Posted by & filed under Biodiversity, Deforestation, Insects, Plant Systems, Trees.

Why are almost all the trees that councils plant exotic species?

by George Monbiot

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.

7 Responses to “Return of the Native (UK)”

  1. Chris McLeod

    Eucalypts don’t suck the soil dry, they are alleopathic and if they survive, they will hybridise within two to three generations. I’ve seen them in the strangest locations from SE Asia to South America.

    We have this argument here in Australia – native versus introduced. It is a non sensical argument because none of us consume a diet composed of plants that originated in the local area. We don’t walk the talk on this issue, so why present it as a valid argument?

    Sure I’ve got some mountain peppers and kangaroo apples which are edible from the local area, but the majority of plants in my food forest, vegetable and herb gardens are from – elsewhere.

    With a world of plants to pick and choose from, why restrict ourselves to just the local plants?

    Reply
  2. Craig Mackintosh PRI Editor

    I hear what you’re saying, in regards to vegetables, etc. Like everyone else, I’m growing things that didn’t originally come from the area I live.

    But, I think there’s a big difference between that discussion, and the discussion George is making – i.e. he’s not talking about veggies, but climax tree species. The key point that’s very interesting to me is that the native trees, like oak, support a much larger range of creatures than exotic tree species which those creatures have never adapted over 1000s of years to live with.

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  3. Vasily Kiryanov

    Most of the plant species, especially – trees, in the northern hemisphere, are native EVERYWHERE – from the Rocky Mountains in the west to Kamchatka in the east. Especially – pines. These species are old enough, some up to 300 million years, and have been growing everywhere across the North America and Eurasia. Same to gingko – ceeds and leaves, found everywhere in sediments tell us of vast gingko forests across the hemisphere. But repeated ice ages of the past half a million years have deprived Europe and Northern Asia of most of its diversity – the abundance of plant species that remained mosly in the North America, and asian Far East, where there was enough space for plants to spread away from oncoming glacier. Everything that is called “manchurian”, “korean”, “ussurian”, “far-eastern”, and sometimes even “japanese” – is practically native in Europe.

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  4. Craig Mackintosh PRI Editor

    Hi Vasily. I don’t disagree with you, but this is still out of the context of this article. You’re talking about the past geographic spread of plants – but before the last ice age(s). i.e. a very long, long time ago. If those trees have not lived in the UK for many thousands of years, and then you suddenly start planting them, then they’re now ‘newcomers’ on the scene. They will have ‘gate-crashed’ a party of adaptation and co-habitation between trees, birds, insects, microorganisms, etc., over the last several thousand years.

    I think the key point is that, with important keystone species – like climax canopy trees – that if you can choose between a species that you know already has a ‘long term relationship’ with the land and its critters, then why would you not choose that over an exotic, which usually comes with unknown consequences, some of which can be positively damaging. Yes, they will adapt over 100s of years, and slowly develop a more integrated relationship, but it all takes time – lots of it.

    Again, we’re not talking about veggies here, but keystone species. And, again, why take the risk? I think it’s important to recognise there are a myriad interactions between species that we don’t see, recognise and/or understand. I think we need to be humbly respectful about our own ignorance with these things.

    It’s a permaculture principle to take ‘small and slow’ steps. Putting a whole bunch of different trees in one place does not a forest make. A truly functional forest is a system of its own. We should meddle carefully.

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  5. Vasily Kiryanov

    Craig, it’s certeanly out of question that indigenous species should be planted, and in large quantities. But still, our forests are some 15-20 000 years old, and given that primary climax canopy trees live up to 50-700-1000 years, it is a fairly young system. And it is up to humans to restore it to full strength of its former diversity. Think on this – old woodlands, for example, in Eastern Europe house some 1 500 plant species, including around 600 trees and shrubs. It is old, well established forest, untouched for hundreds of years. Not much of it left, however :). Average woodland in eastern USA may have 6-9 thousand species. Now that’s some diversity.

    Reply
  6. Aapo Leinonen

    Hi all.

    Like Graig and mr. Monbiot, I too am a bit worried about the enthusiasism about non-native trees (and other plants, and also animals and fungi.) We can never quarantee that a non-native tree (even if it is a relative of a native one) can perform all the functions of a native.

    Also some of the non-native species can become real nuisances and overrun native species. A great example of such a plant is the skin-burning and poisonous giant hogweed(Heracleum Mantegazzianum/Persicum/Sosnowsky).

    Also we have to understand that it is not only native trees that are important for ecosystems. Many other plants perform important function as well. And it is not true what is often said in permaculture courses and discussions that “there are no niches for weeds in complete, functional ecosystems”, because a full ecosystem doesn’t necesserily have any large trees. We have to think not only about diversity of species on any one limited area but also globally. Imporing exotic species from other continents often leads to diminished global biodiversity. If every ecosystem/biotope/bioregion in world is imported, for speculation lets say, ten non-native species and as a result every biotope loses four endemic species, we have globally lost 4x the amount of biotopes/ecosystems/bioregions in the world. The biodiversity might increase locally but it certainly would decrease globally.

    Of course we can import usefull species from over seas “provided that they are not locally rampant or invasive”(quoted from Bill Mollisons “Permaculture: A designers manual”). Permies should pay attention to grand-old-man of permaculture in this issue, as he certainly understood the dangers of importing alien species.

    Areas that have no functional ecosystems, areas that have been completely destroyed, sometimes need non-native species to be able to recover. But we should, even in these projects, devote some areas to conservation of endemic or local species.

    And one more thing : there is scientific research (made by Finnish ecologist Ilkka Hanski and his colleagues) that indicates that the loss of biodiversity makes people prone to allergies and other immune system problems. It is assumed that this due to the lower amount microbical encountering in areas with lower biodiversity. If non-native plants host maller amounts of isects than natives, what about microbes?

    http://www.eurekalert.org/pub_releases/2012-05/uoh-blm050712.php

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  7. Chris McLeod

    Hi Craig. I have no beef with either yourself or Mr Monbiot on this matter. You are both correct. Unfortunately, my prior assertions are also correct. This highlights the complexity of the matter under discussion.

    The difficulty I have with Mr Monbiot’s suggested course of action is that the figures he provides are a best case scenario under ideal conditions. An indigenous tree planted in an urban park or even in an agricultural setting is very unlikely to achieve those figures.

    Will it help? Yes. Is it a panacea for the greater ecological problems we are facing as a civilisation? No.

    Perhaps I am also reading too much into Mr Monbiot’s call for action and he is proposing a far simpler course of action?

    Permaculture has a lot to offer in these sorts of situations and Mr Monbiot would be better advised to call for the planting of mixed species linked forests than simply the planting of indigenous trees. A mixed forest would provide far better food and housing for all sorts of life. There are also some serious issues in relation to the maintenance of genetic diversity to any ecosystems that are an island (whether it is an urban island or a small isolated woodland).

    The tree species (as well as the under story) in the food forest here, with the exception of 2 local trees species are all exotics. I’d be reasonably certain that a similar situation exists at Zaytuna farm. The native animals from the surrounding forest use this place as a haven of nutrition.

    The humble apple tree (malus domestica) has it’s origins with crab apples (malus pumila) which are found growing wild in the forests of the Caucasus in southern Russia, the adjoining parts of Georgia, Armenia and Iran and also well to the east, beyond the Caspian Sea in the mountains of Turkestan *taken from The complete book of fruit growing in Australia by Dr Louis Glowinski (an outstanding book on the subject, although he favours non organic production).

    As an introduced exotic tree to the UK, I’d say that apple trees have a lot of value. This however is inconsistent with Mr Monbiot’s claim.

    I’m not disagreeing with you, I’m simply trying to raise the awareness of what a massively complex and thorny issue it is – and there is no simple answer.

    Chris

    Reply

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