You could save a little money, if you put your straw/pine needles/etc... in a lean-to type bldg.. No use is totally securing these items, since risk of theft would be minimal.
If your trailers are able to be locked @ the gate, you could also store these (plural?) in a similar type setup.
I have a 500SF fully enclosed bldg. with one roll-up. I also have a 12'x16' (fully enclosed) I can lock equip. in. Between them, I have one of those 2 car "garages" that's open on the front and rear (would be open on the sides, but it sits between the two bldgs.). I keep my trailer under the 2 car. I'm also able to lock the rear gate on my trailer, so it won't raise/lower. It would take a cutting torch to drop the gate to steal my heavier equip..
ONLY reason, I'd enclose everything, is so I could heat/cool it (if I wanted to).