This is the borealforest（北方森林）. The largest forest on Earth. 750 billion trees smothered by snow throughout the winter. This is the northermostboundary of an extraordinary world.

I‘m standing at the edge of the Arctic Circle（北极圈）. To the north of me, lies a land dominated for most of the yearby snow and ice. 3,000 miles to the south, the tropics, bathed the year roundin warm sunshine, and in between, a very different world, dominated by relentlesschange.

An endless cycle of four distinct seasons, each with its own challenges. The short, freezing days of winter give way tothe urgent awakening of spring. And the long hot days of summer yield to the cooling of autumn. 

(资料图片仅供参考)

Opportunities will be brief. To survive these extremes, plants have not only got to be hardyand resilient, but many of them have developed special strategies in order to meet the demands ofthis seasonal world.

To succeed, they must get their timing just right. For months now, this world has been asleep. But change is coming.

Seasonal Worlds

Spring is on its way. In Canada, the forests are starting to stir.

Throughout the winter, the sugar maples（糖槭） here have kept stores of nutrients ready for this moment. Now, as temperatures start to rise, those nutrients must be brought up from the roots. Sap is on the rise. Fuel for new green growth.

But just as growth reaches full speed, robbersarrive. A sapsucker（黄腹吸汁啄木鸟）. A thief, in search of maple syrup. 

The tree can tolerate hundreds of wounds, but if they completely encirclethe trunk, the tree wil eventually die. The sapsucker's beak-in hasn't gone unnoticed.

encircle: to form a circle around : SURROUND

Hummingbirds（红喉北蜂鸟）...

squirrels...

and insects all invade the supsucker's territory, trying to steal a meal. But unwittingly, they help the tree. Time spent chasing the competitionmeans less time to drill new holes. The tree gets a chance to repair its wounds, and the sap can rise freely again. 

unwittingly:not knowing : UNAWARE

competition此处：one's competitors

So this year at least, the tree will once more produce its leaves. Hundreds of miniature solar panels that collect energy from the sunlight. Each leaf is covered by millions of microscopic pores, stomata. They take in carbon dioxide and release oxygen and water vapour, in a process that produces high energy fuel. All the trees here can now use that fuel to grow over the coming months.

As spring proceeds, the power of the sun increases.

On riverbank across Europe, many plants have been lying dormantunderground waiting for the earth to warm. And now a race begins.

Among the first to start are nettles（异株荨麻）, the sprinters. Then, only just behind, come the brambles（悬钩子属）. They are much more aggressive and have backward pointed hooks with which to scrambleover their rivals. Last to come, the climbers（攀缘植物）. Hops（啤酒花）...and briony（泻根属）. They lassotheir way towards the light.

scramble:to move or climb hastily especially on all fours

lasso(n.): a rope or long thong of leather with a noose used especially for catching horses and cattle : LARIAT

lasso(v.): to capture with or as if with a lasso : ROPE

Soon every inch of space and every patch of light has been claimed. But hidden in the shadows, another plant has been waiting to make its move. This, is dodder（欧洲菟丝子）.

A hunter with an exceptional sense of smell. Moving swiftly, it searches for its prey. It detects the scent of a young nettle. 

The dodder punctures the nettle's stem and sucks out the nutritious sap. And it doesn't stop at just one victim. Withits energy supply assured, it multiplies.

It's a parasite, with an insatiable appetite.

But remarkably, its victims can now exploit the dodder when other springtimeenemies appear. 

Caterpillars！

When under attack, a leaf sends out a signal telling the rest of the plant to start building defences, foul-tastingtoxins that deterthe caterpillars. 

deter: to turn aside, discourage, or prevent from acting

In an extraordinary twist, the stems of the dodder interconnectingthe plants are now being used as lines of communication to relaythese warning signals from plant to plant. They respond by producing their own protective chemicals. 

relay此处: the act of passing along (something, such as a message or ball) by stages

Eventually the whole riverbank becomes one huge defensive network, and the one timerivals are now better offtogether.

The first warm days of spring encourage some plants to flower and get ahead of the competition, creating beautiful displays throughout the seasonal world.

From the spectacular cherry blossoms of Japan

to the goldfield flowers of California（加州金原菊）

But spring flowering is risky. If they flower too early, it may not be warm enough for pollinating insects to be active, but the common daisy gives them every opportunity.

The warmer the flowers are, the more attractive they are, and they have a remarkable strategy in order to maximisethis.

They were closed up tightly throughout night, as a protection against the elements, but once they feel the warmth and the light of the sun they spring intoaction.

spring此处: to move quickly and suddenly towards a particular space; spring into actoon=start taking action

It's a behaviour called heliotropism（向阳性）. They turn to keep facing the sun, absorbing as much heat as they can.

This is a thermal camera, and it will tell me the difference between the surrounding temperature and the temperature in the centre of a daisy flower. The surroundings, 12 degrees, in the centre of a flower, 21.

Pollinators such as bees and wasps prefer the warmed-upflowers, because they can get a share of the heat for themselves. As a consequence, they can collect more nectar from more flowers, pollinating as they go.

And the daisies, with the help of their pollinators, are able to have a particularly long flowering season. But when the window of opportunityis brief, even more ingenious tactics are necessary. 

window此处:  a period when there is an opportunity to do something窗口期

Here in southwestern Australia. Summer temperatures can soar toup to 40 degrees Celsius.

Now, for a brief moment in the cooler spring, flowering plants must get busy.

And this Hammer Orchid（铅色槌唇兰） with its strange flower is doing exactly that. It needs to attract a pollinator, but has no nectar and doesn't even look like a flower, unlike these neibouring, huge grass trees（刺叶树）.

These produce thousands of tiny white flowers, all drippingwith nectar.

But the orchid has a different strategy.

It synchronisesits emergencewith the brief mating season of the thynnid wasp（刺臀土蜂）. The flightless female wasp produces pheromones（信息素）to attract males.

This male detect a scent that seems to be hers, yet flies right on by.

The orchid not only has the same shape and colour as a female wasp but it sits at the same height as she does. It even mimicsher smell and the beguiledmale attempts to mate with it.

beguile: to persuade, attract or interest someone, sometimes in order to deceive them

He hammersagainst the orchid's pollen sacksand the wasp leaves, doubtless a little shaken, only to be dupedonce again.

hammer此处：to hit or kick something with a lot of force

dupe此处: =duplicate; to deceive or cheat easily

The pollen on his back now sticks to this different orchid and the deception has worked. The orchid's imitation is even more intoxicatingit seems, than the real thing.

intoxicating: making you feel excited, happy, and slightly out of control

Eventually, the male wasps tire of the orchid's enticements. Now the wasps, both male and female, find the flowers of the grass trees to feed, and there, at long last, they mate.

And as for the hammer orchid, pollination is complete.

As summer approaches, the need of flower becomes more urgent and nowhere more so than here in South Africa.

This is the Fynbos（凡波斯）, part of the Cape（好望角）  Floral Kingdom, a great expanse of open healthlands（欧石南灌丛）  just North of the Cape. 

Here, there are more different species of plant than anywhere else in the world. Nearly nine thousand, many needing pollinators.

They compete with one another for such help byflaunting extravagant shapes and vivid colours, all promising nectar as a sugary reward.

flaunt: to show or make obvious something you are proud of in order to get admiration

But there is a plant here that avoids this crowded competition, and the rising temperatures of summer bring just what it needs.

This is the first fire here for 15 years. Nearly all the plants are destroyed. Incinerated. It might seem from the smokes and the still smoulderingembersthat no plant could survive such an inferno.

incinerate: to make fire or damage with fire

smoulder(smolder): to burn slowly with smoke but without flames

ember: a piece of wood or coal, etc. that continue to burn after a fire has no more flames

inferno: a very large uncontrolled fire

But just 4 days after the flames, rising from the ashes, a Fire Lily（垂筒花）.

It has been lying dormant underground for 15 years, but now, awakened by the smoke, it flowers.

The blooms may be small and unshowy, but that's all they need to be. In this charredlandscape, pollinators such as these sunbirds（南方重领花蜜鸟） can spot these little red beacons from great distances.

charred: burned and black

They are the only source of nectar around, and the more visitors the flowers attract, the more likely they are to be pollinated.

It's just in time. 

Within a few months, the whole landscape is alive once more. In fact, all these plants need fire to survive. And as competitors return, the fire lily fades.

It now returns underground and will rest there as a bulbuntil another fire awakens it.

By the time, the long hot days of summer arrive, wildflower meadows are bursting with life, but autumn is not far away.

So now, pollinated plants must use their remaining energy to produce the next generation, seeds.

A dandelion clock contains around 200 seeds, each with its own tiny parachute.

Few seeds can fly as far as these, some are known to have travelled over 60 miles. But to travel any distance at all, they need the wind to be just right.

However, before they can get away, there is a risk. Voracioushunters live in this meadow.

Voracious: very eager for something, especially food

Tiny harvest mice（收割鼠）. 

They love dandelion seeds. But they don't like to share.

Finally, the sun warms the ground, and a gentle breeze blows in, creating an ideal updraft.

updraft: an upward movement of gas(such as air)

Air flowing between the bristlescreate a vortexthat lifts the seeds up.

Heading off on a gentle breeze, this seed now starts its travel.

Plants use a variety of different techniques to spread their seeds as far as possible from themselves, and one of the most remarkable is this one. It's called Ecballium（喷瓜） and it's a relative of the cucumber.

For several weeks now, pressure has been building up inside the pods, and so now they're as tautas a well pumped up bicycle tyre, all they need is just a slight nudge.

taut: tight or completely stretched

nudge: to push something or someone gently, especially to push someone with your elbow to attract the person's attention

The further the plant can fire its seeds the better. If a seedling germinatesclose to its parent, the two will have to compete for nutrients and light.

This Himalayan Balsam（喜马拉雅凤仙花）has seed pods that react rather differently, like catapults（弹射器）.

As they dry out, they begin to strain along precisely positioned lines of weakness and then just the slightest disturbance, even a single raindrop is enough to trigger the mechanism.

The east coast of South Africa. Summer here brings scorchingtemperatures.

This Ceratocaryum（银白镊被灯草） has to get its seeds underground before it gets too hot and it has acquired somewhat unlikely allies.

Dung beetles（蜣螂）.

They have a particular fondness for antelope（羚羊）droppings.

At this time of the year these beetles bury the dung and lay their eggs on it. 

And the fresher the better.

Ceratocaryum's spindlystems send their seeds flying.

spindly: long or tall and thin, and looking weak

They are the same size, shape and even smell remarkably like antelope dung.

The beetles simply can't resist them.

Dung beetles always bury dung balls at the same depth and it's one which suits the seeds very well. So effective is the seeds' deception that the beetles come back for more, time and time again.

Most of the Ceratocaryum's seeds get safely buried exactly where they need to be. 

As the sun begins to retreat, so autumn arrives. The cooler, shorter days are the cuefor many seasonal forests to prepare for the winter shutdown. 

The trees starts to divert their nutrients back into their roots, and there, other organisms await them.

Fungi.

Mushrooms are the familiar faceof fungi but they are merely the fruiting bodies（子实体）, some of them only last for a few days. There are however, evidence of the giant organisms that lies in the soil beneath.

Just a single handful of soil may contain several thousand metres of their microscopic filaments. 

It's only recently that we have discovered the extraordinary role that these fungi play in a forest like this.

Their filaments pluginto the tips of the trees' roots, and nutrients pass between tree and fungus thoughout the year. Some fungi have the ability to link with not just a single tree but with a whole group of trees, so that the entire forest may be linked together by these microscopic threads from what you might call a wood-wide web.

plug: to fill a hole with a piece of fuitable material

Hundreds of trees  can beinterconnectedby these webs. Scientists call it the mycorrhizal（菌根） network, and it might look something like this.

They've discovered that trees not only send nutrients along it but chemical and electrical signals allowing them to communicate with one another. 

But some trees can also be selfish and steal from their rivals or evenwage warby sending out toxins that will harm competitors.

wage: to fight a war or organize a series of activities in order to achieve something

It seems however, that most trees do try to help each other. Theyraise the alarmwhen attacked by leaf eaters, giving other trees time to produce defensive chemicals. Those that are dying may send their foodreserves to their neighbours and some individuals known as Mother Trees（母本树） recognise their own offspring and will channel resources to them so giving their young the best possible start in life.

As the harshnessof winter approaches, temperatures woodlands（温带森林） from Russia to Canada are now in a race to shut down.

The green pigment（叶绿素） in the leaves starts to break downand nutrients are withdrawn into the branches. 

The chemicals substances that are left behind then create one of the most spectacular displays of colour in the whole of nature.

The first freezing nights of winter bring with them a killer...

Frost!

Stems, when frozen,rupture, and flowers of ice form in the chilly air.

rupture: to (cause something to) explode, break, or tear:

For some, winter brings dormancy, for others, it brings death. Another cycle of the seasons comes to an end.

Plants all cross the seasonal world have developed many different strategies for success, but they all depend on the seasons changing reliably year after year.

It doesn't matter whether you live for 3,000 years or just a few months, everything depends on yougetting the timingjust right.

But in our changing world, this is becoming a greater challenge every year, and even the mosthardy and resilient of plants are starting to struggle.

These sequoia trees are the giants of the natural world, the largest living things on the planet. They can grow to almost a hundred metres tall and eleven metres across. Their bark alone can be over a half a metre thick. 

They're also among the oldest, some living to over 3,000 years. And to achieve such age and size, they need very particular living conditions such as are found here in the Sierra Nevada mountains（内华达山脉） of California.

Not only do they need energy from the sun, but critically they also require up to 4,000 litres of water a day, so they are almost entirely reliant onthe seasonal snowmelt.

But recent years have brought longer, hotter summers and their source of water is becoming increasingly unreliable.

Scientists are discovering that even these seeminglyindestructiblegiants are now starting to show signs of vulnerability. Some aresheddingtheir needles（针叶） and branches as a way of conserving percious water. But for others, climate change has already been fatal.

cone（此处）: the hard oval-shaped fruit of a type of evergreen tree (= one that never loses its leaves), called a conifer:松果

10% of them have been lost in just the last few years.

A single giant sequoia（巨杉） in its lifetime can produce a hundred million seeds. These, in my hand from one cone are more than enough to start a completely new forest.

But they can only do such a thing in a world where the seasons change with some reliabilty. Today our climate is changing, bringing an unprecedentedlevel of unpredictability, all across the seasonal world.

The question is, can we curb climate change sufficiently to ensure that the seasons will continue? Only if we can do that, with the future of seasonal plants, including these magnificent trees, be assured.