37 Years of Forest Monitoring in Switzerland: Drought Effects on Fagus sylvatica

European beech is one of the most important deciduous tree species in natural forest ecosystems in Central Europe. Its dominance is now being questioned by the emerging drought damages due to the increased incidence of severe summer droughts. In Switzerland, Fagus sylvatica have been observed in the Intercantonal Forest Observation Program since 1984. The dataset presented here includes 179176 annual observations of beech trees on 102 plots during 37 years. The plots cover gradients in drought, nitrogen deposition, ozone, age, altitude, and soil chemistry. In dry regions of Switzerland, the dry and hot summer of 2018 caused a serious branch dieback, increased mortality in Fagus sylvatica and increased yellowing of leaves. Beech trees recovered less after 2018 than after the dry summer 2003 which had been similar in drought intensity except that the drought in 2018 started earlier in spring. Our data analyses suggest the importance of drought in subsequent years for crown transparency and mortality in beech. The drought in 2018 followed previous dry years of 2015 and 2017 which pre-weakened the trees. Our long-term data indicate that the drought from up to three previous years were significant predictors for both tree mortality and for the proportion of trees with serious (>60%) crown transparency. The delay in mortality after the weakening event suggests also the importance of weakness parasites. The staining of active vessels with safranine revealed that the cavitation caused by the low tree water potentials in 2018 persisted at least partially in 2019. Thus, the ability of the branches to conduct water was reduced and the branches dried out. Furthermore, photooxidation in light-exposed leaves has increased strongly since 2011. This phenomenon was related to low concentrations of foliar phosphorus (P) and hot temperatures before leaf harvest. The observed drought effects can be categorized as (i) hydraulic failure (branch dieback), (ii) energy starvation as a consequence of closed stomata and P deficiency (photooxidation) and (iii) infestation with weakness parasites (beech bark disease and root rots).

Sentinel-2 Analysis of Spruce Crown Transparency Levels and Their Environmental Drivers After Summer Drought in the Northern Eifel (Germany)

Droughts in recent years weaken the forest stands in Central Europe, where especially the spruce suffers from an increase in defoliation and mortality. Forest surveys monitor this trend based on sample trees at the local scale, whereas earth observation is able to provide area-wide information. With freely available cloud computing infrastructures such as Google Earth Engine, access to satellite data and high-performance computing resources has become straightforward. In this study, a simple approach for supporting the spruce monitoring by Sentinel-2 satellite data is developed. Based on forest statistics and the spruce NDVI cumulative distribution function of a reference year, a training data set is obtained to classify the satellite data of a target year. This provides insights into the changes in tree crown transparency levels. For the Northern Eifel region, Germany, the evaluation shows an increase in damaged trees from 2018 to 2020, which is in line with the forest inventory of North Rhine-Westphalia. An analysis of tree damages according to precipitation, land surface temperature, elevation, aspect, and slope provides insights into vulnerable spruce habitats of the region and enables to identify locations where the forest management may focus on a transformation from spruce monocultures to mixed forests with higher biodiversity and resilience to further changes in the climate system.

Can we predict Sphaeropsis sapinea outbreak by monitoring fungal diversity in austrian pine plantations?

Fungal diversity plays an important role in forest ecosystems stability as well in the health status of mycorrhizal forest trees. At research area in Istria, Croatia, pathogenic fungus Sphaeropsis tip blight (Sphaeropsis sapinea (Fr.) Dyko et Sutton) caused substantial damage in Austrian pine (Pinus nigra J. F. Arnold) plantations during the last decades. This research continues on previous done in Austrian pine plantations on predisposition to S. sapinea infections, where site and stress conditions were determined as key factors, but also, the ectomycorrhizal (ECM) fungi were confirmed as biological indicators of the health status in selected plantations. We analyzed fungal diversity of Austrian pine plantations to determine the dependence of species richness and its diversity index, both for total and ECM species only, in relation to S. sapinea infection rate and crown transparency of selected trees. Fungal samples were collected during 2013 at nine permanent research plots in autumn season. Their total and ECM species richness and diversity index were correlated to Sphaeropsis tip blight infection level and crown transparency. Our results did not confirm the hypothesis that Austrian pine plantations with higher total species richness and its diversity index are more resistant to S. sapinea infection nor to crown transparency. Oppositely, ectomycorrhizal species richness and its diversity index might be used as good biological indicators of S. sapinea presence and crown transparency of studied plantations.