Unusually for a eucalypt, Mountain Ash does not regrow after fire. They grow only from seed which are released from their woody gumnuts by heat. The seedlings need a lot of light, much more than reaches the forest floor normally when there are mature trees blocking the sun. Severe fires can kill all the trees in a forest, which causes a massive release of seed to take advantage of the nutrients in the ash bed. Up to 2.5 million seedlings per have been recorded after a major fire. Competition and natural thinning slowly reduces the number of mature trees to about 30 to 40 per hectare. Because it takes roughly 20 years for seedlings to reach become old enough to make seeds, repeated fires in the same area can cause local extinctions. If, however, there are no fires to release seeds, the trees die off after about 400 years and are replaced by other species.