Mary Lou Fairweather
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Aspen Decline in Northern Arizona
Mary Lou Fairweather, Plant Pathologist, Forest Health Protection, USDA Forest Service, Flagstaff, Arizona, Coauthors Kelly Barton and Mike Manthei
An accelerated decline of aspen communities has recently occurred across northern Arizona, following two defoliation events and several years of drought. The Southwestern Region’s 1998 aerial detection survey showed a doubling of defoliated aspen acres (>85,000), compared to previous years, followed by another doubling in 1999 (>170,000). Although 1998 defoliation was caused by foliar insects and pathogens, the 1999 defoliation was due to frost damage following a severe June snowstorm, which occurred across most of Northern Arizona. Since 2000, aerial surveyors have switched from reporting acres defoliated to acres in decline, the latter marked by thinning crowns and mortality. Many small lower elevation clones appear completely dead. We began evaluation and monitoring of affected aspen at the stand level on the Coconino National Forest (NF) in 2003, followed by the Apache-Sitgreaves NF in 2004. This monitoring project describes mortality levels, regeneration condition, and stand and site variables that are influencing decline. Preliminary results show that elevation, which varies from 6,800 to 9,300 feet, is a key factor in both tree species composition and severity of decline. Lower elevation sites (<7,500 feet) are on northerly aspects and are dominated by aspen with a ponderosa pine and oak component. In some sites, more than half the overstory aspen died in the past 3 years and more mortality is expected as many trees have only 10 to 30 percent of the original crown left. Higher elevation sites were on various aspects with a mix of conifer species and a higher ratio of live to dead aspen. Since decline appears to be progressive, remeasurements will determine detrimental levels of crown dieback. Aspen regeneration is slight, especially at lower elevations, but ungulate damage is rampant. A few sites had 100 percent browse damage. The large die-off of mature aspen trees in many lower elevation sites coupled with browsing by ungulates is expected to result in type conversion of many ecologically unique and important sites across the state.