The sub-Antarctic islands are home to strange 'megaherbs'

On New Zealand's Campbell Island near Antarctica, some plants grow surprisingly large despite the inhospitable conditions. We may finally know how they do it

For many people, New Zealand is an exotic destination in a far corner of the world. For New Zealanders, of course, it is just home. But even the locals have to travel a surprisingly long way to reach the most remote part of the country. In fact, they have to sail south of the mainland for 600km.

At the end of a journey that can take almost three days, you would reach New Zealand's most southerly territory: Campbell Island.

Campbell Island received Unesco World Heritage Site status in 1998 for its unique biodiversity, but nobody is offering tour packages. Perhaps it is because the long journey seems hardly worth it.

The patches of megaherbs look like "gardens that someone has put lots of time into", says Lorna Little

The island is cold, wet and windy. Temperatures rarely climb above 10C. Every day is cloudy, and on most days the sun peeks through for less than an hour. Strong winds with speeds greater than 30km/h sweep the island. The drizzle never seems to end.

These challenging conditions shape the plants that live on Campbell Island. High winds and low temperatures typically encourage plants to stay small. Insect pollinators are few too, so flowering plants grow plain-coloured flowers and rely on self- or wind-pollination.

But some of the plants on Campbell Island defy these expectations.

Amidst the fields of low shrubs and tussock grasses, giants stand tall. Some herbs rise up more than a metre, with leaves as large as a sheet of printer paper. Some species have hairy leaves, while flowers bloom in various colours from dark purple to pink to yellow-green. Scientists call these large herbs 'megaherbs'.

They are found on a number of sub-Antarctic islands, including Enderby island where the video above was filmed.

The patches of megaherbs look like "gardens that someone has put lots of time into", says Lorna Little, a botanist who studied the plants for her PhD at the University of Otago in Dunedin, New Zealand. "It's really lush and colourful, a striking contrast. For me, the megaherbs on Campbell Island are the equivalent of jungles in the sub-Antarctic."

"But many people don't realise that there are so many colourful plants in this region. They think it's just ice and rock."

Botanists have long registered the absurdity of richly-decorated flowers and big herbs on far-flung sub-Antarctic islands. In his 1839-1843 expedition to the Antarctic, renowned British botanist Sir Joseph Dalton Hooker described the meadows of megaherbs as "producing a floral display second to none outside the tropics".

However, few have studied the megaherbs, and the plants' colourful flowers, hairy leaves and large sizes have remained a puzzle.

About 25 years ago, the late botanist Peter Wardle reported that one of the megaherb species, Pleurophyllum speciosum, has leaves with a surface temperature that reached 20C – much warmer than the environment.

All but one of the six megaherb species had flowers and leaves that were between 4C and 10C warmer than air temperature

The large leaves of P. speciosum are corrugated, with hairs on the ridges. Wardle speculated that those hairs create "greenhouse spaces" that heat up the leaves.

When Little had a chance to visit Campbell Island in December 2010, she decided to test Wardle's claim that megaherbs heat up their leaves. She also checked whether the flowers heat up.

Over several weeks, Little took thermal images of six megaherb species. She used probes to compare real-time plant temperatures to air temperatures and the amount of sunlight available for photosynthesis. Her study is published in the journal Polar Research.

Little found that all but one of the six megaherb species had flowers and leaves that were between 4C and 10C warmer than air temperature, substantiating Wardle's earlier report.

The plants' bigger leaves might help capture as much heat as possible during the brief windows of sunshine

Little also found that densely-packed, darker flowers and corrugated hairy leaves were warmer, while lighter flowers and smooth leaves showed little heating. In thermal images of P. speciosum's corrugated leaves, the hairy ridges shine as hotspots streaks of red-orange while the hairless valleys stay cool blue.

The idea that hairy leaves and dark-coloured flowers heat up might seem a trivial, even a predictable discovery. After all, other studies have shown that dark colours better absorb solar radiation and leaf hairs trap heat within their folds.

But our knowledge of sub-Antarctic island plants is very limited. Little's thermal images provide concrete evidence of one way in which the megaherbs have adapted to the harsh conditions.

"The leaves and flowers are where all the action is," says Little. "Photosynthesis in the leaves feeds the plants while sex (pollination) occurs at the flowers."

In those harsh conditions it pays to be big, dark and hairy

The plants' bigger leaves might help capture as much heat as possible during the brief windows of sunshine. Extra heating would keep the biochemical processes running in the leaves and flowers.

Megaherbs in the sub-Antarctic are not the only plants that have evolved hairy leaves and dark flowers to cope with low temperatures and scarce sunlight. Similar forms are also found in unrelated large herbs in the tropical alpines of Hawai'i, Kenya and the Andes. These plants are called "giant herbs".

Both giant herbs and megaherbs have close relatives in milder environments, which stay small. Little and her team believe that the sub-Antarctic megaherbs and the Alpine giant herbs have converged on the same set of solutions to survive in cold, dim environments.

In those harsh conditions it pays to be big, dark and hairy.

The video on this story was produced with assistance from New Zealand's Department of Conservation to accompany the Planet Earth II television series.

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