At high elevation - physical action king In valley - chemical action prevalent. 1, Attempt of plants to get foothold in physically broken down rocks - 1. moisture probably limiting factor! Study of rock piles - Selaginella invading new ground - grass follows selaginella. climax plants - grass + sage - order of plants - 1. Selaginella 2. short grass (dead in center) Festuca 3. Tall 11 (Agropyron - Stipa - Koeleria) (Temporary climax at least) pioneer - or sub-climax plants rarely maintain themselves in the same locality. Grass tufts dead in center. Wind is a constant factor in the area - vegetation kept in a constant state of suppression by severity of wind, water and grazing. Flexibility of grasses as to growth forms - usually several grasses present