Why are almost all the trees that councils plant exotic species?
The differences can be stark and remarkable: native trees tend to harbour far more wildlife than exotic species. Indigenous oak species, for example – according to the table extracted from scientific papers by the Offwell Woodland and Wildlife Trust – harbour 284 insect species in the UK. Birch supports 266. But horse chestnut, introduced from the Balkans, hosts only 4.
Scots pine is associated with 91 species, larch, from elsewhere in Europe (or Japan) just 17. Sycamore, which comes from southern Europe, carries 15 species; the London plane tree, which is a hybrid between two exotic species, supports just one.
The highly invasive Rhodedendron species (Rhododendron ponticum) which, introduced from southern Europe or the near east, has colonised many of our woods, gives life to a grand total of zero insect species. Interestingly, this plant lived on the land which later became the British Isles during the last interglacial period. Something must have eaten it, or it would have overwhelmed the rest of the flora and dominated the ecosystem. Could it have been browsed by the straight-tusked elephant, or by one of our two woodland rhino species?
The reason is straightforward: our insects have co-evolved with the trees on which they feed, acquiring adaptations which allow them to cope with the chemicals and other defences with which those trees equip themselves. They have not evolved to feed on trees they have never encountered before. Acquiring this ability can take hundreds of thousands of years or more.
Not all native species harbour a profusion of life. Yew, for example, is native, but so toxic that only four insect species can eat the leaves or twigs. (The berries, on the other hand, are eaten by many species, including the author. They are a little too sweet and glutinous for my taste, but otherwise pretty good. You must spit out the seed the berry contains, however, which is poisonous).
So there are exceptions, but you can see the general point. To help wildlife, we should plant more native trees. So why are nine out of every ten trees planted by local authorities exotic?
That’s not an exact figure: none, at the national level, seems to exist. But everywhere I go, I take a look at the trees planted along the streets and in parks and other public spaces. They are, overwhelmingly, non-native. In some towns and cities you seldom come across a native tree planted in a public place.
My local park is typical. There are scores of young trees, from all over the world. Yet not one of the recent plantings belongs to a species that came here without human agency. And the council doesn’t seem to have selected trees which thrive there either: some of them are doing very badly. I’m pleased to see that the eucalyptus – a tree hostile to life outside its native range, and which sucks the soil dry over a wide radius – have curled up and died.
I’m not suggesting that all the trees councils plant should be native ones. There are sometimes good reasons for planting exotics. The Tree Council, which advises local authorities, explains, for example, that sometimes a space is too narrow for a native species to prosper. This is one of the reasons why the gingko has become popular: it has a particularly slim profile. (I was about to say willowy. But willows are anything but willowy: they tend to have broad crowns).
Making use of exotics allows councils to plant a broader range of trees, which means that there is less chance of a disease wiping out a large proportion of their stock. Some species, especially certain non-natives, are better than others at scouring pollutants from the air: they can make a significant difference to local air quality. Others (the London plane is the most famous example) are extremely resilient, coping with levels of pollution and water stress that would kill most trees. The fruit of some exotic trees attracts large numbers of birds.
But even taking all this into account, councils could still plant many more native trees than they do. Some natives (birch, lime, field maple, hornbeam for example) seem to thrive beside busy streets – and quite a few of the exotics planted by councils fail. In parks, housing estates, town greens, schools, cemeteries and other places for which local authorities are responsible, there is far more scope for planting indigenous trees than along the streets, but the opportunity is usually missed.
What I find particularly frustrating is something I see all over the country: the planting of exotic equivalents of native trees. Instead of planting silver or downy birch, for example, councils throughout the UK seem obsessed by paper birch, from North America. Instead of planting native alder, they plant Italian alder. Instead of planting rowan or whitebeam, they plant an exotic member of the same genus (Sorbus). These trees have broadly the same characteristics, in terms of resilience, beauty and amenity, as the native species, but they are likely to harbour less life. So why not choose the native members of the genus?
A few councils have shown that they understand this issue. Cornwall council, for example, explains that it…
… encourages the use of locally native trees and shrubs for planting in rural areas and around the urban fringes – ‘Bring the countryside into the urban rather than the urban into the countryside!’ Natural regeneration is encouraged where feasible and stock from local provenance … is preferred for planting.
As the countryside becomes ever more hostile to insect life (not least as a result of neonicotinoid pesticides), towns and cities begin to look like sanctuaries. Councils should do all they can to protect our insects, on which so many other species depend. But in many cases they seem to have no interest in or knowledge of the subject. Sadly, I’ve also come across green campaigners who have persuaded their local authorities to let them plant trees in public spaces, then use the opportunity to plant species of little value to wildlife.
The Woodland Trust has published a catalogue of all the commonly-planted trees in Britain, which tells you whether or not they are native. Please take a look at it, then press your council to raise the ratio of the indigenous species it plants. They could be a last refuge for some of our declining insects.