Primary succession occurs when a major, catastrophic disturbance (glacial retreat, volcanic eruptions, etc.) destroys all the plants in an area, allowing other species to colonize. The initial colonists on a severely disturbed site vary depending on the nature of the substrate: smooth, bare rocks will attract lichens, gravel will lure perennial herbs, and sand will be colonized by grasses. These plants increase the nitrogen content of the soil, attracting legumes and other nitrogen-fixers, which further grow the nitrogen pools. Normally, mature ecosystems support soil nitrogen pools of 5,000-10,000 kg/hectare in the topsoil, but colonists on a recently disturbed site may have to build up from scratch. Woody plants cannot invade a successional community until the soil holds nitrogen pools of 400-1,200 kg/ha. This can take up to 100 years. Secondary succession occurs when a disturbance (fire, windstorms, etc.) occurs but does not wipe out all the plants in an area, leaving behind a seedbank or a number of vegetative propagules. Short-lived, fast-growing plants achieve dominance first, followed by herbaceous perennials, short-lived trees, and finally slow-growing, long-lived trees. Source: Crawley, M.J. 1997. Plant Ecology. Blackwell Science, Cambridge, MA.