The future of urban trees & German cities asking citizens to water trees

Throughout the summer of 2015, there have been calls by various German cities and towns for residents to water city trees because city sanitation departments were unable to cope with watering demands in heat waves with sustained temperatures in the 90s (F) and peaks at over 40 C.

Such calls made sense to the blogster. Having lived in dry and hot regions of the U.S., we are no stranger to taking a garden hose to the young trees on the sidewalk as needed during long summer dry periods. Well, at least before water rates got jacked up and mandatory reduction of use was instituted.

Germany does not suffer from widespread water shortages, although local collection and storage tanks ran dry in 2015 in a few places. This does not mean, however, that rising temperatures and increasing atmospheric CO2 levels don't cause problems around here.

German scientists and media have no issue reporting what climate deniers claim is regularly suppressed: higher CO2 levels make plants grow better, trees slowly move northwards beyond the previously accepted line of tree growth in the tundras of the world.
But they are aware that these facts don't mean we are going to see a better world  - far from it. A few extra trees in the Arctic do not compensate for lower crop yields like those experienced in, for example, Germany this year, and widespread drought in some regions while rising sea levels flush humans out of homes..

Rising summer temperatures in temperate climates are also not evenly distributed, with cities warming up faster than rural areas, putting plants as well as humans under more stress.

Symptoms of environmental stress, from air pollution to dog urine and temperature rise, prompted scientists to study urban German trees decades ago in search of remedies. As native tree species began to fail, non-native replacements were investigated, leading to the introduction of the Gingko tree from Asia as early as 30 years ago in some cities. This has caused problems with female Gingko trees in an urban setting, prompting cities like Essen in the Ruhr area, to take the axe to some mature trees. When fruit of Gingko decays, butyric acid is created, and it stinks. Add slippery sidewalks caused by accumulated leaves, and you have an incentive to remove the offending plants.

This recent article in the daily Sueddeutsche Zeitung sums up current research efforts into future urban trees. The major shift the blogster found there for the first time consists of plans to move away from deciduous trees like oak, beech, aspen, and others to conifers or other evergreen species, a widespread policy used in countries like China.

Although science is happy to call the newly re-labeled climate in German cities Mediterranean, lower winter temperatures and higher precipitation prohibit the introduction of typical southern flora to replace dying urban oak and elm populations.

Conservation organizations are, of course, concerned about widespread introduction of non-native tree species and the consequences for local eco systems, but urban areas seem to have little choice if they want trees to continue to be part of their man-made landscape.

Who knows, maybe there will be a day when we leave the lemon tree and the avocado tree in the yard year round to let them fend for themselves and instead start to care of a tiny ash sapling the way we do now for the lemon tree.