Search

During the particularly severe hot summer drought in 2018, widespread premature leaf senescence was observed in several broadleaved tree species in Central Europe, particularly in European beech (Fagus sylvatica L.). For beech, it is yet unknown whether the drought evoked a decline towards tree mortality or whether trees can recover in the longer term.In this study, we monitored crown dieback, tree mortality and secondary drought damage symptoms in 963 initially live beech trees that exhibited either premature or normal leaf senescence in 2018 in three regions in northern Switzerland from 2018 to 2021. We related the observed damage to multiple climate‐ and stand‐related parameters.Cumulative tree mortality continuously increased up to 7.2% and 1.3% in 2021 for trees with premature and normal leaf senescence in 2018, respectively. Mean crown dieback in surviving trees peaked at 29.2% in 2020 and 8.1% in 2019 for trees with premature and normal leaf senescence, respectively. Thereafter, trees showed first signs of recovery. Crown damage was more pronounced and recovery was slower for trees that showed premature leaf senescence in 2018, for trees growing on drier sites, and for larger trees. The presence of bleeding cankers peaked at 24.6% in 2019 and 10.7% in 2020 for trees with premature and normal leaf senescence, respectively. The presence of bark beetle holes peaked at 22.8% and 14.8% in 2021 for trees with premature and normal leaf senescence, respectively. Both secondary damage symptoms occurred more frequently in trees that had higher proportions of crown dieback and/or showed premature senescence in 2018.Our findings demonstrate context‐specific differences in beech mortality and recovery reflecting the importance of regional and local climate and soil conditions. Adapting management to increase forest resilience is gaining importance, given the expected further beech decline on dry sites in northern Switzerland. [ABSTRACT FROM AUTHOR]

Background: The tick Ixodes ricinus is an important vector of tick-borne diseases including Lyme borreliosis. In continental Europe, the nymphal stage of I. ricinus often has a bimodal phenology with a large spring peak and a smaller fall peak. There is consensus about the origin of the spring nymphal peak, but there are two alternative hypotheses for the fall nymphal peak. In the direct development hypothesis, larvae quest as nymphs in the fall of the same year that they obtained their larval blood meal. In the developmental diapause hypothesis, larvae overwinter in the engorged state and quest as nymphs one year after they obtained their larval blood meal. These two hypotheses make different predictions about the time lags that separate the larval blood meal and the density of questing nymphs (DON) in the spring and fall. Methods: Inter-annual variation in seed production (masting) by deciduous trees is a time-lagged index for the density of vertebrate hosts (e.g., rodents) which provide blood meals for larval ticks. We used a long-term data set on the masting of the European beech tree and a 15-year study on the DON at 4 different elevation sites in western Switzerland to differentiate between the two alternative hypotheses for the origin of the fall nymphal peak. Results: Questing I. ricinus nymphs had a bimodal phenology at the three lower elevation sites, but a unimodal phenology at the top elevation site. At the lower elevation sites, the DON in the fall was strongly correlated with the DON in the spring of the following year. The inter-annual variation in the densities of I. ricinus nymphs in the fall and spring was best explained by a 1-year versus a 2-year time lag with the beech tree masting index. Fall nymphs had higher fat content than spring nymphs indicating that they were younger. All these observations are consistent with the direct development hypothesis for the fall peak of I. ricinus nymphs at our study site. Our study provides new insight into the complex bimodal phenology of this important disease vector. Conclusions: Public health officials in Europe should be aware that following a strong mast year, the DON will increase 1 year later in the fall and 2 years later in the spring. Studies of I. ricinus populations with a bimodal phenology should consider that the spring and fall peak in the same calendar year represent different generations of ticks. [ABSTRACT FROM AUTHOR]

Promoting tree species diversity is commonly advocated in the anticipation of predicted increases in drought frequency and severity. However, mixing effects on drought responses vary substantially with site conditions and species identity. We combined annually resolved tree-ring data and repeated forest inventory data spanning the last 90 years to examine the effect of species-specific neighbourhood competition on the drought response (resistance, recovery and resilience) of European beech (Fagus sylvatica), silver fir (Abies alba) and Norway spruce (Picea abies) for six drought events that occurred since the 1970s at three sites in Switzerland. We found predominantly weak neighbourhood competition and tree species diversity effects, with significant interspecific influences only for resistance and recovery of beech. These minor neighbourhood effects were outweighed by tree age and size effects. Although age effects depended on species identity and components of resilience, tree size consistently negatively affected all species. Our results emphasize that diversity effects may vary for each given species combination which makes broader conclusions challenging. This is because species interact through their specific set of traits and interactions vary in space and time. Adaptive management strategies are likely to be more effective when they promote more drought-tolerant species and reductions in stand density. Despite the absence of an unequivocal advantage of tree diversity on drought resilience, striving towards species-rich forests nonetheless allows for a risk spreading among multiple species and the reinforced provision of numerous ecosystem services. [ABSTRACT FROM AUTHOR]

Background: To predict the risk of tick-borne disease, it is critical to understand the ecological factors that determine the abundance of ticks. In Europe, the sheep tick (Ixodes ricinus) transmits a number of important diseases including Lyme borreliosis. The aim of this long-term study was to determine the abiotic and biotic factors driving the annual abundance of I. ricinus at a location in Switzerland where Lyme borreliosis is endemic. Methods: Over a 15-year period (2004 to 2018), we monitored the abundance of I. ricinus ticks on a monthly basis at three different elevations on Chaumont Mountain in Neuchâtel, Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production from the literature, as the abundance of Ixodes nymphs can increase dramatically two years after a masting event. We used AIC-based model selection to determine which ecological variables drive annual variation in tick density. Results: We found that elevation site, year, seed production by beech trees two years prior, and mean annual relative humidity together explained 73.2% of the variation in our annual estimates of nymph density. According to the parameter estimates of our models, (i) the annual density of nymphs almost doubled over the 15-year study period, (ii) changing the beech tree seed production index from very poor mast (1) to full mast (5) increased the abundance of nymphs by 86.2% two years later, and (iii) increasing the field-collected mean annual relative humidity from 50.0 to 75.0% decreased the abundance of nymphs by 46.4% in the same year. Climate variables collected in the field were better predictors of tick abundance than those from nearby weather stations indicating the importance of the microhabitat. Conclusions: From a public health perspective, the increase in nymph abundance is likely to have increased the risk of tick-borne disease in this region of Switzerland. Public health officials in Europe should be aware that seed production by deciduous trees is a critical driver of the abundance of I. ricinus, and hence the risk of tick-borne disease. [ABSTRACT FROM AUTHOR]

The response of forest productivity to climate extremes strongly depends on ambient environmental and site conditions. To better understand these relationships at a regional scale, we used nearly 800 observation years from 271 permanent long‐term forest monitoring plots across Switzerland, obtained between 1980 and 2017. We assimilated these data into the 3‐PG forest ecosystem model using Bayesian inference, reducing the bias of model predictions from 14% to 5% for forest stem carbon stocks and from 45% to 9% for stem carbon stock changes. We then estimated the productivity of forests dominated by Picea abies and Fagus sylvatica for the period of 1960–2018, and tested for productivity shifts in response to climate along elevational gradient and in extreme years. Simulated net primary productivity (NPP) decreased with elevation (2.86 ± 0.006 Mg C ha−1 year−1 km−1 for P. abies and 0.93 ± 0.010 Mg C ha−1 year−1 km−1 for F. sylvatica). During warm–dry extremes, simulated NPP for both species increased at higher and decreased at lower elevations, with reductions in NPP of more than 25% for up to 21% of the potential species distribution range in Switzerland. Reduced plant water availability had a stronger effect on NPP than temperature during warm‐dry extremes. Importantly, cold–dry extremes had negative impacts on regional forest NPP comparable to warm–dry extremes. Overall, our calibrated model suggests that the response of forest productivity to climate extremes is more complex than simple shift toward higher elevation. Such robust estimates of NPP are key for increasing our understanding of forests ecosystems carbon dynamics under climate extremes. [ABSTRACT FROM AUTHOR]

Drought entails important effects on tree physiology, which may result in short- to long-term radial growth decreases. While the majority of studies have focused on annual drought-related variability of growth, relatively little is known about sustained growth decreases following drought years. We apply a statistical framework to identify climatic factors that induce abrupt growth decreases and may eventually result in tree mortality. We used tree-ring data from almost 500 standing dead trees and 200 living trees in eight sites of the Swiss network of strict forest reserves, including four of the most important Central European tree species ( Abies alba, Picea abies, Fagus sylvatica and Quercus spp.). First, to assess short-term growth responses to drought under various climate and site conditions, we calculated correlations and linear mixed-effects models between ring-width indices ( RWIs) and drought based on the Standardized Precipitation Evapotranspiration Index ( SPEI). Second, to quantify drought effects on abrupt growth decreases, we applied distributed lag nonlinear models ( DLNMs), which account for both delayed effects and the nonlinear relationship between the SPEI and the occurrence of abrupt growth decreases. Positive correlations between RWIs and the SPEI indicated short-term growth responses of all species, particularly at arid sites. Results of the DLNMs revealed species-specific growth responses to drought. For Quercus spp., abrupt growth decreases were more likely to occur several years following severe drought, whereas for P. abies, A. alba, and F. sylvatica abrupt growth decreases started frequently immediately in the drought year. We conclude that the statistical framework allows for quantifying the effects of drought intensity on the probability of abrupt growth decreases, which ultimately contributes to an improved understanding of climate impacts on forest community dynamics. [ABSTRACT FROM AUTHOR]

Remote sensing enhances large-scale biodiversity monitoring by overcoming temporal and spatial limitations of ground-based measurements and allows assessment of multiple plant traits simultaneously. The total set of traits and their variation over time is specific for each individual and can reveal information about the genetic composition of forest communities. Measuring trait variation among individuals of one species continuously across space and time is a key component in monitoring genetic diversity but difficult to achieve with ground-based methods. Remote sensing approaches using imaging spectroscopy can provide high spectral, spatial, and temporal coverage to advance the monitoring of genetic diversity, if sufficient relation between spectral and genetic information can be established. We assessed reflectance spectra from individual Fagus sylvatica L. (European beech) trees acquired across eleven years from 69 flights of the Airborne Prism Experiment (APEX) above the same temperate forest in Switzerland. We derived reflectance spectra of 68 canopy trees and correlated differences in these spectra with genetic differences derived from microsatellite markers among the 68 individuals. We calculated these correlations for different points in time, wavelength regions and relative differences between wavelength regions. High correlations indicate high spectral-genetic similarities. We then tested the influence of environmental variables obtained at temporal scales from days to years on spectral-genetic similarities. We performed an uncertainty propagation of radiance measurements to provide a quality indicator for these correlations. We observed that genetically similar individuals had more similar reflectance spectra, but this varied between wavelength regions and across environmental variables. The short-wave infrared regions of the spectrum, influenced by water absorption, seemed to provide information on the population genetic structure at high temperatures, whereas the visible part of the spectrum, and the near-infrared region affected by scattering properties of tree canopies, showed more consistent patterns with genetic structure across longer time scales. Correlations of genetic similarity with reflectance spectra similarity were easier to detect when investigating relative differences between spectral bands (maximum correlation: 0.40) than reflectance data (maximum correlation: 0.33). Incorporating uncertainties of spectral measurements yielded improvements of spectral-genetic similarities of 36% and 20% for analyses based on single spectral bands, and relative differences between spectral bands, respectively. This study highlights the potential of dense multi-temporal airborne imaging spectroscopy data to detect the genetic structure of forest communities. We suggest that the observed temporal trajectories of reflectance spectra indicate physiological and possibly genetic constraints on plant responses to environmental change. • Dense multi-temporal airborne imaging spectroscopy data linked to genetic structure. • Spectral-genetic similarities depend on wavelength region and vary with environment. • Spectral-genetic similarities suggest genetically constrained trait expression. • Accounting for uncertainty improves assessment of spectral-genetic similarities. • Multi-temporal spectral data hold potential for genetic diversity monitoring. [ABSTRACT FROM AUTHOR]

• Use of generalized additive mixed models to test growth differences. • Declining beech trees grew slower for more than 50 years compared to vital ones. • Competition and species diversity not relevant for predisposition to crown dieback. • Early-warning signals and resilience indices cannot predict crown dieback. European beech (Fagus sylvatica L.) has strongly suffered from the exceptional 2018 drought and subsequent dry years that hit Central Europe. While many trees showed severe signs of crown dieback or died following the 2018 extreme drought, other co-occurring and neighboring trees showed no sign of dieback or only minor damage. The reasons why some trees were more severely impacted than others and which predisposing factors make some trees more vulnerable than others are still poorly understood. Here, we analyzed differences in long-term growth trends, neighborhood composition (competition and species diversity), early-warning signals, and growth responses to past severe droughts of co-occurring vital and severely declining beech trees at six sites in Switzerland. We aimed to connect tree vitality after 2018 with past long-term growth trajectories and investigated whether declining trees had already been more susceptible to drought than vital trees before dieback occurred. Overall, trees that showed severe crown dieback had a stronger growth decline than vital trees in the last 50 years. Declining trees exhibited stagnating and then decreasing growth trajectories even before signs of crown dieback occurred. Interestingly, we did not find significant differences in growth response to past severe droughts between the vitality classes, with the exception that vital trees recovered faster from past more severe droughts. Further, we could neither detect any difference in the effect of competition and neighborhood species composition on growth response, nor predict crown dieback based on early-warning signals which try to predict regime shifts by sudden changes in the autoregressive coefficient with lag 1, standard deviation and skewness. Our results indicate that unlike vital trees, declining beech trees showed predisposing signs for crown dieback by having lower growth rates during the last 50 years. [Display omitted] [ABSTRACT FROM AUTHOR]

The estimate of growth losses by ozone exposure of forest trees is a significant part in current C sequestration calculations and will also be important in future modeling. It is therefore important to know if the relationship between ozone flux and growth reduction of young trees, used to derive a Critical Level for ozone, is also valid for mature trees. Epidemiological analysis of stem increment data from Fagus sylvatica L. and Picea abies Karst. observed in Swiss forest plots was used to test this hypothesis. The results confirm the validity of the flux-response relationship at least for beech and therefore enable estimating forest growth losses by ozone on a country-wide scale. For Switzerland, these estimates amount to 19.5% growth reduction for deciduous forests, 6.6% for coniferous forests and 11.0% for all forested areas based on annual ozone stomatal uptake during the time period 1991-2011. [ABSTRACT FROM AUTHOR]

Photosynthetic carbon isotope discrimination of Fagus sylvatica was measured online and under field conditions using branch bags and laser spectrometers. A substantial variability was observed. Its potential drivers were investigated.On-line measurements of photosynthetic carbon isotope discrimination (13Δ) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous 13Δ is scarce, although 13Δ is, explicitly and implicitly, used from leaf to global scales for inferring photosynthetic characteristics. This work presents the first on-line field measurements of 13Δ of Fagus sylvatica branches, at hourly resolution, using three open branch bags and a laser spectrometer for CO2 isotopologue measurements (QCLAS-ISO). Data from two August/September field campaigns, in 2009 and 2010, in a temperate forest in Switzerland are shown. Diurnal variability of 13Δ was substantial, with mean diurnal amplitudes of ~9‰ and maximum diurnal amplitudes of ~20‰. The highest 13Δ were generally observed during early morning and late afternoon, and the lowest 13Δ during midday. An assessment of propagated standard deviations of 13Δ demonstrated that the observed diurnal variation of 13Δ was not a measurement artefact. Day-to-day variations of 13Δ were summarized with flux-weighted daily means of 13Δ, which ranged from 15‰ to 23‰ in 2009 and from 18‰ to 29‰ in 2010, thus displaying a considerable range of 8–11‰. Generally, 13Δ showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of 13Δ was, however, always better predicted by that of net CO2 assimilation, especially in 2010 when soil moisture was high and vapour pressure deficit was low. Stomatal control of leaf gas exchange, and consequently 13Δ, could only be identified under drier conditions in 2009. [ABSTRACT FROM AUTHOR]

Requirements for emission reporting under the Kyoto protocol demand an estimate of the dead wood carbon pool in forests. The volume of dead wood consists of coarse woody debris, smaller woody debris and dead roots. The measurement of dead wood volume was included in the most recent National Forest Inventory in Switzerland. To convert dead wood volume into carbon two conversion factors are required: (a) carbon (C) concentration and (b) wood density. So far internationally accepted default values for C concentration (50%) and for wood density (density of alive trees) were used as default values to estimate dead wood carbon, since local measurements were lacking. However, in a field study at 34 sites in Switzerland, the C concentration and density of CWD from Picea abies and Fagus sylvatica of four decay classes were measured recently. The results showed that C concentration in CWD differed significantly between species but did not change due to decay class. The density of CWD decreased significantly with an increase in decay class and it also differed between species. The decrease in CWD density was more pronounced for F. sylvatica than for P. abies. We assessed correlations between climate attributes and CWD density using regression analysis. The modeled densities and measured C concentrations were then expanded with the help of CWD volume data from the NFI3. Spruce CWD and thus spruce CWD carbon is much more abundant in Swiss forests than beech CWD carbon. The majority of spruce CWD is located in the Alps and Pre-Alps. The CWD volume from P. abies was 10 times higher than that from F. sylvatica. Thus, changes in conversion factors for P. abies CWD affected the overall estimate of dead wood carbon in Swiss forests much more than changes in conversion factors for F. sylvatica CWD. Current improvements in CWD conversion factors decreased the estimated amount of spruce CWD carbon by 23.1% and that of beech by 47.6%. The estimated amount of CWD carbon in Swiss forests is decreased by 31%. Since improved estimation methods are currently not applied to smaller woody debris and dead root material, the estimated amount of dead wood carbon is only reduced by 15%. Improving conversion factors for all dead wood fractions would presumably decrease the amount of dead wood carbon by additional 16%. [ABSTRACT FROM AUTHOR]

For a quantitative estimate of the ozone effect on vegetation reliable models for ozone uptake through the stomata are needed. Because of the analogy of ozone uptake and transpiration it is possible to utilize measurements of water loss such as sap flow for quantification of ozone uptake. This technique was applied in three beech (Fagus sylvatica) stands in Switzerland. A canopy conductance was calculated from sap flow velocity and normalized to values between 0 and 1. It represents mainly stomatal conductance as the boundary layer resistance in forests is usually small. Based on this relative conductance, stomatal functions to describe the dependence on light, temperature, vapour pressure deficit and soil moisture were derived using multivariate nonlinear regression. These functions were validated by comparison with conductance values directly estimated from sap flow. The results corroborate the current flux parameterization for beech used in the DO3SE model. [Copyright &y& Elsevier]

The article presents a study on forest developments using the inventory data derived from six beech reserves in Switzerland. It explores on the observation that the decrease in light is one factor for loss of invasive species in forests. Moreover, the findings intend to refine forest management systems and to develop conservation strategies.

Epidemiological analysis of sequential growth data may be a tool in assessing ozone sensitivity of mature trees. Annual shoot growth of mature Fagus sylvatica in 83 Swiss permanent forest observation plots and of Picea abies in 61 plots was evaluated for 11 and 8 consecutive years, respectively, using branches harvested every 4years. The data were assessed as annual deviation from average growth and related to fructification, ozone, meteorological parameters, and modelled soil water content using a mixed linear model. In beech, a significant association between ozone and shoot growth was observed which corresponded to a 7. 4% growth reduction between 0 and 10ppm h AOT40 (accumulated ozone over threshold 40). This is in the same order of magnitude as the response observed in experiments with seedlings. No interaction was found between ozone and drought parameters. In Norway spruce, shoot growth was neither associated with ozone nor with drought. [Copyright &y& Elsevier]

In a network of permanent observation plots in Switzerland, the storm `Lothar' uprooted 18.7% of Fagus sylvatica and 14.8% of Picea abies. The percentage of uprooting was significantly inversely correlated with actual soil base saturation (NH4Cl-extract). At a base saturation of ≤40% (calculated as an average over 0–40cm depth) uprooting was increased by a factor of 4.8 in beech and by a factor of 3.6 in Norway spruce compared to less acidic soils. In beech, the percentage of uprooted trees was also significantly correlated with nitrogen concentration in the leaves (positively) and with coarse pore volume in the soil (negatively). There was neither a relation with seasonal ozone dose nor a relation with crown transparency, stem diameter, crown size, slenderness, social position and position within the stand. The results suggest that anthropogenic stress factors play an important role in the extent of the `Lothar' damages in Switzerland. [ABSTRACT FROM AUTHOR]

• Douglas fir cannot compete with broadleaves in productive European beech forests. • Abundant Douglas fir regeneration was found on dry and less productive forest sites. • Monitoring of Douglas fir regeneration is recommended on dry, nutrient-poor sites. Douglas fir (Pseudotsuga menziesii) has regenerated naturally in European forests since the middle of the 20th 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. [ABSTRACT FROM AUTHOR]

Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >-0.4 MPa): no detectable hydraulic limitations; II (-0.4 to -1.3): persistent stem shrinkage begins and growth ceases; III (-1.3 to -2.1): onset of native embolism and defoliation; IV (-2.1 to -2.8): onset of crown dieback; V (<-2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)