The type of wood is much less important than the ability of the structure to withstand (sometimes) incredible force of wind.

The 'connections' of the structure to the actual tree must 'give', i.e. be able to contract / expand as necessary and at the same time prevent water from leaking inside.

This can be accomplished with judicious placement of the joists - not where there is considerable and noticeable 'sway' of the branches in the wind - and using Butyl rubber surrounding any tree limbs that you may decide to have protruding through inside) the structure. 

It is also very important to be kind to the tree - after all, you don;t want to kill it - there would go your treehouse!  I suggest consulting an arborist before actuslly proceeding.