Gone Native

A few years ago, the idea of a commercial tea garden in East Sussex would have raised a smile. Tea (Camellia sinensis) is more familiar to the misty hills of Assam, Darjeeling, and the highlands of Sri Lanka, not the fields between Hastings and Tunbridge Wells. Yet at the Mountfield estate near Robertsbridge, that is exactly what is now growing. Banks Tea has swapped hops for tea bushes, betting that a warming climate has quietly made the impossible possible. Its founder, Mark Wyatt, is candid that growing tea in Britain is still very difficult, but he argues that the region has become the perfect place to try, as the seasons appear shift and the estate wanted a crop that reflects the way the climate is changing rather than the way it used to be.

It is a quaint story, and a small one, but it opens onto a much larger question that anyone planting a garden, a hedgerow, a street tree, or a woodland now has to confront. If a subtropical crop can take root in the Sussex Weald, what do we really mean when we say a plant does or does not belong somewhere? The debate over native and non-native planting has become one of the more heated corners of conservation, and trees sit right at the centre of it.

Start with what "native" actually means, because it is less obvious than it sounds. In Britain, the working definition is essentially historical. Native species are those that recolonised the land under their own dispersal after the last glacial maximum and before rising sea levels severed the land bridge to continental Europe around eight thousand years ago. Everything that arrived later, whether carried by the Romans, medieval traders, or Victorian plant hunters, is classed as non-native, however comfortably it has settled in since. By this measure Britain has only around thirty-five native tree species, a modest list for a country so proud of its woodlands. The date is real, but the line it draws is partly an accident of geography and sea level. A tree that missed the crossing by a few centuries is treated as a foreigner, while one that scraped in beforehand is celebrated as part of our heritage.

The case for planting native is strong, and it deserves to be taken seriously rather than dismissed as sentiment. Native trees have shared this landscape with local wildlife for millennia, and that long companionship shows in the food webs built around them. The oak is the classic example. Research associates something in the order of 2,300 species with our two native oaks, the pedunculate or English oak (Quercus robur) and the sessile oak (Quercus petraea), and more than three hundred of those are obligate associates, meaning they are found on oak and nowhere else. The relationship is often finely tuned. Many herbivorous invertebrates, particularly leaf-mining and defoliating Lepidoptera, synchronise their emergence with a specific tree's budburst, and those insects in turn feed the breeding birds whose own timing tracks the same cue. This is what ecologists call phenological matching, and a specialist that has co-evolved with one tree cannot simply transfer its affections to a newcomer from another continent whose leaf chemistry, tannin load, and seasonal rhythm are all unfamiliar.

There is a below-ground dimension to this that is easy to overlook but increasingly central to the debate, and that is the mycorrhizal network. The great majority of trees live in symbiosis with root-associated fungi, exchanging sugars from photosynthesis for water and mineral nutrients gathered from the soil, and in many woodland species these ectomycorrhizal fungi link the roots of neighbouring trees into a shared network through which nutrients and chemical signals can pass. Native trees such as oak, beech, and birch have co-evolved with a particular suite of fungal partners over thousands of years, and the soils of ancient woodland hold established, diverse fungal communities that a young tree can plug straight into. This matters for the native question in two directions. An introduced species may fail to find compatible symbionts and struggle as a result, or, at the other extreme, it may arrive with its own fungal associates and alter the soil community in ways that disadvantage the natives around it. It is a reminder that a tree is never planted in isolation, but into a living, connected system whose relationships took a very long time to assemble.

Native planting also guards against a real risk, which is that some introduced species escape cultivation and cause havoc. Rhododendron (Rhododendron ponticum) smothering Atlantic rainforests, and Japanese knotweed (Reynoutria japonica) buckling foundations, are reminders that "non-native" occasionally shades into "invasive" with expensive consequences. Rhododendron does double damage, because it is also a reservoir for the pathogen Phytophthora ramorum, which has forced the felling of large areas of larch. There is a cultural argument too. Bluebells (Hyacinthoides non-scripta) under hazel (Corylus avellana), or an ancient hedgerow of hawthorn (Crataegus monogyna) and field maple (Acer campestre), are part of a sense of place that many people rightly want to protect.

The other side of the argument has been gaining ground, and much of it flows directly from the same warming that put tea into Sussex. If the climate a tree evolved for is shifting north at a noticeable pace, then a species that was perfectly adapted to a place a century ago may be poorly adapted to it a century from now. Beech (Fagus sylvatica), for instance, is thoroughly native and much loved, yet it is shallow-rooted and drought-sensitive, and it struggles in the hot, dry summers of the kind we are increasingly likely to get in the south of England. Rigid nativism can end up defending trees against the very conditions they will have to grow in. This is why foresters increasingly talk about provenance and assisted migration, sourcing seed from warmer, more southerly populations, or introducing species that will cope with the climate that is coming rather than the one that has gone. Diversity itself is a form of insurance. The great tree diseases of recent memory bear this out. Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi and spread by elm bark beetles of the genus Scolytus, erased the English elm (Ulmus procera) from the countryside, and ash dieback, caused by Hymenoscyphus fraxineus, is now working through our ash (Fraxinus excelsior). Both tore through the landscape precisely because of such a small number of native species. A more varied palette, native and non-native together, can spread the risk and slow the progress of these novel pests and diseases.

It is also worth puncturing the assumption that non-native automatically means harmful or barren. Sweet chestnut (Castanea sativa), a Roman introduction, and sycamore (Acer pseudoplatanus) have naturalised so thoroughly that most people take them for locals, and sycamore in particular is a valuable early nectar source and supports large populations of aphids that feed birds and predatory insects. The London plane (Platanus × hispanica), a hybrid of garden origin, shrugs off pollution and compacted soil in a way few native trees manage, which is why our cities are lined with it. Only a small fraction of introduced plants ever become invasive, and treating every non-native as a threat in waiting is not supported by the evidence. Some ecologists now argue that we should think less about a plant's passport and more about its function and behaviour in a given setting, and that the novel ecosystems assembling around us may be worth working with rather than fighting.

So who is right? The honest answer is that it depends almost entirely on context, and that anyone claiming a universal rule is overreaching. A fragment of ancient woodland, with its irreplaceable web of specialist species and its established mycorrhizal soils, is a place to be conservative and to favour natives that belong to that particular community. A city street, a windbreak on an exposed hillside, or a planting scheme meant to stand for the next hundred years is a very different proposition, where resilience, shade, and adaptability to a changing climate may matter more than provenance. A garden is different again, a place where people can reasonably plant for pollinators, for beauty, or simply for pleasure. The tea bushes at Mountfield are neither a triumph nor a threat in the abstract. They are a sensible response to a particular field in a particular decade, and that is rather the point.

What the debate really calls for is an open mind and a willingness to judge each planting on its own merits. If you are choosing trees, the most useful thing is not a rule handed down from either camp but good information about what a species is, where it comes from, how it is likely to fare, and what it will support both above and below ground. That is exactly what our Tree Tool is built to provide. It sets out both native and non-native options side by side, with the detail you need to weigh them, and it leaves the choice where it belongs, which is with you and the particular patch of ground you are planting.

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