"The tree line" on a mountain is the height at which the climate becomes too dry and cold for trees to survive. From a distance, you see green below the tree line, and bare rock and snow about it.
"As we climbed the trail, at first we could see nothing but the woods around us. Then we got above the tree line, and suddenly we could see a spectacular view of the mountains all around us."
A hard blow in the belly or "guts" affects an area called the "solar plexus." It creates a stunning effect of shock, beyond pain. There is a temporary inability to breathe ("it knocked the breath out of me.") The person who has been hit doubles up and falls down. In your example, something that has happened has caused the person to feel stunned, shocked, and almost unable to breathe. It does not mean "sad."