Instructables
I built a treehouse supported by 4 very tall pine trees. They are so tall that they move considerably when it's windy, even thought the trunks are significantly large to support a good size treehouse.

I came up with the sliding support shown here. It has held up quite well for 4 1/2 years now.

Materials consist of pressure treated lumber (2x8 scraps I had laying around) and some 10" long galvanized lag bolts.

I used a piece of 2x8 approx. 5' long as a vertical support and beam retainer.
The 2x8 was secured low, directly to the tree trunk with a 10" lag bolt.
The upper portion was secured the same way except I used (2) 2x pieces of lumber to as a spacer and to act as a sliding rest point for the (2) 2x beams. The vertical 2x8 was bolted through the (2) 2x pieces of lumber
The piece of lumber directly beneath the (2) 2x pieces was nailed to the tree as a temporary support while I bolted the support in place.
The lumber and tree were pre-drilled to prevent the lumber from splitting.
Galvanized washers were also used.

Disclaimer:
This has worked for me, however, please consider the load limitations of the particular trees and lumber being used as well as the weight of the treehouse your are building. This is a concept which needs to be adapted for actual conditions. 

I love your treehouse, and your support solution.
I have suggestions.
For peace of mind, you could add a 2" x 12" (doubled would be good) on the diagonal exactly the same way you installed (and just below) the 2" x 10" s. During a party, our very similar treehouse survived the onslaught of 20 adults at the same time. If each person weighs 150, that's 3,000 pounds in addition to the weight of the treehouse.
Which is a reminder that we need to build for worst cases.
Our treehouse similarly sits on 2" x 12"s through-bolted to four redwood trees with 1/2" by 12" lag bolts. We cut horizontal slots for the lag bolts, which are capped by 4-inch cast washers.
Trouble is, after about 5 years, the radial growth of the trunks means we had to back out the bolts so they don't break or sink through the supports.
As luck would have it, the first two bolts we attempted to back out snapped in half. The redwood had somehow fused with the lag bolts'  galvanized coatings.
So we bought the largest, longest galvanized bolts the hardware store had, to replace all of them. Turns out only the first two broke, but we replaced all of them anyway.
Photo 4 seems to show that you use only two lag bolts to attach your support to the tree. Please consider adding two or three more lag bolts, capped with the largest cast washers your hardware store offers.
RyansRiggs2 years ago
Anyone else who wants to make a tree house like this, please at least use 2x10 lumber doubled up for the horizontal connectors. What was used here is way under engineered. I am surprised it hasn't broken yet.
marple200 (author)  RyansRiggs2 years ago
What do you mean by horizontal connectors?
Do you mean the beams?
I used (2) 2x10s with through bolts staggered top and bottom.
How do you know if this is way under-engineered?
You don't even know what the loads and spans are.
Look at Picture #5. You can see the amount of bow over the 4 years of the weight bearing down on it. Under engineered....maybe. But this is a very cool solution to a common problem.

marple200 (author)  trustmefada2 years ago
It's bowed primarily due to the splice in the lumber.
Been that way since the beginning, pretty much.
I agree.
blkhawk2 years ago
I would have used a few 4x4's encased in cement, that way I would not damage the trees when building the treehouse.
I am not a fan of tree houses in general, but this solution seems to solve the issues as to why I never cared for them. Bravo on working it out this way.
mikeasaurus2 years ago
I like this solution.