What is an alliance?

A category of vegetation classification which describes repeating patterns of plants across a landscape. Each alliance is defined by plant species composition, and reflects the effects of local climate, soil, water, disturbance, and other environmental factors. For example, Ceanothus cordulatus (Mountain Whitethorn) Shrubland Alliance commonly occurs in the mountains of California after fire or other disturbance, and stands can transition to montane and subalpine forests of conifers that overtop the shrubs in time. Alliances are commonly used in vegetation mapping.