The history and influence of tires was studied at the forest- grassland ecotone in high valleys ofsouthwestern Montana. Inves- tigations were focused upon several sites having early landscape photographs and modern retakes that allow for detection of vege- tational changes. Fire intervals were determined for these sites by analyzing fire scars on trees. Prior to 1910, mean tire intervnls at Pseudofsuga forest-grassland ecotones were 35 to 40 years, and probably shorter in grassland proper. Notires were detected on the study areasafter 1918. Photographiccompnrisonssnd field inspec- tions show a substantial increase in mountain bigsagebrush (Arte- misia tridentata subsp. vaseyana) and conifers since 1900.