I am unsure of the interpretation of this question. However, small tree species disappear over time due to human activity and natural causes.
Humans have a big impact in the endangerment and extinction of a species. We cut down trees and deliberately commit to deforestation in an attempt for a better economy for ourselves. In none of these decisions are the inhabiting species considered, and for that reason, numbers in their populations significantly decrease. Natural causes, such as wildfires and droughts, can also affect the absence of a residing species because of their reliance on the resource.