The Green Planet: Seasonal Worlds

• Synopsis
• Key stories
• Filming Locations
• Filmed Species
• Behind the scenes
• Interview with producer Rosie Thomas

Synopsis

Between the tropics and the frozen poles lies a region dominated by relentless change, in the form of the four seasons. Each offers plants brief opportunities and challenges, but to survive they must get their timing right. Sir David Attenborough kicks off the episode by showing of the most extreme examples of seasonal plant life in the Arctic Circle - Finland.

In spring, rising sap in Canada's Maple trees fuels the growth of new leaves. Hungry Sap Sucker Woodpeckers hammer holes in the bark - wounds that leak pure maple syrup. The tree seals the holes and the bird drills more - it's a life-and-death battle.

A different enemy awaits early growing nettles. The Dodder plant has no leaves, just a stem that smells for prey! It pierces its chosen victim and sucks out what it needs.

Summer means flowering - and heat. The South African Cape is coloured by millions of flowers, all competing for pollinators. A huge summer fire wipes the land clean, but it's not a disaster for everything. A new shoot appears through the blackened earth - the Fire Lily, which hasn't flowered since the last fire 15 years ago. It emerges into a world with no competitors at all.

Seeds need to spread before winter and they have many tricks. Parachutes to catch the wind, mimicry to trick animals, explosive mechanisms to fire seeds away.

In autumn, forest fungi are the fruiting bodies of a huge underground network that connects tree to tree - the woodwide web. Trees use it to communicate and to pass vital nutrients to relatives and others in need.

The approach of winter means rapid shutdown. Some that don't get it right grow stunning ice-flowers. Looking like delicate sculptures, they emerge from stems that rupture in the first frosts.

Seasonal plants depend on the predictability of the seasons. Their lives are becoming ever harder with the disruptions by caused climate. Sir David Attenborough visits the ancient giants like Giant Sequoia Trees, in California, which are thousands of years old, are starting to suffer from the consequences of climate change

Key stories

Frostweed Verbesina virginica freezes, bursting the stem and growing out as ice sculptures (USA)

New/remarkable behaviour

Ceratocaryum - The plant whose seeds mimic antelope dung to fool dung beetles, has not been filmed before.

New/remarkable behaviour

Frost weed ice flowers - these have never been filmed before

New/remarkable behaviour

Dodder - filmed in unprecedented detail. The final twist, in which the victim plants use the dodder stems as communication cables is a new story.

A riverbank in England bursts into life in the spring, as various plants push and struggle against each other in their race to grow tall and get the sunlight. But one plant waits until they have grown tall before it emerges. It has no leaves, just a long, flexible stem which can smell the other plants as it grows, until it detects the one it wants. Then it loops around it, sending out shoots that grow into the plant. It's a parasite and it sucks out all it needs. Now it grows many more stems that seek out and plug into other plants.

New/remarkable behaviour

Fire Lily - the Fire Lily has never been filmed before. The Fynbos region of South Africa boasts the densest aggregation of flowers in the world, all competing for the attention of pollinators. Occasionally summer fires break out and burn up whole hillsides, destroying all the flowers in matter if hours. The smoke even percolates into the soil, where it is detected by a bulb and triggers it to send up a shoot. This grows into a red flower - the Fire Lily. It is the only flower in the blackened, burned landscape and so it has all the pollinators to itself.

Tykky forest

Sir David Attenborough in the snow-covered Boreal Forest of Finland

Sir David Attenborough opens the episode in Finland, just inside the Arctic Circle, where it is -20C

Hammer Orchid

In Australia the Grass Tree produces long spikes covered in tiny white, nectar-filled flowers. At the same time the Thynnid wasp emerges from underground. The female is wingless and relies on a male wasp to carry her to the Grass Tree flowers, where she can feed before they mate. But another flower has also grown at the same time. It has the same texture and smell as a female wasp, but is even more attractive to a male. So instead of searching out female wasps, the male instead heads for the flower.

When it lands the flower flicks back on a hinge, wasp attached, and back of the wasp is hit against the pollen-holding part of the plant. Pollen sticks to the wasp, which then carries it away to the next female-mimicking flower, where the same thing happens. The pollen on the wasp now sticks to this flower, pollinating it.

Exploding seeds in the UK

Plants have many ways to distribute their seeds as far from the parent plant as possible. The Himalayan Balsam seed pod is made of several parts held together along seams. Each part is under high tension. At the slightest disturbance, such as a falling raindrop, the pod explodes open along the seams, and fires the seeds into the air, many metres from the parent plant. The Squirting Cucumber builds up high water pressure inside its seed pod. At the slightest touch a jet of liquid shoots out carrying the seeds many metres away.

Maple Trees in Northern Canada

In early spring Maple trees are the first to wake from their winter hibernation. Their task is to quickly grow new leaves so that they can photosynthesise and create the nutrients needed to produce flowers and seeds. To do that they draw up water and sugars stored in their roots over winter and send it up into the branches, where the leaf buds will grow.

The flow of this sap up the tree is very attractive to animals. Sap Suckers hammer into the tree to create wells which leak sap - a rich food. When they have finished Hummingbirds appear. They are on migration and hungry for the high energy sap, for which they are prepared to fight each other.

The Woodwide Web

In autumn mushrooms and toadstools are the visible evidence (the fruiting bodies) of a vast network of fungi that spread through the soil in the form of fine filaments. These grow around the tips of the finest, smallest roots of trees and connect one tree to another. The trees use this network to send signals to one another, warning of attack by herbivores. They also use it to share nutrients with their own kind and to battle enemy trees. A dying tree will even send nutrients into the fungal network that living trees can use.

Filming Locations

Giant sequoia Sequoiadendron giganteum in the mountains of California

• Finland - Boreal forest
• Canada - sugar maple trees, sap suckers (yellow breasted), Ruby-throated humming birds and red squirrels
• UK - Nettles, brambles, hops, bryony, European dodder, daisies, dandelions, harvest mice, fungi, exploding seeds (Ecballium Elaterium), Himalayan Balsam
• Australia - Warty hammer orchids, grass trees, thynnid wasps
• South Africa - Fynbos, Fire Lilly, ceratocaryum, dung beetles
• Austria - autumn colours, European Larch
• America - Aspen forests, Bald Cyprus, Giant sequoia, frost weed

Filmed Species

• Giant sequoia
• Dodder
• Sugar Maple
• Ruby-throated Hummingbird
• Yellow bellied sapsucker
• Ceratocaryum
• Dung beetles
• Grass tree
• Hammer orchid
• Thynnine Wasp
• Dandelion
• Harvest Mouse

Behind the scenes

To film giant sequoias the team used a motorised rope ascender to follow climbers up the tree with a camera, the kit is normally used by the military and for search and rescue to haul casualties up/down ropes.

Filming Maples in spring the crew arrived three weeks later than normal, meaning they didn't see a hummingbird visit a maple tree until their final day of filming. So they had to return the following year, only spring was two weeks early this time!

In Missouri where the team filmed the ice flowers, they only got the right frosty conditions for the ice to form on one morning, and the structures melted just an hour after sunrise, so they had to be lucky to find them at just the right moment.

To capture the fire lily pollination the camera operator spent over 50 hours in a hide waiting for birds to visit the flower, often in mid 30s heat.

Interview with producer Rosie Thomas

Sir David Attenborough being filmed by The Green Planet crew in the snow-covered Boreal forest in Finland, while temperatures were -18 degrees celcius

What's the theme of your Seasonal Worlds episode?

The theme is timing. It's a constantly changing world, and nothing ever stands still. For a plant to be successful in that world, they have to get the timing right. One of the things that we experienced when we were filming was that climate change is really messing up the seasons, so for plants to get their timing right is becoming much more difficult. The episode takes you through these challenges.

A lot of the plants are real opportunists, so there are a lot of risk-takers, con artists and fraudsters in there. They have to go to extreme lengths to succeed, because some of them may only have three weeks to do what they need to do. There's a whole world of these characterful plants that have to go above and beyond in order to succeed.

Which stories stand out for you?

One of the most precise plants is the warty hammer orchid in southwestern Australia. They have to time their emergence absolutely bang on with the thynnid wasp, which comes out in the spring. The flower needs the male wasp to pollinate it, which it does so by mimicking the female wasp. The orchid mimics the female wasp - it's the same colour and size, enticing the male wasp in. It then uses its perfectly designed hinge to flip the male wasp over onto its pollen sacs. The wasp then picks up the pollen sacs on his back and flies off to try and find another female. It's so incredibly clever!

There's another deception story that we've filmed for the first time ever, and that's a plant in South Africa called ceratocaryum. It's a long, spindly grass that stands about two metres tall. Its seeds perfectly mimic antelope dung - they even smell like it. In the height of summer, they flip the seeds to the ground for a dung beetle to come along, roll the seeds away, and bury them. I find it fascinating how these relationships evolved to allow the plant to take advantage of the animal, when normally it's the other way around.

You can't do a programme like this without mentioning climate change, can you?

We talk about it during a sequence about the giant sequoias in California. Because they live for 3,000 years, and their life cycle is so long that they can't adapt very quickly. A tree might produce seeds now, but it needs another 3,000 years for those seeds to be fully mature.

We also had multiple shoots where the seasons didn't do what they were meant to do. We had a plan to shoot Sir David amongst snowy trees at the top of a mountain, but about a couple of days before we were meant to go on the shoot, everybody was calling us to say the snow was melting! Luckily, there were some areas in the higher mountains that had fresh snowfall.

The level of unpredictability and unreliability of seasons is really becoming difficult. If we can't predict it, how can a plant predict it?

BDW