Biodiversity in the City

Native vs. Exotic Tree Planting

Biodiversity is the term used to describe the degree of variation of life forms within a given territory, or ecosystem (see this Wikipedia article for a comprehensive account: http://en.wikipedia.org/wiki/Biodiversity). So, the Brockley Conservation Area could, at a stretch, be described as an ecosystem. How we support local biodiversity is something that your BrocSoc Tree Wardens (and others) are trying to keep in mind. 

We already subscribe to the mantra: "Right Tree, Right Place". Trees are best planted with an eye to their location and the eventual size of the mature tree. We wouldn't for example plant an oak on a narrow kerbside; in the city they are really parkland trees, needing the space to spread and extend their wide, bushy canopy. They are also very slow growing and would not survive well in the relatively harsh environment of the kerbside. 

But an article by George Monbiot in today's Environment Guardian raises really interesting points about the pros and cons of planting native and exotic (foreign) species: 

http://www.theguardian.com/environment/georgemonbiot/2013/aug/15/trees-native-exotic-wildlife-councils-insects

It turns out that native species like oak and birch support a vastly greater number of insects (and mites, and lichens) than imported (exotic) trees. This is mostly because insects have 'co-evolved' for millennia alongside our native trees. So, our native Oak species typically hosts some 284 insect species; birch, 266. Yet the Horse Chestnut (imported from the Balkans) hosts only four. Surprisingly, the London Plane (a hybrid of two exotic species), hosts only one!

To see the complete table (extracted from various scientific papers by The Offwell Woodland & Wildlife Trust) go here: http://www.countrysideinfo.co.uk/woodland_manage/tree_value.htm

So think twice if you are planting trees in your back gardens. Is this a good tree for the location? How large will it grow? In what time? How big will the mature canopy be? How invasive are the roots? Is it a thirsty tree (like the Eucalypts)? And then, just when you thought you were done, think about favouring one of our glorious natives, like this veteran beech spotted in Devon last autumn: