transition zone

from The Collaborative International Dictionary of English v.0.48

Transition zone \Tran*si"tion zone\ (Biogeography)
   The zone lying between the Boreal and Sonoran zones of North
   America. It includes an eastern or humid subdivision and a
   western arid one of corresponding temperature comprising the
   northern Great Plains and the lower slopes of the mountains
   of the western United States and Mexico. Called also {Neutral
   zone}.
   [Webster 1913 Suppl.]