How do oak and pine cope with edge effects along railway tracks?

Germany has one of the most extensive railway networks in Europe. Due to the effects of climate change and the increasing frequency of extreme weather events, there is a growing risk to railway infrastructure posed by poor vitality of trees along rail tracks. Due to the forest edge effect, these trackside trees are exposed to greater variation in temperature and humidity, and are more strongly affected by weather extremes than their equivalents within a forest stand.

The project RailVitaliTree (Tree vitality monitoring and modelling of drought-related risks along railroads with remote sensing and dendroecology) has a multidisciplinary approach, using remote sensing, dendroecological, and hydroclimatic analyses, to study tree vitality and microclimatic conditions along the German railway network. For this, increment cores of Quercus robur and Pinus sylvestris were extracted at four sites per species, where each site consists of a subsite along the railway and a corresponding reference in the forest.

Our results show that trees along the railway had higher radial growth than reference trees in the forest. In fact, although mean series produced by pooling all trackside and all reference trees display that the growth trend of trackside and reference trees is highly synchronous (Q. robur GLK = 0.87; r = 0.83 and P. sylvestris GLK = 0.82; r = 0.60), the mean ring width and basal area increment of trackside trees were higher than that of the reference trees. So why do these trees seemingly grow better along railway tracks?

Despite more radial growth, trackside trees of either species did not show a notably stronger response to climate parameters than the reference. However, there was a greater relative decrease in ring width and basal area increment of trackside trees in both species during known drought years. In order to investigate this difference in sensitivity and growth of trackside trees during drought events we use a high-resolution, species-specific drought-stress index developed by the German Meteorological Service, identifying when plant-available soil water is below drought thresholds. Through this work, we aim for a deeper understanding of this special type of forest edge, so to better assess its possible impacts on the railway system.