James J. Worrall
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Sudden Aspen Decline in Southwestern Colorado
James J. Worrall, Roy A. Mask, Thomas Eager and Leanne Egeland, USDA Forest Service, Gunnison, CO, and Wayne D. Shepperd, USDA Forest Service, Fort Collins, CO (retired)
Sudden aspen decline (SAD) has increased rapidly in recent years, approaching 350,000 acres in Colorado in 2007, or 13% of aspen cover type. We documented landscape and stand factors associated with SAD. There was a strong inverse relationship between elevation and damage. Damage tended to occur on south and southwest aspects and was most severe in open stands with large trees. Regeneration was poor in damaged stands. Five biotic agents were most frequently associated with SAD: Cytospora canker, poplar borer, bronze poplar borer, and two bark beetles. We proposed a causal hypothesis in a decline context: predisposing factors are low elevations, south/west aspects, low density, and stand maturity; inciting factors are warm drought conditions; contributing factors are the secondary, biotic agents mentioned above. We then conducted an intensive field survey with 76 plots on four National Forests. Preliminary analyses indicate that 1) regeneration has not responded significantly to crown loss, 2) root mortality varied from 0 to over 90% of root volume and was correlated with crown loss, and damaged plots had significantly higher volume of dead roots than healthy plots, 3) regeneration decreased significantly as root mortality increased in damaged plots, but not in healthy plots, 4) crown loss did not vary significantly with depth of soil mollic layer, and 5) crown loss did not vary significantly with average or oldest age of sampled codominant/dominant trees. The rapidity of mortality, landscape scale, mortality agents involved, and probably other causal factors distinguish SAD from the long-term loss of aspen cover caused by successional processes operating in an altered fire regime (and often exacerbated by ungulate browsing). There are significant management implications and there may be loss of aspen cover type where aspen stands are declining and regeneration is inadequate. Marginal regeneration may be further compromised by such factors as amount and duration of ungulate browsing.