Dendroecological detection of spruce bark beetle outbreaks in northwesternColorado

Episodic outbreaks of Dendroctonus rufipennis (Kirby), the spruce bark beet le, have greatly influenced the structure of subalpine forests in northern Colorado. During the 1940s, much of the subalpine zone of northwestern Colo rado was severely affected by beetle outbreak; also, tree-ring and photogra phic evidence suggest that large-scale outbreaks occurred in the 19th centu ry. The present study focused on tree-ring methods to examine the regional extent and synchrony of pre-20th-century beetle outbreaks in northwestern C olorado. Results from examination of both live and dead Engelmann spruce (P icea engelmannii Parry ex Engelm.) tree rings in nine stands were compared with results of previous tree-ring studies in the same region. Evidence of past canopy disturbance included episodes of tree mortality in conjunction with sustained increases in radial growth rates. We identified regional out breaks of spruce beetle by synchronous and sustained growth release in tree s from disjunct stands. These new tree-ring records, along with previously published records, indicate that severe and widespread canopy disturbances, probably spruce beetle outbreaks, affected northwestern Colorado in 1716-1 750, 1827-1845, 1860-1870, and 1940-1960. These results support earlier fin dings that large-scale outbreaks of spruce beetle have long been an importa nt component of the dynamics of subalpine forests in Colorado.