We had a 'bush' wedding, had a marquee with a wooden floor, and people wore the same thing they would wear to a 'city' wedding. The marquee, floor and everything else was all brought in from offsite and set up- All we needed was a flat space big enough for the marquee.

I would check before wearing jeans, black pants and a shirt might go better. I didn't mind what people wore, but i would be uncomfortable if i thought i was underdressed.