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.
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.