Investigating drought effects on forest edges along railway tracks within the project RailVitaliTree

How is tree vitality affected by conditions near railway tracks? Evapotranspiration can be higher, through increased sunlight exposure and wind, higher air temperature and lower air humidity than in a closed canopy. The extent, impact and occurrence frequency of more drought-prone conditions are investigated in the project “RailVitaliTree – Tree vitality monitoring and modelling of drought-related risks along railways with remote sensing and dendroecology”. The four most common tree species in Germany, pedunculate oak (Quercus robur), European beech (Fagus sylvatica), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), are examined.

The project follows a multidisciplinary approach, aspiring to develop a nation-wide tree vitality monitor along the German railway network to support early detection of potential damage to railway infrastructure and further ensure railway safety. Tree vitality is investigated through dendroecological methods, digital orthophotos and satellite imagery analysis, hydroclimatic measurements and a forest-focused climate analysis.

Herein we focus on the hydroclimatic investigations of the project, which consist of two parts: (1) Regional climate change effects on tree vitality are analysed via the plant-available water content computed by the forest water balance model LWFBrook90 from 1961 until the present. After applying a literature-based threshold for drought indication, the findings are compared with relative tree vitality changes computed from satellite data (https://forestwatch.lup-umwelt.de/) and dendroecological time series. As a further step, the lengths of continuous periods with a drought indication and their frequency over time are initially evaluated only for oak. An increase in period length and frequency (for the time period 1961 to 2020) can be observed so far.

(2) Additionally, instrumental measurements are carried out at selected, exemplary sites along the German railway network, to investigate microclimate conditions at forest edges. At a total of three sites, mobile weather stations measure standard meteorological parameters (air temperature, humidity, precipitation, wind, etc.) as well as soil moisture and matrix potential over one or two vegetation periods. These stations are installed as pairs, one station at the edge and one as a reference within the forest stand. The collected data is used to identify differences in the local water balance and compared to selected existing meteorological products of the German Meteorological Service. Preliminary results show small measurement differences between the reference and forest edge stations. Averaged over the meteorological summer months, the air temperature is highest and the humidity is lowest at the forest edge at midday.