Floristic provinces

desert in northwestern Mexico and the southwestern United States

thumb|240px|Macaronesia

large depression in western United States

In phytogeography, a phytochorion is a geographic area with a relatively uniform composition of plant species. Adjacent phytochoria do not usually have a sharp boundary, but rather a soft one, a transitional area in which many species from both regions overlap, called a vegetation tension zone, or ecotone.