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From the Avocado to Pando, we love trees! They do so much for us, from making oxygen so we can breathe, to cooling urban environments, to literally holding the ground together to prevent erosion! The SciShow team is joining #TeamTrees to help plant 20 million trees by 2020. You can help too by donating at and helping to spread the word. For every $1 raised, the Arbor Day Foundation will plant one tree.

For the original videos and sources in this video:
Why Avocados Shouldn't Exist

Bringing Back the American Chestnut

Weird Places: Europe's Dancing, Crooked Forests

Why do Leaves Change Color in the Fall

What's The Oldest Tree in the World?

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I, you may not know this, love trees they're tall and they're skinny just like me and they do so much for us from making oxygen so we can breathe to cooling urban environments with their shade the literally holding the ground together to prevent erosion so when we here at SciShow heard that Mr Beast and Mark Rober were assembling a team of tree lovers to help them plant 20 million trees by the end of 2019 we were all in everybody on the SciShow team agreed we are #TeamTrees and we want you to join us. For every dollar you donate at the Arbor Day Foundation will plant a tree the goal is to get to 20 million by.

December 31st and we've put together this compilation of our favorite tree episodes to inspire you to donate. So kick back enjoy the show and be sure to head to afterwards to help us plant trees. [ ♪INTRO ]. First up we're going to talk about what is arguably the most delicious tree out there: avocado trees.

Don't eat the tree part, though, but who doesn't love their tasty green fruit mashed and spread on a piece of toast? But it turns out it is a bit of a miracle that avocados are still around we very nearly lived in a world without them. Here's Michael to explain their almost tragic fate. whether it's sliced on top of a salad tucked into California sushi roll or mashes guacamole in a burrito people seem to love avocados in fact people in the United States munched through 4 billion of them in 2014 alone they taste great they're good for you but one of the most amazing things about avocados is that they still exist see they had a special relationship with huge beasts that lumbered around Central America tens of thousands of years ago and when these animals went extinct avocados could easily have gone down with them but luckily for us they were saved by some prehistoric farmers the word avocado comes from the Aztecs specifically the Nahuatl word avocado which means testicle I mean you can kind of see where they got the name it probably has something to do with the you know the shape and texture of avocados the way they hang from trees anyway before they became popular in the rest of the world they were cultivated in Mesoamerica for thousands of years avocados are a fruit basically swollen plant ovaries but nutritionally they're very different from other fruits you'd find in the supermarket first like apples and oranges are composed mostly of water and sugar and in general fruit is probably better for you than say a bag of sweets or a sugary drink because it contains fiber which slows down the sugar absorption and makes you feel fuller faster by comparison avocados have much less sugar but more protein in fat that gives them that smooth creamy texture but it also puts them on the calorific side for a fruit anyway they also contain high levels of potassium and folate nutrients as well as vitamins c e and k and technically avocados are berries like grapes and blueberries rather than holding lots of little seeds the avocado goes all-in on one big seed that massive ball at the core of each fruit and avocados with their huge seeds evolved alongside equally huge guts tens of thousands of years ago during the.

Pleistocene epoch a menagerie of mega fauna or giant animals roamed the. Americas while woolly mammoths chilled out in the North ground sloths weighing three tonnes and armadillos the size of cars lived in the warm equatorial forest sneeze giant sloths and armadillos a lot of avocados their digestive systems would break down the tough skin and absorb the high-energy pulp then the indigestible seed which contains bitter toxins that kept the animal from chewing it up passed right out the other end the animals got a tasty meal and the avocado trees got to scatter their offspring throughout the Mesoamerican forests plus the seeds got some nice warm fertilizer to give them a nutritious boost and with these mega fauna around to eat the fruit avocado trees could keep growing berries with increasingly massive seeds a bigger the seed the more nutrients could be stored inside as a starter kit for the baby tree this is especially useful in dense tropical forests where canopies of older trees block out much of the light for the saplings below so instead of depending entirely on sunlight for energy the avocado seedlings could supplement photosynthesis with the nutrients in their seed to survive this happy evolutionary match didn't last though eventually the megafauna suffered a mass extinction around ten to thirteen thousand years ago we don't know exactly why but scientists think the warming climate at the end of the last ice age was partly responsible though it was also suspiciously close to the time humans began spreading across the Americas no doubt enjoying lots of giant mammal meat along the way this meadow vacarro's were in trouble without their large gutted evolutionary partners the trees stopped thriving the fruit fell to the ground and the seeds mostly just became food for mold but more hungry creatures were nearby the new human arrivals love the avocados flesh as much as the ground sloths did they also had the tools to eat them and the brains to figure out how to grow them avocados were all set for domestication the avocados we eat today are probably a little different than the ones that grew tens of thousands of years ago for example thanks to artificial selection they probably have more pulp than their ancestors but they've kept their huge seeds ready and waiting for the guts of long-dead beasts so we're lucky that thousands of years ago some farmers decided to plant a bunch of avocado trees and hey I bet that thousands of years from now our descendants will be pretty happy if we plant a whole bunch of trees too so don't forget to go to team trees org after this episode to help us plant 20 million trees and speaking of planting trees avocados aren't the only tree whose fate is in our hands the American chestnut is also struggling to survive our modern world though that's because of a deadly fungus not the lack of seed spreaders time for Olivia to explain picture a forest full of gigantic trees soaring 30 meters into the sky with 5 meter wide trunks you probably envisioned something like the giant sequoias and redwoods that grow on the western coast of the United States but a little over a century ago the east coast of America was also home to giant trees so somewhat smaller than their Western counterparts American chestnuts were huge and they were all over the eastern US at the dawn of the 20th century then within a few decades they were almost extinct the culprit a fungus that strangled the trees from within brought by accident from Asia since their demise scientists have been trying to figure out if there's a way to bring the American chestnut back and thanks to technological advances they may finally have a solution if they can convince the government to let them plant genetically modified trees to understand what happened to the American chestnut we have to go back in time to the end of the 19th century back then. American chestnut trees were known as the Sequoias of the east because they had huge trunks and were tall like the West Coast Giants and they were all over in 1900 around 1/4 of the hardwood trees east of the Mississippi were American chestnuts in some places they made up as much as 40% of the forests but by the 1940s they were all but gone the first signs of trouble were seen in the Bronx.

Zoo in 1904 when Soares called cankers were discovered on a stand of dying trees scientists soon realized that the disease was widespread and by 1912 botanists had managed to identify both the fungus responsible and it's point of origin the chestnut blight fungus gets under the trees bark by hitching a ride on insects the fungus then attacks and feeds off of the trees water transmitting cambium tissues essentially choking the tree the blight fungus probably arrived in New England in the 1870s when Japanese chestnut trees became popular ornamental plants the imports are resistant to the blade so it's likely they carried it to America where the chestnut trees were totally susceptible and by the 1940s it's estimated that nearly 4 billion trees had died but they didn't go extinct entirely a few scattered populations still exist mostly trees that people planted outside of their original range there are also smaller specimens along the east coast that were isolated enough from their kin to avoid infection and it turns out that like the Dread Pirate. Roberts even the dead trees are only most we did while the blade destroyed their trunks their root systems remained and even decades later these living stumps occasionally eke out a chute of new growth but it's usually in vain because the blight is still around although it isn't doing much damage to them it's still lurking in oaks that took over after the chestnuts were wiped out so before any chestnut shoots can reach a reproductive maturity they catch the blight but where there's growth there's hope so scientists have been trying to figure out a way to bring American chestnuts back to their former glory since the 1980s forestry specialists and geneticists have tried all sorts of things to make blight resistant trees they attempted a technique called back crossing for example we're surviving specimens and their offspring were carefully bred together to select for natural resistance genes but while this method seems to work for European chestnuts it hasn't worked as well with the American ones probably because the European ones were more resistant to begin with researchers have also tried hybridizing. American chestnuts with blight resistant Chinese chestnuts but so far they haven't been able to get the resistance traits to reliably pass down from generation to generation but one method that does seem to work is genetically modifying the trees it turns out that we trust a fungal disease of wheat has a similar mechanism of infection to chestnut blight both use a compound called oxalic acid to soften up important structural tissues while also attacking their host cambium by stimulating the growth of calcium oxalate crystals blocking the flow of nutrients resistant forms of wheat produce an enzyme called oxalate oxidase which breaks down the acid thereby blocking the dispersal of the disease and preventing the growth of those crystals scientists have introduced this wheat gene into American chestnuts and in 2014 they revealed that they produced a 100% resistant tree that passed that trait onto its offspring success but the trees haven't been planted yet the researchers have conducted some preliminary studies to show the trees don't cause any unexpected harm to the organisms that live in the environments that they once inhabited and then they requested permission from the US.

Department of Agriculture to release the transgenic trees into the wild but they're still waiting for the green light and that could take a while if it's ever granted at all aside from the general anxiety that accompanies the development of any GM some ecologists worry that a return of the American chestnut would disrupt a century-old ecosystem that's developed without it on the other hand if successfully put in action this method could also work for restoring other wild tree populations beleaguered by fungal invasives like elm trees I guess only time will tell if the. Sequoia of the east will once again stand tall it's really sad that billions of chestnuts just died so suddenly even today we're losing trees at an alarming rate which is why it's more important than ever to plant more and you can help us do that if you go to team trees org after this episode it would be a shame if we didn't have all the wonderful weird trees we have today like for example the ones in Europe's dancing forests oh look it's a younger version of me here with the deets on those the dancing forest of Kaliningrad is exactly the kind of place where you'd expect to find a werewolf creeping through the mist located in a place called the Caronian spit off the Baltic Sea on the border of Russia and. Lithuania the strange forest is known to locals by a jollier name the drunken forest because well the stand of pine trees looks more than a little schnockered as they twist and curves stretching upward and contorted loops to find their way to the sky and here's the thing no one knows why these trees look like they're grinding to Marvin Gaye of course theories abound some suggesting unstable soils the cause or beetle damage or even nuclear radiation local legends say that crawling through one of these tree loops in the right direction will earn you an extra year of life a more popular non-magical theory suggests spousal winds were the original shaping force and there is a precedent for that if you've ever hiked into an Alpine zone forests you've probably seen patches of stunted twisted supercooled mini trees called Krumholtz they get so thoroughly clobbered by a harsh cold winds that they end up growing more horizontal than vertical but some people think that the trees in the dancing forest have been trained to grow that way humans have long been manipulating trees for commercial or aesthetic purposes in mr.

Miyagi and his bonsais he was all about tree shaping humans can train a young tree to grow in unconventional ways by laying a heavy object on its skinny trunk sometimes for years the tree just like the house plant in your windowsill wants to grow toward the Sun really bad and no weight is going to stop it from reaching the light a process called phototropism and whether plants are made to bend intentionally or not the effects of phototropism can change the character of its tissues in trees the wood that forms under the pressure of weight is called reaction wood or in conifers compression wood it's created when the layer of tissue beneath the bark called the cambium thickens below the source of the pressure to support the horizontal weight of the tree in time the funny shape of the bend becomes permanent and it leaves behind a record of oval or oblong instead of more circular rings in the case of the dancing of forests local historians have no recollection of any human manipulation to create this effect but there is another forest in Northwest. Poland called the crooked forest made of about 400 pine trees that all have uniform 90-degree bends at base of their trunks the trees are all the same age and they all bend north because of this uniformity many people believe that this forest was manipulated by humans perhaps to grow uniquely shaped wood for oxen yokes ship hulls or for furniture making that particular theory maintains that the trees were shaped before 1930 but were abandoned before they could be harvested with the outbreak of World War two but ultimately even the cause of the crooked forests odd tree shapes remains a mystery and they could also be attributed to some powerful force like strong winds heavy snow and ice pack or even the result of one of my favourite theories being run over by Nazi tanks as young trees during the war you know all this reminds us that while scientific explanations of natural phenomena are usually pretty cool and often necessary sometimes it's maybe a little bit cooler for it just to be a mystery oh those twisty trees are very cool you know what else would be cool if team trees successfully plants 20 million trees in the next two months you know you want to be a part of that and you can be if you go to team trees org to donate and speaking of cool things it's fall here in the northern hemisphere which means the temperature is falling and leafy trees are painting the landscape with beautiful yellows oranges and reds if you've ever wondered why that happens we'll wonder no more. Michaels got the skinny on autumn leaves the changing leaves of autumn are really awesome to look at but they're also a really striking example of nature taking extreme measures to protect itself you're probably familiar with photosynthesis it's the process plants use to turn carbon dioxide water and light energy into sugars and oxygen and you probably also know that photosynthesis depends on a pigment a colored compound called chlorophyll but you may not realize that plants contain lots of other pigments as well some of the most important are the carotenoids yellow orange and brown pigments they give color to things like corn carrots pumpkins and sweet potatoes and the anthocyanins which give red and purple color to cherries berries pomegranates and red apples to name a few all of these pigments play an important role in the plant's functions but there's usually far more chlorophyll in a plant than anything else because photosynthesis is a plant's number-one job however many trees are less active in the winter because they grow at northern and southern latitudes that get less sunlight during those months these trees are called deciduous from the Latin word that means to fall off since deciduous trees don't do much photosynthesis in the winter it doesn't really make sense to spend of energy maintaining big green leaves so when the days get shorter and the temperature gets cooler they send less of their limited resources to the leaves and start using what water and nutrients they have to keep the rest of the tree alive the chlorophyll in the leaves breaks down and the green color gradually goes away and when that happens the other pigments which were there all the time are better able to show off their colors before the leaves die entirely and fall off the tree so the leaves aren't actually changing pigments they're just losing their strong green pigment to reveal the other colors in the tissue after the tree stops supply of food and water to the leaves all that's left is for the tree to cut them off the tree forms a special layer of weakly bound cells near the base of the Leafs stock then another layer of cells at the very bottom of the stalk expands to push the leaf away eventually the leaf can be knocked off easily even by a light wind and then it's your job to rake them up it's pretty weird when you think about it that deciduous trees just discard huge chunks of themselves every year to make it through to spring it's just like oh I don't need these hands anymore I'll grow new ones in a few months so here's a really young me to talk about the oldest trees in the world well when you started talking about the oldest or biggest or almost any other superlative in nature you're unlikely to find a cut and dry answer there are in fact two contenders for oldest tree and it depends on how you define the term the oldest known individual tree was discovered in 2012 in the white mountains of east central.

California a great northern Bristlecone pine that's 5060 three years old that's older than the pyramids here's a photo of a similar Bristlecone pine now it doesn't look exactly alive and that may be part of its secret to success the high cold arid climate of the White Mountains turns out to be the perfect environment for fostering these ancient trees strangely the higher you go in those mountains the older the trees get and several studies have suggested that the longevity of pines there is directly related to how bad the growing conditions are not only is the average rainfall in the White Mountains less than 30 centimeters per year but most of the trees are growing on dolomite a type of limestone in highly alkaline soil with very few nutrients but over time bristlecones have adapted to this alkalinity unlike other trees which has left them to grow without much if any competition whistle guns also don't expend a lot of energy on their growth in a good year the trees girth will increase by about 0.25 millimeters so instead they can make the most of their meager resources as a result bristlecones tend to have a pretty high proportion of dead to live wood but this has its advantages to reducing respiration and water loss and it also helps that there aren't many other trees around which makes it less likely that they'll fall victim to a forest fire over the millennia researchers are able to determine these trees precise age thanks to a process called cross dating which involves taking core samples from both living and dead trees and then matching up the patterns of their rings to come back with a timeline that goes back thousands of years for our second contender we're going to Fish Lake National Forest in south-central Utah here it lives a clonal colony of quaking aspen that may very well be the oldest living thing on earth it's been named Pando and every tree or stem as they're called in the half square kilometer colony is genetically identical although no individual tree in the colony is older than 200 years they're all connected by a single root system that's at least 80 thousand years old and possibly much older at over 6,000 metric tonnes it also holds the distinction of being the heaviest known living organism on earth so how did bando get so old clonal colonies like panda can reproduce either by flowering and producing seeds or by producing a clone of themselves in this case cloning just means extending the enormous network of roots enforcing a new stem up through the ground because the heart of Pando is so far beneath the ground it can't be killed by a forest fire recent studies have found that. Pando hasn't reproduced sexually in more than 10,000 years that's quite a dry spell and not that surprising given its age that just means that it's up to the root system to continue producing clones and letting the forest fires burn to keep invading conifers at bay so thanks for the evolutionary tips world's oldest trees I'll be sure to keep them in mind when I turned 5000 years old and want to go for another five thousand 80,000 years just imagine what Pando has witnessed in its lifetime it must feel like every new clone tree grows into a totally different world and it's not just Pando of course lots of trees can live for centuries if not millennia trees planted today could last long after you and I are gone they'll be witness to the future we're creating with the choices we make so let's make good choices for them and for us by planting trees we can make the world a better place in all sorts of ways so I hope you'll join us you can be part of team trees by donating at team trees org every dollar donated plants a tree thanks to the Arbor Day Foundation you.