A Tree-Ring Reconstruction of Western Spruce Budworm History in the Southern Rocky Mountains

Washington D.C.

1989

S. 962-986

7 Abb., 5 Tab., zahlr. Lit. Ang.

Bandaufführung

64480

Tree ring width chronologies from ten mixed-conifer stands in the Colorado Front Range and New Mexico Sangre de Cristo Mountains were used to reconstruct the timing, duration, and radial growth impact of past outbreaks of western spruce budworm (Choristoneura occidentalis Freeman). At least nine outbreaks were identified in the stands from 1700 to 1983. Severity and timing of outbreaks was highly variable. The average duration of reduced growth periods caused by budworms was 12.9 years and ranged from 5 to 26 years. The average interval between initial years of successive outbreaks was 34.9 years and ranged from 14 to 58 years. The average maximum and periodic radial growth reductions were 50% and 21.7%, respectively. There was a relatively long period of reduced budworm activity in the first few decades of the 20th century, and since that time outbreaks have been markedly more synchronous among the sampled stands. It was hypothesized that the increased synchroneity of outbreaks in the latter half of the 20th century is due to changes in age structure and species composition following harvesting and fire suppression in the late 19th and early 20th centuries.