It begins with the outliers. Small, sneaky, they hide behind bigger bushes or wedge themselves onto cliff-faces, surveying the landscape with the rapacious eye of a thin-ribbed colonialist. As your descent from the high ground continues, larger, long-limbed gangs of them appear, poking at you from the side of the path. Then the sunshine fades, curtained off behind great ranks of the monsters – pine trees, eucalyptus … the unspeakable wattles.

Do not be fooled by the reassuring balmy bathroom smell of the alien trees; the scent of shiny-bowled sterility promises the same for the natural environment. Birdsong ceases in these forests, as does the buzz and hum of insects. All the things you came to the mountain for – views, flowers, wildlife, clear-running water – are gone: choked, desiccated and poisoned by the relentless march of the aliens.

Hysteria? I think not. The damage caused by alien vegetation to both the natural world and agriculture is astounding. Dealing with it costs African farmers an estimated USD 60 billion a year while in South Africa, alien vegetation has invaded an area of land equivalent to KwaZulu Natal, one of its provinces. It gets worse: in the Western Cape, a province forever plagued by drought, alien trees account for a third of total water use. A third.

Everything beneficial that you can think about trees – with the exception of shade on a hot day – does not apply here. Here in the Western Cape, alien trees do not hold crumbly mountainsides together; their big roots accelerate erosion, punching through the loose, gravelly soil. They are even more destructive when they line up along rivers, breaking up the riverbanks and sending water rushing everywhere but downstream. And remember that it is phytoplankton in the oceans, not trees in forests that produce the overwhelming bulk of the oxygen we breathe.

So why are they here? Well, there is indigenous forest in the Cape, but there wasn’t much of it when the European colonialists arrived in the 17^th century and before long there was even less of it – you know the story.

The answer for the Europeans was to import hardy pine trees from the Mediterranean, drought-resistant eucalyptus and fast-growing wattles from Australia, all species that come from a similar environment to the Western Cape – dry and fire-prone. And like similarly imported cockroaches, rodents and flies, the alien trees found the local landscape much to their liking and, in the absence of their natural predators, have been multiplying con gusto ever since.

Did someone just mention fire? Ah, now we can zero in on the Big One – the aspect of alien vegetation that brings the issue right to our front door. Through it, if you are unlucky.

The natural vegetation of the Western Cape is mostly fynbos, and this is a biome that – as I hardly need remind you – is built around a fire cycle; read here why fynbos must burn.

Fynbos fires, in a perfect world, occur every 20 years or so and are of varying intensity but generally fairly cool and short-lived. But in our imperfect world of alien infestation, the ensuing increase in biomass, and the blurring of the interface between the natural world and suburbia, the game changes and natural fires become deadly furnaces.

It took seven lives and 800 destroyed buildings to make the point in South Africa’s coastal town of Knysna in 2017. A natural fynbos fire was sent roaring into the town’s suburbs, bouncing from pine tree to pine tree and into people’s gardens and then their houses. Drone research during the fire showed beyond any doubt that alien forests (and fynbos that had been infested with aliens) burn much more aggressively than natural fynbos vegetation – flames are bigger, the temperature is higher, and the showers of wind-borne sparks more prolific. A glowing ember of eucalyptus can easily travel over 30 kilometres on the wind to start a new fire.

But the message went unheeded until 2021 when Table Mountain sent a fire rippling across Devil’s Peak and into the thick belt of alien trees that grow around the University of Cape Town, set in the city suburbs. BOOM! Onto the campus it went, igniting (alien) palm trees and running up (alien) ivy-clad walls into windows and onto roofs like a cat burglar. A library was lost, buildings were gutted, irreplaceable cultural and academic material burnt to cinders.

Then things got worse.

A change in wind direction and the fire came charging towards Cape Town itself, gorging itself on large stands of pine and eucalyptus that have been allowed to remain on the very edge of the city. Panic and hasty evacuations ensued, and the fire was beaten back only by human intervention in the form of the fire services.

So you get the point: alien trees = bad. Easy – or is it? Have a look here.

The above scene is in the Cederberg mountains. It should be pure fynbos but two clumps of alien trees blot the landscape: on the right, stone pines (Pinus pinea) and on the left, English oaks (Quercus robur). Now I’m all for chopping down the pines – wretched horrible things – but at the risk of incurring hate mail, I admit that I did recently spend five very pleasant days camping in the shade of the oaks. In fact, I’d be horrified if someone removed them – there’s not much shade around in the Cederberg and I’d justify my apparent hypocrisy by pointing out that oak trees not invasive alien trees. After all, in the absence of squirrels, an oak tree’s acorn cannot travel far, and the modest stand of oak trees you see above will probably stay much like that for another hundred years.

But it does raise the question of who decides what stays and what goes? In Cape Town, the debate finds expression in opposing groups of citizens, some of whom want to keep alien trees in some contexts for the benefit of people (shady walks, a soothing forest ambience) while others want nothing more but the sound of the chainsaw and the eradication of them all. 

The answer probably lies in the middle – I’m sure you like a shady walk as much as I do – but whatever answer it is, we’d better find it sooner rather than later. Much of Table Mountain, for example, has not burned for forty or fifty years. The fynbos in these areas is dead and dying, heavily infested with alien vegetation and holding decades-worth of flammable biomass, and all of this is within metres of dense residential housing.

It’s a legacy of the poor understanding we have of local ecology and inadequate forest management plus the visceral reaction we have to fire (put it out! put it out!!) but for those residents on the front line, those living in the shadow of these aliens, whether it’s in Cape Town or California or Spain or Greece or anywhere else that’s affected by this alien blight, my advice is simple: you are going to need that chainsaw.