I was always threatened by the changing light and shadows when considering painting plein air. I was very dependent on photos to capture the shadow. But, I took the advise of people doing plein air on a regular basis and roughly painted in all dark areas first. It looked really weird, but when I started doing the trees, I knew exactly where one tree started and another one ended.
I found that I didn't worry about the shadows changing, because I had already marked their placement in. It meant that I didn't have to focus so much on tree placement as I painted, but more on what colors and shapes I was seeing.
I also found that I could compare values of light as I went along and make adjustments on the spot.
This could be addictive especially if the weather is as pleasant as it was today.
Judy