, die sich in ab- nehmenden pH-Werten und niedrigen Verhältnissen von basischen Kationen zu Aluminium zeigt. Die Böden puffern die sauren Depositionen durch die Auswaschung von Nährstoffen und die Freisetzung von Aluminium. Diese Veränderungen in der Bodenlösungschemie können lange anhalten und die Nährstoffverfügbarkeit für die Bäume beeinträchtigen. Dies ist einer der Faktoren, die zu einem signifikanten Rückgang wichtiger Nähr- stoffe wie Stickstoff, Phosphor und Schwefel in den Blättern geführt haben. Dies weist auf eine Verschlechte- rung der Baumernährung hin. Zusammengefasst zeigen die Ergebnisse, dass trotz des Rückgangs saurer Depo- sitionen die langfristigen Auswirkungen auf die Waldböden und die Baumernährung bestehen bleiben, was sich auf die Gesundheit und Vitalität der Schweizer Wälder auswirken kann.

Il monitoraggio e lo studio dei boschi al Sud delle Alpi ci hanno consentito di approfondire le conoscenze sugli effetti di cambiamenti climatici e inquinamento atmosferico: frequenza e intensità di eventi siccitosi sono au- mentate; la qualità dell’aria negli ultimi 30 anni è sensibilmente migliorata in seguito all’adozione di misure per la riduzione di emissioni, ma, in certe zone, deposizioni azotate e ozono destano ancora preoccupazione.

Litterfall, typically referring to needles/leaves, may stand for >50% of the total mercury (Hg) deposition in forest ecosystems. By detailed categorisation, we reveal for the first time that the contributions through lichens and fine litter, together 9.98 mu g Hg m- 2 yr -1, could be as high as that in needle litter (9.96 mu g m- 2 yr -1) to the annual total Hg deposition (44.6 mu g m- 2 yr -1) in a subalpine forest in Switzerland. Noticeably, needle litter had the highest contribution (53%) to total Hg in the autumn litterfall but lichens and fine litter together predominated in other seasons (47-59%). Such a seasonal pattern is caused by the high ability of lichens and fine litter to accumulate Hg and the high needle litterfall in autumn, which is related to a good rainfall in summer followed by a dry period in autumn. The constantly higher Hg levels in lichens and fine litter than in needle litter together with similar seasonal patterns of litterfall during 2009-2019 and rainfall during 1980-2019 suggest that our finding can be generally valid. Here, we highlight not only the considerable role of non-needle litterfall in Hg deposition but also the association with weather for seasonal Hg dynamics in different litterfall components.

The effects of tree pollen on precipitation chemistry are not fully understood and this can lead to misinterpretations of element deposition in European forests. We investigated the relationship between forest throughfall (TF) element fluxes and the Seasonal Pollen Integral (SPIn) using linear mixed-effects modelling (LME). TF was measured in 1990–2018 during the main pollen season (MPS, arbitrary two months) in 61 managed, mostly pure, even-aged Fagus, Quercus, Pinus, and Picea stands which are part of the ICP Forests Level II network. The SPIn for the dominant tree genus was observed at 56 aerobiological monitoring stations in nearby cities. The net contribution of pollen was estimated as the TF flux in the MPS minus the fluxes in the preceding and succeeding months. In stands of Fagus and Picea, two genera that do not form large amounts of flowers every year, TF fluxes of potassium (K+), ammonium-nitrogen (NH4+-N), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) showed a positive relationship with SPIn. However- for Fagus- a negative relationship was found between TF nitrate-nitrogen (NO3−-N) fluxes and SPIn. For Quercus and Pinus, two genera producing many flowers each year, SPIn displayed limited variability and no clear association with TF element fluxes. Overall, pollen contributed on average 4.1–10.6

Leaf morphological traits (LMTs) of forest trees have been observed to vary across space and species. However, long-term records of LMTs are scarce, due to a lack of measurements and systematic leaf archives. This leaves a large gap in our understanding of the temporal dynamics and drivers of LMT variations, which may help us understand tree acclimation strategies. In our study, we used long-term LMT measurements from foliar material collections of European beech ( Fagus sylvatica ) and Norway spruce ( Picea abies ), performed every second year from 1995 to 2019 on the same trees within the Swiss Long-term Forest Ecosystem Research Program LWF. The 11 study plots (6 beech, 4 spruce, and 1 mixed) are distributed along gradients of elevation (485–1,650 m a.s.l.), mean annual precipitation (935–2142 mm), and mean annual temperature (3.2–9.8°C). The investigated LMTs were (i) leaf or needle mass, (ii) leaf area or needle length, and (iii) leaf mass per area or needle mass per length. We combined this unique data set with plot variables and long-term data on potential temporal drivers of LMT variations, including meteorological and tree trait data. We used univariate linear regressions and linear mixed-effects models to identify the main spatial and temporal drivers of LMT variations, respectively. For beech LMTs, our temporal analysis revealed effects of mast year and crown defoliation, and legacy effects of vapor pressure deficit and temperature in summer and autumn of the preceding year, but no clear long-term trend was observed. In contrast, spruce LMTs were mainly driven by current-year spring conditions, and only needle mass per length showed a decreasing long-term trend over the study period. In temporal models, we observed that LMTs of both species were influenced by elevation and foliar nutrient concentrations, and this finding was partly confirmed by our spatial analyses. Our results demonstrate the importance of temporal analysis for determining less recognized drivers and legacy effects that influence LMTs, which are difficult to determine across space and species. The observed differences in the temporal drivers of beech and spruce LMTs suggest differences in the adaptation and acclimation potential of the two species.

Die atmosphärische Stickstoff-Deposition hat bis Mitte der 1990er-Jahre zugenommen und ist seither rückläufig. Eine genauere Betrachtung zeigt, dass der Verlauf regional unterschiedlich ist je nach orografischer Lage und Nähe zu Ammoniak-Quellen. Die Nitratauswaschung aus den Waldböden ist ein Indikator für die Sättigung des Waldbodens mit Stickstoff und zeigt das Risiko einer Versauerung des Waldbodens an. Dazu kommt, dass zu viel Nitrat im Grund- und Trinkwasser für Mensch und Tier eine gesundheitliche Bedrohung darstellt.

Resource allocation to different plant tissues is likely to be affected by high investment into fruit production during mast years. However, there is a large knowledge gap concerning species-specific differences in resource dynamics. We investigated the influence of mast years on stem growth, leaf production, and leaf carbon (C), nitrogen (N), and phosphorus (P) concentrations and contents in Fagus sylvatica , Quercus petraea , and Q. robur at continental and climate region scales using long-term data from the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) and similar datasets. We discussed the results in the light of opposing resource dynamics hypotheses: (i) resource accumulation before mast years and exhaustion after mast years ( resource storage hypothesis ), (ii) shifting resources from vegetative to generative compartments ( resource switching hypothesis ), and (iii) investing resources concurrently in both vegetative and generative compartments ( resource matching hypothesis ). Linear mixed-effects modelling (LMM) showed that both stem growth and leaf production were negatively influenced by weather conditions which simultaneously lead to high fruit production. Thus, the impact of generative on vegetative growth is intermixed with effects of environmental factors. Superposed epoch analyses and LMM showed that for mast behaviour in F. sylvatica , there are indicators supporting the resource storage and the resource switching hypotheses . Before mast years, resources were accumulated, while during mast years resources switched from vegetative to generative tissues with reduced stem and leaf growth. For the Quercus species, stem growth was reduced after mast years, which supports the resource storage hypothesis . LMM showed that leaf C concentrations did not change with increasing fruit production in neither species. Leaf N and P concentrations increased in F. sylvatica , but not in Quercus species. Leaf N and P contents decreased with increasing fruit production in all species, as did leaf C content in F. sylvatica . Overall, our findings suggest different resource dynamics strategies in F. sylvatica and Quercus species, which might lead to differences in their adaptive capacity to a changing climate.

Verfrühter Fruchtabwurf in Schweizer Buchenbeständen im Hitze- und Trockensommer 2018 Die Rotbuche (Fagus sylvatica) zeigt mit ihrer Fortpflanzungsstrategie ein klares Mastverhalten, das heisst, sie produziert nur in einzelnen Jahren sehr grosse Mengen an Blüten und Früchten (Bucheckern). Bisher wurde angenommen, dass die Buche zu den Baumarten gehört, die bei erfolgter Bestäubung in aller Regel auch Früchte produziert. Im Schweizer Hitze- und Trockensommer 2018 reagierte die Buchen an einigen Standorten aber anders: Nach einer sehr starken Blüte entwickelten sie nur wenige Früchte, oder sie warfen die unvollständig entwickelten Früchte bereits Mitte des Sommers ab. Die Analyse von Messdaten der letzten 15 bis 19 Jahre von drei Buchenuntersuchungsflächen der Langfristigen Waldökosystem-Forschung (LWF) zeigt, dass starke Hitzewellen und langanhaltende Trockenheit während der Fruchtentwicklung zu einem Abbruch der Fruchtproduktion führen können. Diese extremen Wetterverhältnisse funktionieren somit als «Umweltveto». In Jahren mit vergleichbaren Bedingungen während der Fruchtentwicklung waren die Durchschnittstemperaturen im Sommer um 1.5 °C höher, und der Niederschlag war um 45% niedriger als im langjährigen Mittel. Der angenommene Zweijahreszyklus der Buchenblüte kann somit nicht nur durch ungünstige Wetterbedingungen während der Bestäubung, sondern auch während der Fruchtentwicklung unterbrochen werden.

European beech is known to be a masting species, i.e. fruit production does not occur every year. It is thought to be a species which is flowering controlled, i.e. that after successful pollination, fruits and seeds would be produced. In the last two decades, years with high fruit production occurred every two to three years in Middle Europe, which may be indication for an inherent biennial cycle. However, successful fruit production can be hampered by disadvantageous weather conditions, such as frost events, during the pollination season.In Switzerland, after high beech pollen concentration was measured in spring of 2018, high fruit production was expected. However, during the extremely hot and dry European summer of 2018, beech produced no, or only small amounts of beechnuts in two of three long-term monitoring beech stands in Switzerland, which are part of the Swiss Long-Term Forest Ecosystem Research Programme. We observed that beechnuts were aborted in early summer already. Over the last decades, we found similar examples of mast failure and fruit abortion in years with hot and dry summer conditions. These extreme conditions can thus act as an “environmental veto”, similar to frost events during flowering. In years with fruit abortion, summer mean temperatures were 1.2°C higher, and precipitation sums were 45% lower than the long-term average. Our findings are evidence for a biennial masting cycle in European beech, which can be interrupted by extreme weather conditions such as extreme summer heat and drought or frost during flowering.

Climate change and the expected increase in frequency of dry summers are likely to affect the growth of important tree species. We investigated relationships between morphological and chemical leaf traits of European beech (Fagus sylvatica L.) and environmental factors along a water availability gradient consisting of 12 sites located throughout Switzerland. We found that leaf dry mass and leaf area tended to decrease with increasing long-term mean annual precipitation (MAP) and actual to potential transpiration (AT/PT), two correlated variables that form the water availability gradient. These results contrast with those of several other studies, and might be explained by favorable temperature and humidity conditions during leaf formation in spring at the dry sites. Although the relationship was not linear over the whole gradient, the drier sites were characterized by beech with a lower specific leaf area, along with higher foliar potassium (K) and lower foliar nitrogen (N) concentrations. These patterns likely reflect strategies developed as an adaptation to reduced water availability, but they also result from variation in the availability of nutrients in soil across our sites. In the case of N, there are indications that atmospheric deposition plays a role in foliar concentrations.

Years with high fruit production, known as mast years, are the usual reproduction strategy of European beech. Harsh weather conditions such as frost during flowering can lead to pollination failure in spring. It has been assumed that mast is controlled by flowering, and that after successful pollination, high amounts of fruits and seeds would be produced. However, the extremely hot and dry European summer of 2018 showed that despite successful pollination, beechnuts did not develop or were only abundant in a few forest stands. An in-depth analysis of three forest sites of European beech from the Swiss Long-Term Forest Ecosystem Research Programme over the last 15–19 years revealed for the first time that extreme summer heat and drought can act as an “environmental veto”, leading to early fruit abortion. Within the forest stands in years with fruit abortion, summer mean temperatures were 1.5 °C higher and precipitation sums were 45% lower than the long-term average. Extreme summer heat and drought, together with frost during flowering, are therefore disrupting events of the assumed biennial fruiting cycle in European beech.

Increased atmospheric nitrogen (N) deposition is driving nutrient imbalances, soil acidification, biodiversity losses and the long-term reduction in stability of sensitive ecosystems which previously had limited N. In this study, we analysed the concentrations of seven different N compounds in precipitation and in the air at 34 sites across Switzerland. We calculated the N deposition by precipitation (bulk deposition) and applied the inferential method to derive dry deposition (gases, aerosols) from air concentrations. We then quantified the total inorganic N deposition by adding together the bulk and dry deposition. Finally, the total inorganic N input into the sensitive ecosystems of the 34 sites was compared to the critical loads of these ecosystems. N deposition by precipitation was the main contributor to the total N load in 16 out of 34 sites, especially into open ecosystems such as alpine/subalpine grassland, mountain hay meadows, and raised bogs. Dry deposition of ammonia (NH3) was the second most important pathway, in particular for forests close to agricultural activities, due to high NH3 concentrations and the higher deposition velocity. The N deposition exceeded the lower limit of the Critical Load of Nitrogen (CLN) range at most sites, and at many sites even surpassed the upper limit of the CLN range. No, or minor, exceedances of the critical loads for N were found only at remote sites at higher elevation in the Central Alps. Annual inorganic N deposition between 2000 and 2017 revealed a significant decline in oxidised N compounds at four of five sites (-1.6-1.8% per year), but reduced compounds only decreased at two sites (-1% and -1.4% per year) and even increased at one site (+1.2% per year), despite adopted abatement strategies for agricultural practices. This emphasises that most sensitive ecosystems in Switzerland continue to be exposed to excessive N loads through atmospheric deposition, with detrimental consequences for the biodiversity and stability of these ecosystems.

Atmospheric nitrogen (N) deposition in terrestrial ecosystems is difficult to quantify, especially in forests. In this study, we compared three approaches for determining the wet and dry deposition of nitrogen (total deposition) at 17 intensively monitored forest sites in Switzerland. Specifically, we considered approaches based on: 1) measurements of bulk deposition and throughfall in 2014 (throughfall method); 2) measurements of bulk deposition and measurements of air concentrations of ammonia (NH3) and nitrogen dioxide (NO2), to which deposition velocities were applied, also in 2014 (inferential method); and 3) a model developed for Switzerland at a high spatial resolution, run for the five-year period 2013-2017 (emission based model). In addition, changes over two decades were assessed using continuous measurements of throughfall and bulk deposition. Further, air concentrations of NH3 and NO2 measured in 2014 were compared with concentrations measured in 2000 at 10 of the sites. The three approaches generally yielded comparable estimates of total deposition, with some notable differences at some sites. For both the model and the inferential method, uncertainties were related to the deposition velocities that were applied to air concentrations of N compounds, especially for NH3. The throughfall method provided a minimum estimate of the total N deposition in the forest, but the fraction of the deposited N that is directly taken up by the canopy remained difficult to quantify. Nitrogen deposition has decreased since the mid-1990s at the majority of the sites. However, deposition of the reduced forms of N seems to have stagnated at the local level. Furthermore, N deposition is still too high in comparison with the range of empirical critical loads of N (CLN). The minimum deposition estimated from throughfall exceeds the lower limit of CLN at all sites except those in the Central Alps. Deposition estimated with the model and the inferential method exceeds the lower limit of CLN at all sites and even exceeds the upper limit at several locations.

This paper describes meteorological measurements collected since 1997 at 16 ICP Forests Level II sites across the complex topography of Switzerland, both under the canopy and in the open-field nearby.The data offer detailed comparisons of deciduous, mixed, and coniferous forest microclimatic conditions with standard meteorological conditions in the open-field.Contrary to the open-field stations, those under the canopy do not fully correspond to the WMO criteria.

Key message Exceedance of critical limits in soil solution samples was more frequent in intensively monitored forest plots across Europe with critical loads for acidity and eutrophication exceeded compared to other plots from the same network. Elevated inorganic nitrogen concentrations in soil solution tended to be related to less favourable nutritional status . Context Forests have been exposed to elevated atmospheric deposition of acidifying and eutrophying sulphur and nitrogen compounds for decades. Critical loads have been identified, below which damage due to acidification and eutrophication are not expected to occur. Aims We explored the relationship between the exceedance of critical loads and inorganic nitrogen concentration, the base cation to aluminium ratio in soil solutions, as well as the nutritional status of trees. Methods We used recent data describing deposition, elemental concentrations in soil solution and foliage, as well as the level of damage to foliage recorded at forest plots of the ICP Forests intensive monitoring network across Europe. Results Critical loads for inorganic nitrogen deposition were exceeded on about a third to half of the forest plots. Elevated inorganic nitrogen concentrations in soil solution occurred more frequently among these plots. Indications of nutrient imbalances, such as low magnesium concentration in foliage or discolouration of needles and leaves, were seldom but appeared more frequently on plots where the critical limits for soil solution were exceeded. Conclusion The findings support the hypothesis that elevated nitrogen and sulphur deposition can lead to imbalances in tree nutrition.

Forest growth is affected by various concurrent and counteracting climate change related factors and the overall impact of environmental changes on forest growth is still uncertain. In order to evaluate the variability of tree and forest growth and the possible impact of climate change, we analysed 15 years (1995-2010) of tree growth data from 18 Level II plots in Switzerland, spanning a wide range of altitude, temperature and precipitation conditions.Stem diameter of all trees within the Level II plots was measured every 5 years. Other above- and belowground parts of the trees were modelled by allometric relationships and validated with measurements if available. We analysed individual tree growth (basal area increment, bai) and the whole forest net primary productivity (NPP), here the sum of carbon gain by tree growth within the plot, during three inventory periods. Additionally, annual stem diameter increment, assessed on single trees at each plot since 2001, was used to approximate annual NPP of the forest.Temporal patterns of NPP could not easily be related to climate conditions, since forest management and disturbance events (e.g. storms, diseases) overshadowed the climate impact on NPP. However, when looking at the individual trees, a clear decrease of bai by 2-30% (mean 19.3%) was observed for most of the sites during the second inventory period (2000-2005), which could not solely be explained by increasing stand density overtime. Tree growth was most likely reduced due to the dry conditions during this period that included the extreme year 2003. An increased tree growth during the third inventory period (2006-2010) at a few sites could be clearly related to growth enhancement after stand density reducing events, such as storms or thinning. Thus, understanding climate impact on forest growth requires detailed site history knowledge and available long-term data sets.The variability of mean NPP levels could be described with a function of the sites' climate conditions and nutrient deposition (adj. r(2) = 0.86), with N deposition enhancing forest productivity up to a threshold of 20-25 kg ha(-1) yr(-1) of N and with no further growth increase beyond that threshold. (C) 2013 Elsevier B.V. All rights reserved.