I've built a few as an adult. Any lumber that was attached to the tree was done by using a kerf cut. We would use a circular saw to make a kerf (cut a slot) about 8 inches long in the lumber. We would then use a lag screw and a washer and place the lag about midway in the slot, screwed into the tree.
Even when they are tightened, the screw can move within the slot as it grows and sways.
I watched a recent episode of Treehouse Masters, they used some sort of slide to keep the house level and still while allowing the tree to sway...what that was called? I have no idea! For growth, they left a gap between the tree and the floor and actually laid big rope along the space to keep the wind out.
The secret my friend is find a huge tree that has pretty grown as much as it can here and then let your imagination go wild. I believe on HGTV they have a series called Wild Tree Houses where you may get some ideas if you get this channel. Good luck or even look up some of the bigger woodworking websites or even the larger Lumber stores may have ideas and tips on the websites like Lowes and Home Depot
I built one as a kid. I built it in the crotch of the tree. I was more concerned with the tree swaying than growth. It lasted for years
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