Felix Weisbrich scratches the compact earth under a buckthorn with blackish branches devoid of leaves. This species of tree may well be widespread in the Mediterranean areas, but this example did not resist on this piece of Berlin sidewalk, near the main boulevard of Mehringdamm, in the heart of the Kreuzberg district.
“The shaft trick is extremely hard. It can barely absorb water yet. The roots probably can’t get deep enough because underneath there are layers of WWII rubble, debris, drains,” explains the Borough’s green spaces manager. “The environment is completely concrete, with strong heat reverberation from the buildings. In addition, rainwater is evacuated by the pipe and cannot irrigate the tree. »
Rainfall deficit
Here are summaries of the evils from which many trees in the German capital suffer. For several years, Berlin has been one of the driest regions of the country. Since 2018, the rainfall deficit is estimated at 500 liters per square meter, i.e. almost a full year of rain. These repeated droughts weigh on the million trees listed in Berlin, of which nearly 430,000 are planted in the streets. Some residents take out the watering can, “but it remains a drop in the ocean. An adult tree evaporates between 150 and 300 liters of water per day.says Felix Weisbrich.
If the trees do not simply die of thirst, the fungi and parasites end up finishing off the most fragile. To protect passers-by and motorists, the authorities must have them cut off. “Of the 42,000 trees in the borough, we usually cut down 800 to 900 each year. In recent years, the trend has gone up to 1,000, 1,200, 1,300 trees. We fear we will have to cut down up to 2,000 trees this year,” regrets Felix Weisbrich, energetic forty-something.
Planting too fast
For the forest engineer, the city of Berlin, of which 30% of the surface is made up of green spaces, pays “a bad investment policy. We quickly planted trees. We didn’t have enough money to plant them deep enough. We have the backlash now with climate change”. Cramped, the trees, in the ground for decades, cannot adapt to the frequent rise in temperature.
What to do when heat waves are set to increase in Europe? For Felix Weisbrich, the answer is simple: multiply the trees in the streets. Their shade will lower ground temperatures by a few degrees. To avoid the mistakes of the past, “You have to take the problem literally at the root and give them space”, warns the Berlin official. He recommends 12 to 36 m3 of soil against the usual 1 to 3 m3. His proposal involves reviewing urban planning in favor of vegetation. It is part of the recommendations of an official report, published in June, to implement Berlin’s 2030 climate plan.
Conflict of use with the automobile
To explain his thoughts, Felix Weisbrich walks towards a sparsely leafed lime tree on the Boulevard de Mehringdamm. The tree, typical of the streets of the German capital, is wedged between a cycle path and parked cars. Victim of drought, it must soon be felled. “The square is most often found in town where cars park,” details Felix Weisbrich. He is aware of the conflict of use that is looming in a city where the automobile is omnipresent despite the large supply of public transport.
The resistances to be broken will also be of a financial nature. Better planting would cost the City of Berlin more, which currently spends €2,500 per tree. “We need green infrastructure, believes Felix Weisbrich. Highways cost billions, in the future trees in streets and cities will also have to cost billions. »