Derby have perhaps had more of the ball and created the better chances, but find themselves behind at the break.
The hosts controlled possession in the early parts, with Derby happy to play on the break.
Bryson fired a long-range effort just wide after that, before a clash of heads left Jamie Mackie needing a bandage.
Derby continued to keep the ball well and threatened the home goal on a couple of occasions through Jamie Ward before, against the run of play, Forest took the lead.
Jack Hobbs was left unmarked with minutes to go until half time and met Reid's delivery with a powerful header into the corner to send the City Ground into raptures.