Stressed street trees: Lindens, oaks, or maples facing premature death in the future

16.07.2018

Rambling through Hamburg and its environs, you will discover a landscape sculpted by the ice ages. Glaciers of the past pushed large amounts of scree shaping today’s hilly countryside northeast of the City. Meltwater leached the glacial Elbe valley and wind-blown sand deposits piled up into dunes such as the Boberg Dunes nature reserve.

Newly planted trees in Hamburg are expected to last no more than 50 years.
Annette Eschenbach explores what trees need to live in the city.

From glacial materials and recent sedimentation—for instance, due to the Elbe’s tidal conditions—about 30 different soil types have developed, representing almost all soils occurring in Germany. Very few people have heard their names: Regosol, for example, is a plain, carbonate-poor soil, whereas Podzol is acidic and sandy, and Gley groundwater-saturated.

Together with my team from the Center for Earth System Research and Sustainability (CEN) at Universität Hamburg I am exploring how the various soils react to climate change and how their roles in the ecosystem and human well-being are altering. The rising temperatures and longer dry spells in summer predicted for North Germany will dry out local soils with poor water storage and transmission abilities such as the sandy areas of the Lüneburg Heath. Dehydration will also make them susceptible to wind erosion. More extreme rainfall may effect waterlogging in the less permeable soils in northeast Hamburg — plus water-induced erosion.

These changes will also impact the City’s vegetation, particularly urban trees. They are already forced to withstand difficult conditions: A warmer local climate than in the countryside, higher pollution levels, and sealed, compacted soils. Currently, 60 percent of urban Hamburg are covered by settlements and areas constructed for traffic, more than one third is sealed.

My team and I collected soil samples across the city and analyzed them. In 9 out of 10 places we found substances dumped by humans, mostly mineral-poor sand that can scarcely store any water. Almost every third sample contained building rubble, waste, slag, and ashes.

Young trees can hardly grow roots in such soils. Thus, street tree seedlings are often placed into pits with a planting depth of 1.5 meters. But these pits will soon be too small; after all, a tree root system requires about as much space as its visible part, its crown. If tough conditions worsen on account of climate change, future street trees will be prone to short life spans. We expect newly planted trees to last no more than 40–50 years.

This would be tragic for Hamburg. No prospect of regrowing a well-established stock of trees; a dramatic loss, as ancient and mighty linden, oak, or maple trees help humans thrive by producing oxygen, providing shade, and cooling their surroundings through evaporation.

Hence, I am investigating how we can improve living conditions for trees in urban settings. We need, for instance, suitable growing mediums for planting holes which can store or drain water as needed. Assigning specially trained soil guards to roadside construction sites might help, too. We should all learn how to better protect and value soils; they are the bread and butter of plant life and animals aplenty—an all-too-exhaustible resource.

This content was first published as a guest article in the newspaper Hamburger Abendblatt in July 2018.

Annette Eschenbach is a professor of soil protection and soil technology and a member of Universität Hamburg’s Center for Earth System Research and Sustainability (CEN).

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