As I've lived most of my life in the countryside, I've never had trouble having a 'tree wee' (that's what the rangers at our local outdoor activity centre call it ?) if necessary, and still have to do it quite regularly.
Both of my young granddaughters struggle to do this, however. Early attempts led to the wee either running down their leg or into their lowered pants/trousers. Female anatomy can be very impractical in this regard! Not surprisingly, this has made them very reticent to repeat the experience.
Like a poster upthread, I have been caught out on one occasion. I was in a vast wood. Not a sound to get heard. Knowing I couldn't hold it in until I got back home, I stepped into a shallow ditch. As I reached the point of no return, a family appeared as if from nowhere! There was no way they could have not seen me, in my bright red anorak. 