Swiss Long-Term Forest Ecosystem Research Research Articles

Variation in Leaf Morphological Traits of European Beech and Norway Spruce Over Two Decades in Switzerland

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.

Extreme summer heat and drought lead to early fruit abortion in European beech

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.

Meteorological data series from Swiss long-term forest ecosystem research plots since 1997

Meteorological data series from Swiss long-term forest ecosystem research plots since 1997

Impacts des dépôts atmosphériques acides sur l'eau des sols forestiers

Impacts of atmospheric acid deposition on soil solutions in forests After a massive input of acidifying components on the environment in the middle of the 20th century, atmospheric acid deposition has decreased as a result of sulphur emission reduction. The continuous acid input might affect the chemistry of soils and drainage waters and accelerate soil acidification. In the framework of the Swiss Long-Term Forest Ecosystem Research (LWF), we examined whether acid deposition has continued to decline in the last ten years in different forest ecosystems and how the chemistry of soil water reacted to the improvement in air quality. Acid deposition decreased significantly at only three out of the nine study sites. Sulphur deposition declined at all sites, but due to the relatively low sulphur load compared to nitrogen deposition, it did not contribute to decrease acid deposition. Chemistry of soil solution remained quite constant since the beginning of the measurements about ten years ago. We did not observe any acidification of soil solution in six out of eight sites. In contrast, changes in soil solution chemistry at two sites showed a rapid acidification. At three sites, the deeper soil layer released large amount of sulphate coupled with base cations, which likely contributed to deplete the soil in nutrients. The analysis of the base saturation in 1039 soil profiles across Switzerland shows a high risk of relatively fast acidification of soil solution in almost 20% of sites.

A decade of monitoring at Swiss Long-Term Forest Ecosystem Research (LWF) sites: can we observe trends in atmospheric acid deposition and in soil solution acidity?

Trends in atmospheric acid deposition and in soil solution acidity from 1995 or later until 2007 were investigated at several forest sites throughout Switzerland to assess the effects of air pollution abatements on deposition and the response of the soil solution chemistry. Deposition of the major elements was estimated from throughfall and bulk deposition measurements at nine sites of the Swiss Long-Term Forest Ecosystem Research network (LWF) since 1995 or later. Soil solution was measured at seven plots at four soil depths since 1998 or later. Trends in the molar ratio of base cations to aluminum (BC/Al) in soil solutions and in concentrations and fluxes of inorganic N (NO(3)-N + NH(4)-N), sulfate (SO(4)-S), and base cations (BC) were used to detect changes in soil solution chemistry. Acid deposition significantly decreased at three out of the nine study sites due to a decrease in total N deposition. Total SO(4)-S deposition decreased at the nine sites, but due to the relatively low amount of SO(4)-S load compared to N deposition, it did not contribute to decrease acid deposition significantly. No trend in total BC deposition was detected. In the soil solution, no trend in concentrations and fluxes of BC, SO(4)-S, and inorganic N were found at most soil depths at five out of the seven sites. This suggests that the soil solution reacted very little to the changes in atmospheric deposition. A stronger reduction in base cations compared to aluminum was detected at two sites, which might indicate that acidification of the soil solution was proceeding faster at these sites.

Ground vegetation monitoring in Swiss forests: comparison of survey methods and implications for trend assessments

At Swiss long-term forest ecosystem research sites, ground vegetation was assessed during the period 1994-2003/2008 following two approaches: (1) visual assessment of the cover of species occurring in sixteen 1 m(2) quadrats, distributed over a 43 × 43 m area, and (2) phytosociological relevés in concentric circular plots of 30, 200, and 500 m(2). We first compared the two approaches with respect to diversity assessment. The number of species recorded in the 16 quadrats was in general higher than in the 30 m(2) plot and it represented 42% to 108% of the number of species recorded in the 500 m(2) plot. In a second step, we tested whether any temporal trends were apparent. In a few cases, a decrease or increase in Landolt's mean indicator values for light, nitrogen availability, soil pH, soil moisture, or temperature was found to be significant. However, these changes were usually restricted to one approach or one area. The only clear trend was detected in an unmanaged former coppice beech stand, for which all survey approaches indicated canopy closure. At another site, vegetation reacted to the local opening of the canopy following windthrow. In a third step, we compared the leaf area index (LAI), measured with an LAI-2000 instrument (Licor, Inc.) over each quadrat, with the indicator value of the vegetation for light (L). Within a site, there was no clear relationship between LAI and L values per quadrat. In contrast, across all sites, the relationship between LAI and L, averaged per site for all available years, was highly significant.

Preface—long-term ecosystem research: understanding the present to shape the future

This special section of Environmental Moni-toring and Assessment presents peer-reviewedcontributions from the international conference“Long-term ecosystem research: Understandingthe present to shape the future” that was heldon 7–11 September 2009 in Zurich, Switzerland atthe occasion of the 15th anniversary of the SwissLong-Term Forest Ecosystem Research LWF.Since 1994, the Swiss Long-Term Forest Ecosys-tem Research Programme has been addressingglobal change as a driving force on forest biotaand ecological processes. On this landmark date,the international community was invited to reflecton the current state of long-term (forest) ecosys-tem research and its scientific value.The selected papers compiled in the presentspecial issue highlight important insights into thelatest aspects of long-term ecosystem research.Graf Pannatier et al. (2010) demonstrate the greatvalue of the monitoring data from Swiss Level II

Estimating leaf area index in different types of mature forest stands in Switzerland: a comparison of methods

Leaf area index (LAI) was estimated at 15 sites in the Swiss Long-Term Forest Ecosystem Research Programme (LWF) in 2004–2005 using two indirect techniques: the LAI-2000 plant canopy analyzer (Licor Inc.) and digital hemispherical photography, applying several exposure settings. Hemispherical photographs of the canopy were analysed using Hemisfer, a software package that offers several new features, which were tested here: (1) automatic thresholding taking the gamma value of the picture into account; (2) implementation of several equations to solve the gap-fraction inversion model from which LAI estimates are derived; (3) correction for ground slope effects, and (4) correction for clumped canopies. In seven broadleaved stands in our sample set, LAI was also estimated semi-directly from litterfall. The various equations used to solve the gap-fraction inversion model generated significantly different estimates for the LAI-2000 measurements. In contrast, the same equations applied in Hemisfer did not produce significantly different estimates. The best relationship between the LAI-2000 and the Hemisfer estimates was obtained when the hemispherical photographs were overexposed by one to two stops compared with the exposure setting derived from the reading of a spotmeter in a canopy gap. There was no clear general relationship between the litterfall and the LAI-2000 or the hemispherical photographs estimates. This was probably due to the heterogeneity of the canopy, or to biased litterfall collection at sites on steep slopes or sites subject to strong winds. This study introduces new arguments into the comparison of the advantages and drawbacks of the LAI-2000 and hemispherical photography in terms of applicability and accuracy.

Does exceeding the critical loads for nitrogen alter nitrate leaching, the nutrient status of trees and their crown condition at Swiss Long-term Forest Ecosystem Research (LWF) sites?

Nitrogen (N) deposition exceeds the critical loads for this element in most parts of Switzerland apart from the Alps. At 17 sites (8 broadleaved stands, 8 coniferous stands, and 1 mixed stand) of the Swiss Long-term Forest Ecosystem Research network, we are investigating whether N deposition is associated with the N status of the forest ecosystems. N deposition, assessed from throughfall measurements, was related to the following indicators: (1) nitrate leaching below the rooting zone (measured on a subset of 9 sites); (2) the N nutrition of the forest stand based on foliar analyses (16 sites); and (3) crown defoliation, a non specific indicator of tree vitality (all 17 sites). Nitrate leaching ranging from about 2 to 16 kg N ha−1 a−1 was observed at sites subjected to moderate to high total N deposition (>10 kg ha−1 a−1). The C/N ratio of the soil organic layer, or, when it was not present, of the upper 5 cm of the mineral soil, together with the pool of organic carbon in the soil, played a critical role, as previous studies have also found. In addition, the humus type may need to be considered as well. For instance, little nitrate leaching (<2 kg N ha−1 a−1) was recorded at the Novaggio site, which is subjected to high total N deposition (>30 kg ha−1 a−1) but characterized by a C/N ratio of 24, large organic C stocks, and a moder humus type. Foliar N concentrations correlated with N deposition in both broadleaved and coniferous stands. In half of the coniferous stands, foliar N concentrations were in the deficiency range. Crown defoliation tended to be negatively correlated with N concentrations in the needles. In the majority of the broadleaved stands, foliar N concentrations were in the optimum nutritional range or, on one beech plot with high total N deposition (>25 kg ha−1 a−1), above the optimum values. There was no correlation between the crown defoliation of broadleaved trees and foliar concentrations.

Seasonality of the Na/Cl ratio in precipitation and implications of canopy leaching in validating chemical analyses of throughfall samples

The temporal variation of the Na/Cl ratio in bulk precipitation and throughfall samples was analysed using an extensive data set based on measurements over several years at 11 sites of the Swiss Long-Term Forest Ecosystem Research Programme (LWF). This analysis was prompted by the results of the application of the criteria recommended for validating chemical analyses of precipitation samples in the Integrated Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). One of these criteria involves verifying that the sodium to chloride (Na/Cl) ratio of the analysed sample is within a restricted range, assuming that most of the chloride originates from sea-spray, and that the contribution of marine ions in atmospheric deposition is in the same molar ratio (0.86) as in sea-water. The range of possible Na/Cl values was defined between 0.5 and 1.5 by ICP Forests in order to account for other possible sources of Na + and Cl − (natural or anthropogenic). When all sites were considered, approx. 85% of our Na/Cl values were within the proposed range, both for bulk precipitation and throughfall samples. In some cases, low Na + or Cl − concentrations close to the detection limit were responsible for the Na/Cl ratios outside the range of acceptance. Plotting the Na/Cl ratio versus time revealed a seasonal pattern, which was clearer in the throughfall than in the bulk precipitation samples. This could also account for Na/Cl values higher or lower than the defined limits. The seasonality of the Na/Cl ratio and its components (Na + and Cl − fluxes) was tested using a regression model. For throughfall, the seasonal pattern of Na/Cl could be ascribed to the seasonally driven canopy leaching of Na + and Cl −, the intensity of which depended on the tree species.

Atmospheric Deposition and Ozone Levels in Swiss Forests: Are Critical Values Exceeded?

Air pollution affects forest health through atmospheric deposition of acidic and nitrogen compounds and elevated levels of tropospheric ozone (O3). In 1985, a monitoring network was established across Europe and various research efforts have since been undertaken to define critical values. We measured atmospheric deposition of acidity and nitrogen as well as ambient levels of O3 on 12, 13, and 14 plots, respectively, in the framework of the Swiss Long-Term Forest Ecosystem Research (LWF) in the period from 1995 to 2002. We estimated the critical loads of acidity and of nitrogen, using the steady state mass balance approach, and calculated the critical O3 levels using the AOT40 approach. The deposition of acidity exceeded the critical loads on 2 plots and almost reached them on 4 plots. The median of the measured molar ratio of base nutrient cations to total dissolved aluminium (Bc/Al) in the soil solution was higher than the critical value of 1 for all depths, and also at the plots with an exceedance of the critical load of acidity. For nitrogen, critical loads were exceeded on 8 plots and deposition likely represents a long-term ecological risk on 3 to 10 plots. For O3, exceedance of critical levels was recorded on 12 plots, and led to the development of typical O3-induced visible injury on trees and shrubs, but not for all plots due to (1) the site specific composition of O3 sensitive and tolerant plant species, and (2) the influence of microclimatic site conditions on the stomatal behaviour, i.e., O3 uptake.

Atmospheric Deposition on Swiss Long-Term Forest Ecosystem Research (LWF) Plots

Atmospheric deposition of the major elements was estimated from throughfall and bulk deposition measurements on 13 plots of the Swiss Long-Term Forest Ecosystem Research (LWF) between 1995 and 2001. Independent estimates of the wet and dry deposition of nitrogen (N) and sulfur (S) on these same plots were gained from combined simplified models. The highest deposition fluxes were measured at Novaggio (Southern Switzerland), exposed to heavy air pollution originating from the Po Plain, with throughfall fluxes averaging 29 kg ha(-1) a(-1) for N and 15 kg ha(-1) a(-1) for S. Low deposition fluxes were measured on the plots above 1800 m, with throughfall fluxes lower than 4.5 kg ha(-1) a(-1) for N and lower than 3 kg ha(-1) a(-1) for S. The wet deposition of N and S derived from bulk deposition was close to the modeled wet deposition, but the dry deposition derived from throughfall was significantly lower than the modeled dry deposition for both compounds. However, both the throughfall method and the model yielded total deposition estimates of N which exceeded the critical loads calculated on the basis of long-term mass balance considerations. These estimates were within or above the range of empirical critical loads except above 1800 m.

Total deposition of nitrogen on Swiss long-term forest ecosystem research (LWF) plots: comparison of the throughfall and the inferential method

Abstract The total deposition of nitrogen was estimated at 10 plots in the Swiss Long-Term Forest Ecosystem Research Programme for 1 year (1999/2000) using two methods: (1) the throughfall method, using bulk precipitation and throughfall measurements as input data for the canopy budget model; and (2) the inferential method, based on measurements of ammonia and nitrogen dioxide with passive samplers and deposition velocities taken from literature. The inferential method was coupled with a regression model deriving the other terms of the total deposition of nitrogen (wet deposition, dry deposition of nitric acid and dry deposition of particulate nitrogen) from measurements of the bulk deposition of nitrogen. The two methods gave estimates of the total deposition that correlated significantly with each other, in the range of 5–38 kg ha −1 a −1 , but the inferential method tended to yield higher estimates than those obtained with the throughfall method (median difference +2.4 kg ha −1 a −1 ). In view of the overall agreement of the two methods, the use of passive samplers in the open, coupled with the measurement of bulk deposition, appears, under certain conditions, to be an interesting alternative to the throughfall method.

Tree mortality in an unmanaged mountain pine ( Pinus mugo var. uncinata) stand in the Swiss National Park impacted by root rot fungi

We investigated mortality patterns and their possible causes in a Swiss mountain pine forest. Although the period of observation was only 4 years, it was possible to estimate spatial relationships between tree conditions and past and future stand dynamics. The 2 ha research plot, established in 1995 as part of the Swiss long-term forest ecosystem research programme (LWF), is located in the Swiss National Park and has not been managed since 1914. Tree assessments in 1995 and 1998 gave an annual mortality rate for standing trees of 1.5% and annual fall-down rates of 0.7%, while ingrowth was 1.4% of the initial tree number in 1995. Over the past 4 years the percentage of standing dead trees with stem diameters of at least 12 cm has increased by 0.8% annually to more than 20%. The trees that died during the last 3 years were significantly smaller than the surviving trees. However, the available growing space, as indicated by Thiessen polygons, of all living trees in 1995 was not different between the two groups of trees. Mortality due to competition alone was therefore discounted. Kernel estimation showed clusters for living and dead trees, with high densities of dead trees in areas of low densities of living trees and vice versa. Nearest-neighbour distances and K-functions showed that the spatial distributions of both living and dead trees were significantly clustered. The clustering of dead trees was higher than expected under random mortality. Sixty-one trees that died in the last 2 years were analysed for root rot. More than 90% were infected by root rot fungi ( Armillaria spp . and/or Heterobasidion annosum). Two thirds of the trees were infected by H. annosum and 50% by Armillaria spp., with 18% of the trees having infections by both pathogens. Trees with H. annosum were on average larger, and the growing space of trees with H. annosum tended to be larger than for trees without H. annosum. This suggests that H. annosum was a primary cause of tree death. Future studies are planned to quantify the effects of both Armillaria and annosum root disease in this mortality process. In summary, our study suggests that pathogens play an important role in the dynamics of these natural mountain pine ecosystems.