The Resilient World

The Sustainable Cities Collective blog carries a post today about a German effort to identify the trees most likely to thrive in tomorrow's environment. Obviously, models show different conditions for different places. What trees will thrive in African cities, for example, where they often provide essential shade and shelter?

It is issues like this that are likely to be overlooked when we think about climate adaptation. Without people of vision like Klaus Körber, no one will pay attention to trees until theirs are dying, at which point it is a little too late to plan.

Philips, the electronics conglomerate, has a competition underway for its "Livable Cities" award. One of the finalists, James Kitoya, proposes creating 45 ‘Shade Stands’ across Uganda’s capital, Kampala, to provide such shelter. (Sadly, it is running last in the voting).

The streets of Bobo Dioulasso, Burkina Faso, are lined with shade trees. Photo courtesy of Guillaume and Pauline via Flickr, licensed under Creative Commons with some rights reserved (non-commercial use only, attribution required, no derivative works).

 

One thing that troubles me about the discussions of good tree species to use is that there doesn't appear to be any thought of invasiveness. For example, in the Sustainable Cities Collective article, Ailanthus is mentioned as a very "resistant" species. Is Ailanthus the tree of the future? Perhaps, but let's not make it a self-fulfilling prophecy just yet - it is highly invasive and corrosive to natural systems in North America (evidently less so in Europe). It undermines the resiliency of natural habitats by replacing a diverse system with monotypical stands (it is allelopathic). Nothing grows under it, it is almost impossible to eradicate, and it colonizes quickly (in Washington DC it is called the "ghetto palm" because vacant lots in the inner city quickly fill with the palm-like shoots of this rapidly growing tree species).

So to plant Ailanthus is in a sense to opt for the "nuclear option". Not to say it isn't the right choice - but it is important to chose deliberately and carefully and to know what you're getting, because there's no turning back.

What other features contribute to resilient cities that are being overlooked?

 

 

Amplify’d from sustainablecitiescollective.com

"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.

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. Read more at sustainablecitiescollective.com