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  • Hey everybody, got a special video for you today. 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 teamtrees.org 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 teamtrees.org 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