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