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Wanted: The city tree of the future

Image"Climate change will not leave the green lungs of our cities untouched. We already know that some species will not get along with the associated weather extremes in the long run, "said Klaus Körber of the Bavarian State Institute for Viticulture and Horticulture in Veitshöchheim, Germany.

Therefore, the research organization tries to find the best urban trees for the future in a large-scale attempt. Currently on a five acre test site near the German town Würzburg 450 trees are being planted, some of them more than five meters tall. "As one of the few locations in Europe, we will have the opportunity to observe, compare and evaluate the reactions of 130 different tree species to certain weather conditions for years and in one area," explained Körber. Since the spring of 2010, there is another project in three Bavarian towns, where 480 trees of 20 species are tested under field conditions.

The climate forecasts for Germany and the neighboring countries predict milder winters with the persistent danger of frost. In summer, the precipitation will generally decrease, while heavy rainfall will increase. According to the forecasts the number of hot days is expected to rise up to 20%. "For our city trees this means more frequent droughts and higher water and heat stress," said the expert Klaus Körber. "Therefore in the future trees will grow well in Germany, which come from climatic regions of the world, where it has always
been dry and hot in summer, but cold in winter. One important area for us with many plants suitable to climate change is the Southeast European Mediterranean and the Caucasus region - for example, countries such as Italy, Greece, Turkey and the northern Iran." On very dry city locations there may also be a future trend toward smaller trees or cut shapes, because smaller treetops generally require less water.

With the predicted increase in extreme weather events the following aspects have to be considered more in the selection of trees:
• a firm fastening in the ground by a strong root system,
• the danger of wind damage and falling branches,
• the regeneration by shoots after storm damage and
• an extensive root system to prevent soil erosion.

Another important aspect: drought stressed plants are more susceptible to disease. "With the globalization of trade the infestation of plants with new diseases and pests is enormous. Previously robust, local species can be infected, too, "said Körber. According to the expert, it must be the goal, not always put on the same five or six main tree species, but to increase the diversity. "Only a broad base of suitable plant species and varieties reduces the risk that more new diseases and pests reduce the available range." With this claim there have been problems in the past by the debate on nature conservation and autochthony. Also exotic, recommended
species and varieties often weren't available in the nurseries because there was little reliable demand from the municipalities. "For the city trees of the future, it will not be about which species have been growing here before, but what types thrive right under the changed conditions and work good as the green lungs of our cities in the long run," stressed Körber. "And for the nursery the change to new species on the one hand is a challenge, but on the other hand, it's a great opportunity!"

Extremely resistant to the urban climate are trees such as Ginkgo, Gleditsia Tree-of-heaven (Ailanthus), Honey Berry (Celtis australis), Turkish Hazel (Corylus colurna) and Sophora. "But from past experience one shouldn't not put too much on such exotica," Körber stated. "First, in their homeland a number of diseases and pests exists that could follow their hosts. Second, there are significant differences between cultivars with respect to the resistance, too. Third, the growth form in some cases doesn't meet the requirements of a street tree." Generally, the proportion of a certain tree species in a city should never be too high.