This exact topic is covered in Wittgenstein's Philosophical Investigations, and the answer is that the concept of forest is not defined by such a condition. It

This exact topic is covered in Wittgenstein's Philosophical Investigations, and the answer is that the concept of forest is not defined by such a condition. It is a basket of examples of forests and identifying a new example in the basket is done by means of analogy to the existing examples. It is therefore possible for a concept to grow to include contradictory examples, at which point people will typically split the basket into two distinct concepts, like a forest vs a grove. The point at which that happens is somewhat arbitrary, but it also doesn't exist a priori. Someone has to try to use the word ambiguously. Words don't have meanings until they are used by people to mean. It is even possible for a distinction to not be made, and for someone to experience cognitive dissonance when two contradictory examples remain in the same basket.