Environmental portraits, by my own definition, are usually captured with a wide angle focal length to show a subject's "environment". But this girl's riding gear tells me the full story.
The strong magnification of a telephoto lens makes the distant trees a nice blurry backdrop that makes the subject pop.
Note that I did not need a 1/1600 shutter speed to stop the action in this image, but the difference between ISO 160 and ISO 100 is not significant. I simply used the action settings I had been using.