Urban ecology: Revisiting weeds?

Urban landscapes seem very different from natural, 'pristine' landscapes, but many similar principles apply to understanding both of them. Those of us who are city-dwellers from birth, like most Singaporeans, have a very skewed vision of what urban vegetation 'should' be like, conditioned by heavily manicured and landscapes parks and plantings. We tend to look down on weeds and wildflowers as plant pests and ugly eyesores to be managed rather than celebrated. But weeds, or to use a more neutral term, 'spontaneous vegetation', have an important function in the urban environment, as Peter del Tredici argues.

Del Tredici is a scientist at the Arnold Arboretum and lecturer at the Harvard Graduate School of Design, and has just written a book titled Wild Urban Plants of the Northeast: A Field Guide, where he explains his philosophy of urban ecology in detail, along with descriptions of common species in the American Northeast. I attended a talk he gave in conjunction with a booksigning event, and was struck by how many of the things he observed about urban vegetation also applied to what we find in Singapore:

• Plants that thrive in urban open lots tend to be native to harsh environments. In the US, the 'Tree of Heaven' (Ailanthus altissima) is a cliff-dwelling species from East Asia and common in cities like Boston; in Singapore, Adinandra dumosa, which is common on wasteland, is actually native to limestone hills on the Peninsula.
  
• Bird-dispersed species with small seeds spread quickly and aggressively. Most of the successful species in urban landscapes are early successional species in 'nature'.
  
• The growth of spontaneous vegetation is inversely proportional to how wealthy the neighborhood is. In shrinking cities with declining economies, like Detroit, abandoned lots are being left to be grown over by wild grass and shrubs, and some companies even run pheasant hunts in the city because those birds are starting to colonize these spaces.
  
• Wayside trees planted for ornament and shade can escape and invade native habitats, displacing native species. Norway Maple is a big problem in Massachusetts, where it's illegal to plant this species, just like Albizzia might be in Singapore.
  

Ultimately what he argues for is a change of perspective - we can see these as ugly weeds, or we can incorporate them into our appreciation of the urban landscape, using and manipulating them to suit our needs. Even in their unmanaged form, urban wild plants provide many important services, not least among them shade and temperature regulation, soil consolidation, and even phytoremediation. With the economic downturn, less money is available for urban landscaping and planting, but if we can make use of these erstwhile invasive species, by weeding out the uglier and less cooperative species, making paths and borders, we could produce wild urban woodlands that enhance the urban experience, instead of detracting from it.

Looking at the plant list in his book, it is also striking how many of the species listed are actually common to both places. At the booksigning, he wrote in my copy that I "won't find any of these in Singapore," but at the genus or family level, there are quite a number of familiar names: Composites, Legumes, and Crucifers for sure, but also Euphorbiaceae, Scrophulariaceae, and Vitaceae. Familiar genera abounded especially among the grasses: Eragrostis, Digitaria, Poa. I'm sure someone has had similar thoughts before, but it would be instructive to look at the comparative community phylogenetics of weeds around the world - are certain groups of plants particularly predisposed to be successful in the urban landscape? If so we should start getting familiar with them, because these are the species that will be most familiar to most of the human population in time to come, if not already.