Portland, OR gardener here. We cut our own natural additive-free Xmas trees every year. Got a noble fir last year ($10!) and I cut off the branches and threw the whole trunk in my new hugel bed. (Would've used the branches, too, but I needed them for erosion control.)
I also used a lot of big leaf maple logs and branches, and a few Doug fir logs, but left the western red cedar and holly tree out. Added a few inches worth of half-composted kitchen scraps and oat
straw, and some Azomite that was just laying around, and then a few inches of clayey soil excavated from the hugel pit (the bed is dug into the
yard about 15" deep), and topped it all off with half compost/half garden soil mix from the
local nursery. Harvest was good.
I say go for it, assuming no nasty chemicals on the tree (flock anyone?).
