A bushfire is a wildfire that occurs in the bush (collective term for forest, scrub, woodland or grassland of Australia, New Zealand, New Caledonia). In southeast Australia, bushfires tend to be most common and most severe during summer and autumn, in drought years, and particularly in El Nino years. In the north of Australia, bushfires usually occur during winter (the dry season), and fire severity tends to be more associated with seasonal growth patterns. In the southwest, similarly, bushfires occur in the summer dry season and severity is usually related to seasonal growth. Fire frequency in the north is difficult to assess, as the vast majority of fires are deliberately started by humans. Plants have evolved a variety of strategies to survive fires, (possessing reserve shoots that sprout after a fire, or developing fire-resistant or fire-triggered seeds) or even encourage fire (eucalypts contain flammable oils in the leaves) as a way to eliminate competition from less fire-tolerant species. It is also a method of reproduction for eucalypts as their seed pods explode in the intense heat. Many native animals are also adept at surviving bushfires.