ecotone
n.— «In most places under natural conditions there seems to be an ecotone, a place of equal aggressiveness, betwen this formation and the Bouteloua formation.» —“A Study of the Vegetation of the Mesa Region East of Pike’s Peak: The Bouteloua Formation. II. Development of the Formation” by H.L. Shantz Botanical Gazette Sept., 1906. (source: Double-Tongued Dictionary)