Each Christmas, families gather around evergreens, real or fake, to celebrate the season.
But what holiday revellers may not realise is just how incredible these spruce, fir and pines can be.
In the wild, evergreen conifers survive drastic temperature swings, grow to towering heights and create ecosystems that shelter strange and wonderful creatures.
Here are some the secrets of Christmas trees and their tough, tenacious lives.
Christmas trees can turn to glass
Here's a party trick not to try without the proper safety equipment: drop a sprig of Siberian spruce (Picea obovata) or Scots pine (Pinus sylvestris) in a vat of liquid nitrogen, at a temperature of -196 degrees Celsius. Providing you've pre-chilled the plant to -20 degrees Celsius or so, the sprig will survive.
This incredible cold tolerance serves conifers in the boreal forests of Siberia, where winter temperatures regularly get below -60 degrees Celsius. How these trees do it, though, is only partially understood, says Richard Strimbeck, a plant physiologist at the Norwegian University of Science and Technology.
"Once the molecules are in this glassy state, they can't move around
What seems most likely is that the tree tissue turns to glass.
In this sense, the word glass means a solid without a crystalline structure — just like window glass, but made of sugars, protein and water molecules rather than silica.
"Once the molecules are in this glassy state, they can't move around, and that means they can't react," Strimbeck said. Essentially pre-frozen, the trees' metabolism drops to zero, and their cells aren't damaged by extreme cold. When winter approaches, trees also pull water from their cells to the surrounding tissue, so that the swelling ice crystals don't burst the cell walls.
This process of cold-weather preparation is called "hardening," and trees appear to rely on a combination of the seasonal light cycle and temperatures to determine when to prepare. The exact mechanisms remain largely a mystery, Strimbeck said.
Christmas trees are home to tarantulas
Far south of Siberia, on the other side of the world, Christmas trees and tarantulas go together like bread and butter.
In the southern Appalachian mountains of eastern Tennessee and North Carolina, in the US, the spruce-fir forests above 1645 metres in elevation are home to one of the world's tiniest tarantulas, the spruce-fir moss spider (Microhexura montivaga).
Too much sun will dry the spiders out
At only a quarter of a centimetre in length, these brown spiders are endangered and elusive. They live on only a handful of peaks, and even there, only in tiny communities. On one mountain in North Carolina, they're found on only a single rock outcrop and a nearby boulder, according to the US Fish and Wildlife Service.
Fraser firs (Abies fraseri) and red spruce (Picea rubens) provide the shelter needed to keep the spruce-fir tarantulas' rocky, mossy habitats cool and damp. Too much sun will dry the spiders out; too many water droplets can flush out their tube-like webs, which they weave in the miniscule spaces between rocks and moss mats.
Christmas trees are as tall as trees can get
Most of the Christmas trees scraping the ceilings of sitting rooms are a mere 10 years old or so. Given the right time and conditions, though, Christmas trees can become true behemoths.
The Herculean effort is even visible in the tree's anatomy
The tallest Douglas fir tree in the world, for example, rises 99.7 metres above the soft forest floor in Coos County, Oregon. It's the tallest non-redwood conifer in the world. The coast redwood, also an evergreen, is the tallest tree on record. "Hyperion," a coast redwood in California's Redwood National and State Park, towers above all its kin at 115.55 metres in height.
Trees likely can't get much taller, according to a 2004 study published in the journal Nature. At between 122 metres and 130 metres, the effects of gravity make it too difficult for a tree to push water any higher up though its tissues, the authors reported.
The Herculean effort is even visible in the tree's anatomy, said Strimbeck, who was not involved in that research: the cells in the needles at the tops of extremely tall trees are smaller than cells in needles lower down, because plant cell expansion is driven in part by water pressure.
Christmas trees create their own ecosystem
A tree, as long as a soccer pitch is tall, is a world unto itself.
"It's like a different biome when you're at the top of the tree," said Brian French, a professional tree-climber and co-founder of Ascending the Giants, a non-profit organisation in Oregon, US dedicated to measuring and preserving the world's oldest and largest trees. French and his co-founder and climbing partner Will Koomjian regularly ascend some of the biggest evergreens on the West Coast of the US, including the Doerner fir, the record-holder for tallest Douglas fir in the world.
What they find is a layered world of life. For some of the tallest trees, the branches don't even begin until 30 or 60 metres up, French said, requiring crossbows or giant slingshots to set climbing ropes. At the base of the tree, the forest floor is damp and spongy, but at the crown, near-constant wind makes everything cool and dry.
When we have these old, big trees, they offer places that take hundreds of years to establish
"It just feels like a harsh environment, or like an unwelcoming environment sometimes," Koomjian said.
Unwelcoming doesn't mean lifeless, however. The hollows and branches of giant evergreens provide shelter for birds such as the endangered spotted owl and mammals such as flying squirrels and red tree voles, which can live whole generations in one tree without ever touching the ground.
Lichens that thrive only on old-growth trees provide winter forage for flying squirrels. In 2008, Ascending the Giants climbers even discovered two clouded salamanders living in a cavity 76 metres above the ground in a Douglas fir.
Younger, smaller trees are no substitute for these old, stately habitats.
"When we have these old, big trees, they offer places that take hundreds of years to establish," French said.
Christmas trees may struggle with future climates
Christmas trees are quite adaptive, and capable of growing well outside their native range. The Fraser fir, native to only the highest peaks of the southern Appalachian mountains in America, has become one of the most popular species for Christmas in North America; it is now grown by tree farmers in such far-flung spots as Michigan and Oregon in the US and has recently been introduced to the United Kingdom.
Still, researchers worry about what climate change might mean for native evergreens. If winter temperatures don't get cold enough, some tree species might not get the necessary messages to "wake up" in time for spring, Strimbeck said.
We at least get information for the Christmas tree farmers here about how elevation effects the growth of their trees
Temperature might also affect growth. Researchers led by Howard Neufeld of North Carolina's Appalachian State University, US are using Christmas trees to test this very question. By measuring trees at tree farms of different elevations, Neufeld and his colleagues hope to gauge how cooler and warmer temperatures alter tree physiology. For example, the older the needle on a tree, the less effective it is at photosynthesis; converting sunlight into energy. Temperature might alter how fast needles age, Neufeld explained.
Too much heat might also harm spruces, firs and their relatives because these trees are simply not made to withstand warmth. At about -2.8 degrees Celsius, photosynthesis slows, Neufeld said. Meanwhile, water loss through respiration speeds up.
The research matters for understanding the physiology of these beloved evergreens, Neufeld said. It could also save Christmas by making sure farmers prepare for coming climactic shifts.
"We at least get information for the Christmas tree farmers here about how elevation effects the growth of their trees," Neufeld said.