Which is a primary disturbance?

Forest fire? Snow melting? Tree falling? Landslide?

Forest fire is primary disturbance.

Explanation:

A forest disturbance is any occurrence that lead to change in the structure and composition of the ecosystem, which might result to death and destruction of forest life.

Fire is a primary disturbance because it is an agent that affect the forest negatively. It lead to destruction and death of living organisms in it by wiping healthy living things and destroying their shelter.