A small red circle with a white exclamation mark in the train app. Travelers know that this doesn’t mean anything good: “Connection fails.” The warning has been flashing even more frequently than usual in the app and on the railway’s website since Saturday, since almost half a meter of snow fell in a short space of time in Munich. The masses of snow paralyzed rail and road traffic in southern Germany. “Trees keep falling onto tracks and overhead lines under the weight of ice and snow,” a railway spokeswoman told the DPA news agency. Clearance crews and technicians then have to come in with heavy equipment to clear roads and tracks.
If the weight of the snow presses too hard, the snow can break: branches or crowns break off, or the trunks buckle. “Even healthy trees can collapse under the snow load if it gets too high,” says Gernot Hoch from the Austrian Federal Forest Research Center (BFW). Especially when snowfalls last for a long time and bring with them a lot of frozen precipitation, the risk of snow breakage increases.
Last weekend it snowed with temperatures around freezing point. A lot of wet snow fell – and it weighed particularly heavily on trees. “The wetter the snow, the heavier it becomes,” says Hoch, who heads the Institute for Forest Protection at the BFW. Wet snow can weigh many times more than freshly fallen, dry powder snow.
“A tree breaks more easily if the crown is damaged or the root system is weakened”
In addition, there was very little wind in the south of Germany. Otherwise the wind would have easily blown loose fresh snow off the trees. If snow freezes on the trees, it stays on them for a relatively long time. “Especially when a lot of snow sticks to the trees in cold temperatures, the snow load can become so high that the trees can no longer withstand it mechanically,” says forestry scientist Hoch.
And there was something else: the autumn dragged on for a long time, the onset of winter came suddenly, and in many places deciduous trees still had leaves. As the leaves grow, the area on which snow can remain increases and with it the weight that presses on the trees and branches.
So everything is completely normal? Or have the drought and mild winters of recent years left their mark? “It is not possible to say in general terms what role climate change plays. A tree breaks more easily if the crown is damaged or the root system is weakened. But this can have various causes, such as fungal infestation,” says Hoch.
Tanja Sanders researches forest ecosystems at the Thünen Institute in Eberswalde. She also examines latewood, which gives the tree stability. It has become narrower due to the drought of recent years. “In the medium term, we assume that the trees will become a little more unstable,” says forest adaptation expert Sanders. “But we have been observing this effect more and more since 2018; for trees that are 80 to 100 years old, these five years of thinner latewood are not crucial.”
In areas with regular snowfall, trees can adapt to snow loads by changing their crown shape. “Trees that rarely have a high snow load are more vulnerable,” says Sanders.
She sees the problem elsewhere: Where there is a lot of reforestation, the trees are susceptible to snow breakage. Young trees initially shoot up and increase in size over time. They can handle the mechanical stress worse than older stock. Very old, rotten or damaged trees, on the other hand, break more quickly when pressure is applied.
According to the German Weather Service, the wintry situation will largely remain intact in the coming days. But in the southwest half it will be milder at three to seven degrees. Thaw – so relief for the trees? “When it thaws now, the water flows away and the load on the trees decreases,” says Gernot Hoch. “It would be problematic if it snowed or rained heavily again on the trees.” Then the load increases enormously.