frequent stand replacing wildfires. Canopy openings Forest canopy gaps are essential in creating and maintaining mixed-age stands. Also, some herbaceous plants only become established in canopy openings, but persist beneath an understory. Openings are a result of tree death due to small impact disturbances such as wind, low-intensity fires, and tree diseases. Old-growth forests are unique, usually having multiple horizontal layers of vegetation representing a variety of tree species, age classes, and sizes, as well as "pit and mound" soil shape with well-established fungal nets. Because old-growth forest is structurally diverse, it provides higher-diversity habitat than forests in other stages. Thus, sometimes higher biological diversity can be sustained in old-growth forests, or at least a biodiversity that is different from other forest stages. Topography The characteristic topography of much old-growth forest consists of pits and mounds. Mounds are caused by decaying fallen trees, and pits (tree throws) by the roots pulled out of the ground when trees fall due to natural causes, including being pushed over by animals. Pits expose humus-poor, mineral-rich soil and often collect moisture and fallen leaves, forming a thick organic layer that is able to nurture certain types of organisms. Mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive. Standing snags Standing snags provide food sources and habitat for many types of organisms. In particular, many species of dead-wood predators such as woodpeckers must have standing snags available for feeding. In North America, the spotted owl is well known for needing standing snags for nesting habitat. Decaying ground layer Fallen timber, or coarse woody debris, contributes carbon-rich organic matter directly to the soil, providing a substrate for mosses, fungi, and seedlings, and creating microhabitats by creating relief on the forest floor. In some ecosystems such as the temperate rain forest of the North American Pacific coast, fallen timber may become nurse logs, providing a substrate for seedling trees. Soil Intact soils harbor many life forms that rely on them. Intact soils generally have very well-defined horizons, or soil profiles. Different organisms may need certain well-defined soil horizons to live, while many trees need well-structured soils free of disturbance to thrive. Some herbaceous plants in northern hardwood forests must have thick duff layers (which are part of the soil profile). Fungal ecosystems are essential for efficient in-situ recycling of nutrients back into the entire ecosystem. Definitions Ecological definitions Stand age