If you parse using GLR parsing (Generalized LR), and if the parse of the input is ambiguous (there are multiple possible ways to parse the input), then the result of the parse can be thought of as a parse DAG, rather than a parse tree. The parse DAG compactly encodes many possible parses: multiple possible parse trees.

However, the bottom line remains that if you have a context-free grammar, and if your input string is unambiguously parseable (there is only a single production in the grammar that produces this input string), and if the job of parsing is to produce that production ... then under these conditions, the output of parsing will always necessarily be a parse tree, because any production of a context-free grammar inherently has a tree structure.

If you parse using GLR parsing (Generalized LR), and if the parse of the input is ambiguous (there are multiple possible ways to parse the input), then the result of the parse can be thought of as a parse DAG, rather than a parse tree. The parse DAG compactly encodes many possible parses: multiple possible parse trees.

However, the bottom line remains that if you have a context-free grammar, and if your input string is unambiguously parseable (there is only a single derivation in the grammar that produces this input string), and if the job of parsing is to produce that derivation ... then under these conditions, the output of parsing will always necessarily be a parse tree, because any production of a context-free grammar inherently has a tree structure.