The basic purpose of the other-things-equal assumption is to

Other things equal assumption is a common assumption used in economic analysis that frequently goes by the technical Latin term, ceteris paribus. This assumption is used when recognizing the relation between two precise variables, such as price and quantity for the law of demand. The basic purpose is it allows one to reason about the relationship between variables X and Y without the intrusion of variable Z.

this statement is true.

forests are public goods, available for everybody. when usage of such a good becomes interesting for consumers and its usage increases, there can be a risk of overexploitation. it is a non rival good therefore, if one person consumes the good it does not reduce the availability for subsequent consumers.

these caracteristics require the establishment of measures in order to prevent the destruction of a commonly owned space, for example, to restrict the amount of people that have access to the same time.