Christmas trees have approximately a 2 to 1 ratio. Every 2 feet tall is roughly 1 foot of width. Even if he started with rows 10 feet apart, kind of a waste of land, in a few years the trees would be too wide for most equipment. By the time a tree reaches 8 feet it's eating 2 feet of row on each side, so is the next row so the 10 foot row is down to 6 feet wide. Most experts advise 6x6 spacing, I don't know how you maintain the rows with such narrow spacing. One guy I know buys every Cub Cadet lgt he can find; keeps 1 or 2 running using the others for parts. Sounds like a lot of work. While I have a 6 foot wide utility tractor for bull work I had to buy a little 4 foot wide Deere and a 4 foot bush hog for mowing and spraying, I couldn't find another reputable manufacturer making that narrow a machine unless I went Orchard/vineyard and those babys start about 65 horse and are priced accordingly. I'm still considering fenders for front and back tires to push lower branches out of the way.
Strange how most tree farmers can tell you exactly how wide their equipment is.
Other alternatives are a self propelled sickle bar or field and brush mower; but most guys get real good with chemical weed control.