I'm buying a Christmas tree to grow in a pot year round and thinking about how I'll keep it healthy in the long run. The tree needs to be pruned to keep a height of 5 feet ( 150cm ) and I'm thinking of buying a fast growing fir. Would putting the tree in an airpot inside a decorative box work long term to promote a healthy root system?
Well you can try it if you like, but most conifers tend not to produce a long taproot (with the exception of Pine, or Pinus varieties) so there doesn't seem to be much advantage in using an airpot. Conifers are not listed as suitable plants for growing in airpot containers. Using one won't mean you can keep it in a pot for longer anyway (see other question and answer).
The air pot will prevent root circling, and should encourage radial root growth, so this will make root maintenance a lot easier for you. Taking the tree out for repotting will also be much easier as you can just undo the screws to remove it from the pot when you want to prune any dead roots, and replace the soil.
This person has planted a noble fir in his air-pot.