Risk of weather-related tree and branch fall – now and in the future

Tree and branch fall is a risk to traffic infrastructure, forestry, buildings and the energy system. Next to biological and ecological factors like tree species, tree size or soil type, meteorological factors influence tree fall and branch fall risk. While both these types of tree damage are caused by high wind speed tree fall is also connected to storm lengths and changes in root anchorage caused by soil moisture and soil frost. Branch fall is reported to increase with high temperatures.
We aim to identify meteorological factors influencing branch and tree fall, analyse potential most extreme events and estimate future changes in weather-related risks.

In a first step we developed a logistic regression model predicting tree fall risk in winter based on a dataset of tree and branch fall events provided by Germany's national railway company (Deutsche Bahn) and meteorological data derived from ERA5. Here, we used a stepwise model selection process and 10-fold cross validation. Our findings suggest that high wind speeds, a low gust factor, and prolonged duration of strong winds, especially in combination with wet conditions (high precipitation and high soil moisture) and high air density, increase tree fall risk. While severe winter wind storms cause the highest daily numbers in tree fall events, we found that a quarter of all trees fall on days when ERA5 wind speeds are below 11 m/s. 

In a second step we are currently extending this existing model for tree fall to the summer season. Furthermore, we are developing a model for branch fall. We will test if additional predictor variables based on tree species and biomass data, convective events and the drought index SPEI will improve these models. 

In the near future we will use these models to investigate potential most extreme tree fall hazards and the changes in tree and branch fall risks in future climate scenarios based on climate model output data from CMIP6.