The dragon has not seen you, that much is correct. However, that is not his only sense. He knows you're there, because he can smell you. And though you're hiding amongst the trees, he's not willing to play hide and seek. He saves you both the trouble, and rears back, breathing in a deep breath, lunging forward and lets loose a searing hot gout of flame, setting the forest ablaze right in front of you. Barely safe yourself, the tree you're hiding behind is lit up like a roman candle, but it does its job and you yourself do not get torched. There are two rivers of flame beside you though, one to either side. You're trapped in a sea of flame! It turns out this dragon likes his humans well-done!!
Now what?