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"I've heard everything between 1500 to 3000 years as an age estimate for the largest welwitschias," says Sussman about these strange plants. She photographed this example – probably around 2000 years old – in the Naukluft desert in Namibia.

"The welwitschias are strange and unique" she says. "Surprisingly enough, they're part of the conifer family and live only in the very specific climate along the coast of Namibia and Angola where coastal fog and desert meet."

To capture moisture from sea fog, the welwitschia, also known as tree tumbo, has evolved special leaves. These leaves become tattered and frayed with time – earning the plant the description of being "octopus-like". In addition, it has long tap roots that sip water deep underground and anchor the plant as it is buffeted by desert winds.

Its unusual appearance caused Friedrich Welwitsch, the botanist who discovered it for science in 1859, to "do nothing but kneel down and gaze at it, half in fear lest a touch should prove it a figment of the imagination".

This may look like a clump of many different plants – but each "ring" is actually a single individual. This is the Mojave yucca (Yucca schidigera), a plant that clones itself by sending out underground rhizomes that sprout into genetically identical stalks.

Each ring, which can be as large as 6 metres across, forms as old stalks in the centre age and die. This particular specimen is probably over 12,000 years old, estimates Sussman. Whilst most rings of Mojave yucca contain around five stalks, older plants can have 10 or more.

Also known as the Spanish dagger, the Mojave yucca is found only in the Mojave desert in the south-western US. It grows at the leisurely rate of just 1 centimetre per year.

This llareta plant, parts of which are over 3000 years old, calls Chile's Atacama desert home. Llaretas can be found throughout the Andes in Peru, Bolivia, Chile and Argentina.

A relative of parsley, its moss-like appearance belies thousands of flowering buds on long stems which are so densely packed together they can take the weight of a human.

"When I saw the llareta for the first time I immediately recognised it from photos I had seen," says Sussman. "Many of them dotted the hillside, some more strangely formed than others, sort of like mutated topiary on steroids."

Because the llareta is dry and dense, it burns well, like peat. "Its function as fuel is endangering its survival, as even park rangers charged with protecting it have been known to burn it to keep warm on cold nights."

Some of the bristlecone pines found in the White mountains of California are over 4500 years old. A specimen known as The Old Man, is 4676 years.

Bristlecone pines receive very little water and food throughout the year: average annual rainfall in the White mountains is less than 30 centimetres, and the trees stand on dolomite, a form of limestone that contains few nutrients.

To survive on this ascetic diet, Pinus longaeva invests very little energy in growth. As a result, they're quite small despite their immense age – the tallest bristlecone is just 18 metres in height – and the trees' girth increases by just 0.25 millimetres a year.

"It shuts down all its non-essential processes," says Sussman. "This looks half dead most of the time, perhaps with just one branch that appears to be alive."