The Response of Subalpine Forests to Spruce Beetle Outbreak in Colorado

Spruce beetle (Dendroctonus rufipennis Kirby) outbreaks are important disturbances affecting subalpine forests of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern Rocky Mountains. However, little is known about the influences of these outbreaks on overall forest dynamics. We used age—structure analyses and dendrochronological techniques to investigate the effects of a major spruce beetle outbreak on stand composition, dominance, tree age and size structures, radial growth, and succession in subalpine forests in Colorado. This outbreak, which occurred in the 1940s, caused a shift in dominance from spruce to fir and a reduction in average and maximum tree diameters, heights, and ages. The outbreak did not favor new seedling establishment of the seral lodgepole pine. Thus, in seral stands spruce beetle outbreak accelerates succession towards the shade—tolerant tree species. The predominant response to the outbreak was the release of previously suppressed small—diameter spruce (not attacked by the beetle) and subalpine fir (a non—host species). Following the 1940s outbreak, growth rates of released trees remained high for >40 yr. The relative increases in growth rates were similar for both species. Both spruce and fir will continue to codominate the affected stands. The predominance of accelerated growth following a spruce beetle outbreak, instead of new seedling establishment, is a major contrast to the pattern of stand development following fire. In some Colorado subalpine forests the effects of disturbance by spruce beetle outbreaks appear to be as great as those due to fire.