Grassland vegetation is either the most widespread vegetation type or an otherwise significant vegetation type in all four inventory and monitoring networks of the American Southwest. For example, in the Southern Plains Network it is the most widespread vegetation type, and in the Chihuahuan Desert Network it comprises about 30% of the land area. Grasses are uniquely able to thrive without the benefit of a canopy and with major disturbance processes such as drought, fire, and grazing. In contrast to most plants, which grow at their tips, grasses grow from their base and have an extensive root system. Sensitive growth tissues remain below the soil and are protected from disturbance. The extensive root system allows for regrowth after disturbances and can extract moisture deep below ground during time of drought. Exotic species invasions, overgrazing, fire suppression, and soil erosion have all contributed to arid grassland degradation.