The climate of Rabbit Ears Pass is generally semi-arid to humid. Vegetation consists mostly of pine, spruce, and aspen trees. Sagebrush grows on the Niobrara, Pierre and Coalmount formations. Undergrowth is dense in most places. Like many places in the West, Rabbit ears Pass experienced a decline in Englemann spruce (Picea engelmannii) trees due to episodic outbreaks of the spruce bark beetle (Dendroctonus rufipennis). During the 1940s much of the subalpine zone of Northwest Colorado was severely affected by beetle outbreak. Spruce beetle outbreaks have taken place in this area in 1716-1750, 1827-1845, 1860-1870, and 1940-1960. Large-scale outbreaks of spruce beetle have long been an important component of the dynamics of subalpine forests in Colorado. During outbreaks, spruce beetle may attack and kill lodgepole pine (Pinus contorta) as well as spruce. Nearly all attacked trees are killed