Actually pretty easy. Trees don’t reject tissue from other trees in the same family. You bend the tree to another tree when it is a sapling, scrape off the bark on both trees where they touch, add some damp sphagnum moss around them to keep everything slightly moist and bind them together. 
Then wait a few years- The trees will have grown together. 

You can use a similar technique to graft a lemon branch or a lime branch or even both- onto an orange tree and have one tree that has all three fruits.

Frankentrees.

As a biologist I can clearly state that plants are fucking weird and you should probably be slightly afraid of them.

On that note! At the university (UBC) located in town, the Agriculture students were told by their teacher that a tree flipped upside down would die. So they took an excavator and flipped the tree upside down. And it’s still growing. But the branches are now the roots, and the roots are now these super gnarly looking branches. Be afraid.

But Vi, how can you mention that and NOT post a picture? D:

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I am both amazed and horrified of nature as we all should be

I love how trees are like “fuck it, I’ll deal” at literally everything. Forest fire? Cool, my seeds’ll finally grow. Upside down? Branches, suck, roots, leave. What’s this new branch? Eh, welcome to the tree buddy.

I need to be more like tree

I continue to fear and respect out arboreal overlords.

what kind of professor did these students have that they needed to prove him wrong so badly that they literally dug up a tree, flipped it and put it back in the ground?

Sounds like y’all’ve never heard about the Tree of 40 Fruits. Well, it’s exactly as it sounds. Sam Van Aken, an artist based in New York, decided to try his hand at grafting (e.g. the process by which you attach the branches of a different tree to a host tree).

As artists are inclined to do he decided to push some limits and over the course of a few years he grafted over 40 different fruit onto the host “ including almond, apricot, cherry, nectarine, peach and plum varieties.”

It has a fruiting period lasting from July to October and this is what it looks like when blossoming.

Shit’s tight yo.

Also we have a group called the Guerrilla Grafters. A group who started in San Fransisco with the goal of grafting fruiting branches onto non-fruiting trees of the same type.

Most cities have fruit trees that simply don’t produce fruit because having all these would be a mess and inadvertently providing unregulated food to people comes with a lot of legal risks I suppose. These grafters seem to think otherwise and have taken it upon themselves to try and bring fruit trees back to urban areas.