Ash trees are being removed when they show signs of the ash borer, a parasite that affects the trees and kills them off. What best explains why the ash trees are being cut down? The ash borer is a density-independent limiting factor, and humans need to clear cut to control tree density. The ash borer is a density-dependent limiting factor and will start to spread to nearby healthy trees. Ash trees are outcompeting other trees and are thus more affected by the ash borer. Long periods of drought have caused an increase in the ash borer population.