The term 'Poisonous Tree' is a metaphor for a situation in which a small mistake or wrong decision can lead to a much larger problem. The two crossword solutions, UPAS and YEW, are both types of trees that are known to be poisonous. UPAS is a type of tree native to Southeast Asia and is known for its toxic sap. YEW is a type of evergreen tree found in Europe, North America, and parts of Asia and is known for its poisonous berries.