Climate change will not leave the green lungs of our cities untouched
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.
The Great Epidemic
A great post on an ecological holocaust that happened in my own back yard. The loss of this vast economically important forest resource was a blow to Appalachian communities and may have driven many into poverty. Its cause was horticulture - it is believed that the blight was introduced in nursery stock from Asia for plantings in the New York Botanical Garden.
Modern introductions can be prevented through high standards for handling of nursery stock, and early detection and rapid response. But how effective are our quarantine measures? Seen any stinkbugs lately? The presence of these introduced pests strongly suggests that our systems are not working. It is only a matter of time before another economically devastating epidemic occurs without vigilance. And are we prepared to pay for that vigilance? In light of the current economic distress, there is every reason to expect that budgets for control of invasive species will be cut to the bone. This is not a strategy that promotes resilience.Amplify’d from schaechter.asmblog.org
When you read the title—The Great Epidemic—what came to mind? The Black Death (Yersinia pestis) that in two years killed 20 million people in Europe—approximately 30-60% of the population? The 1918 flu pandemic with its tally of 50 million dead in three years? AIDS, with a death toll projected to reach 200 million by 2025? Or perhaps that 20th century epidemic that struck down over three and a half billion in North America in the space of a few decades—the American chestnut blight? These chestnut trees, Castanea dentata to be precise, were stately giants often 100 feet or more in height with crowns that spanned 100 feet. Their straight trunks provided billions of dollars worth of beautiful, rot-resistant wood, and the bountiful nuts provided far more than the traditional stuffing for Thanksgiving turkeys. Combined they had made up a quarter of the forest canopy from Maine to Mississippi.
What does the future hold for the American chestnut? Several strategies are being pursued in hopes of countering the blight. One is to employ the hypovirulence plasmid as a biological control agent. Hypovirulent strains had spread naturally throughout many parts of Europe in the wake of the blight itself, reducing mortality but not eliminating the virulent strains. Since the 1970s this process has been helped along in European chestnut orchards by inoculating virulent cankers with a hypovirulent strain. This proved to be quite successful in spreading the hypovirulent strains and reducing the severity of the blight in Europe, but similar efforts in America failed. Why? The answer is not known, but one can make various conjectures here. It could be that the European hypovirulence plasmid can't compete with other genomic elements already present in the American chestnut or its spread by anastomosis is hampered by vegetative incompatibilities between fungal strains that prevent hyphal fusion. Also, since the hypovirulent fungal strains produce fewer spores, they may lose out in the competition for new hosts.
