lower sonoran

from The Collaborative International Dictionary of English v.0.48

Sonoran \So*no"ran\, a. (Biogeography)
   Pertaining to or designating the arid division of the Austral
   zone, including the warmer parts of the western United States
   and central Mexico. It is divided into the

   {Upper Sonoran}, which lies next to the Transition zone, and
      the

   {Lower Sonoran}, next to the Tropical.
      [Webster 1913 Suppl.]