Let M be the number of trees he plants on Monday.
On Monday the forest was three times larger from the trees that were originally there and the M trees: 3*30 = 30 + M trees.
So, 90=30+M.
And, M = <<60=60>>60 trees.
On Tuesday, he planted 60 trees / 3 = <<60/3=20>>20 trees.
In total, he has planted 60 trees + 20 trees = <<60+20=80>>80 trees.
#### 80