Ok, apparently a Christmas tree lasts longer if you cut it down yourself and get it into a bucket of water; 2 gallons at least, seeing as how the outer layer of the bark consumes about 3.79 liters a day. If its left too long, sap will cover the bottom cut making it impossible for it to draw in water and you have to re-cut the bottom a second time to open it up again. Using aspirin or other tree preservatives helps as well as does drilling small holes at the base of the trunk and keeping the tree away from any heat source.
Here is what I've found out so far for preserving natural cat trees. First, I can't use terms like 'cat trees' in my search; it brings up all the wrong stuff! So I tried 'preserve natural cat trees' as a search term and found the following information. Its better if the tree itself is thicker than the pencil cat tree we have now and striped of its bark so it can dry out some. Then treated with a non-toxic wood sealer to protect the wood and repel insects.
You have a personal choice as to how tall your cat tree is and how many perches or boxes you have on it; you can design them to fit exactly what you need and where it will go in your home. Its better, I think, to use trees that have come to the natural end of their life cycle and are already somewhat dried out. Then you can strip it, treat it and build a nice, sturdy stand for it. Build cat-sized platforms with a washable rug or other material or simply secure cat-sized boxes. Or do both!
I found this one online and liked the design.
My hubby just decided one day to make one for our cats as a test and its worked well for the half month we've had it, but now its time to get more serious about making a more long-term version that we can have around for awhile.
Of course another alternative is if you're handy with wood and tools, you can simply build one from sheets and long two by fours and add a rug or two.
These are some ideas we are looking at for now.