Defying the summer heat, providing shade, producing oxygen: avenue trees have many tasks.
But prolonged drought makes life difficult for them.
Hohenkammer
- On State Road 2054 between Hohenkammer and Eglhausen, the Freising State Building Authority recently planted seven winter linden trees and 15 Norway maple trees. The trees complement the avenue along the state road, which had a few gaps. They should later defy the summer heat, provide shade and produce oxygen. So that the trees can withstand prolonged drought, the state building authority has adapted its planting and care concept. Planting 19 trees on a federal highway takes about a day. It takes around 20 to 30 years for the trees to be reasonably large, provide sufficient shade and serve as a habitat for animals - provided that long, dry and hot summers are not too hard for them.
The summer of 2020 was reasonably relaxing, at least for the trees on the street. The temperatures in southern Bavaria were below the long-term average, there was no permanent drought. And spring 2021 was also rather cool and humid. For the trees this means a short rest.
But if you look closely, you will notice: More and more trees are suffering from the increasing heat in summer and the increasingly longer dry periods. This means a lot of stress for trees in urban areas and along roads. The sycamore maple in particular suffers from heat and drought in road locations. Because when the air shimmers with heat, the effect on the streets is intensified by the reflection from the asphalt. If too little rain then falls, the shoot tips dry out first, and then the tops of the trees later. Branches die off, first smaller ones, then bigger ones too. If there are several drought years in a row, this can cause the tree to die completely. In any case, roadsides are extreme locations for the trees. The soil quality is usually poor,because either the topsoil layer on the gravel slopes is significantly less than in the open landscape, or the soil next to the road shoulder is heavily compacted. This makes root growth difficult. The root space of street trees is also often less than in natural locations and often narrowed. And finally, the trees have to struggle with salt in the ground from winter road clearance and with the spray of salt on the bark and buds. The damage that occurs to the trees multiplies when it is dry and hot for several years.And finally, the trees have to struggle with salt in the ground from winter road clearance and with the spray of salt on the bark and buds. The damage that occurs to the trees multiplies when it is dry and hot for several years.And finally, the trees have to struggle with salt in the ground from winter road clearance and with the spray of salt on the bark and buds. The damage that occurs to the trees multiplies when it is dry and hot for several years.
So what to do Before replanting, the state building authority improves the soil with special substrates that have a high water storage capacity. So it is ideal for new plantings during longer dry periods, and for some time now only more heat and heat-tolerant native tree species such as winter linden, field maple and norway maple have been planted on street locations. In addition, in the so-called completion and development maintenance, woody plantings are watered more frequently. So far, these water courses are planned for two to three years, it is conceivable to extend this to five years in the future.
At the Bavarian State Institute for Agriculture and Horticulture in Veitshöchheim, scientists are examining various, including exotic tree species as part of the Urban Green 2021 project for their suitability in the extreme city. But a red maple from eastern North America or a ginkgo will not be found along a state or federal highway outside of built-up areas for the foreseeable future. Because since 2020 it has been mandatory to use only local seeds and planting material in the open countryside. However, other types of wood or specially cultivated varieties may also be planted within a locality.
But at the Freising State Building Authority, they usually stick to the native species and use particularly urban climate-friendly varieties or varieties that form compact or columnar crowns.
Thomas Jakob
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