[Is carbon dioxide killing off local aspens, or is it some combination of drought, frost, bugs, fungus, browsing, fire exclusion, etc?]

"Aspen in Arizona have been hit by a multitude of stressing agents over the last decade," said Forest Service Southwest Region Plant Pathologist Mary Lou Fairweather. "In addition to drought, there was a late-season frost event in 1999 and several years of defoliation by the western tent caterpillar. Aspen became so stressed that a canker causing fungus and a wood borer, normally considered opportunistic organisms that only infest weakened trees, were associated with mortality of entire stands of trees."
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Ironically, on a place called Aspen Hill south of Williams, researchers have been unable to find any aspen except in a small grove that has been fenced off from animals, because the deer and elk aren't helping the aspen any either. "We found more than 98 percent of aspen trees surveyed are browsed by ungulates," said forester Mike French of the Kaibab National Forest.
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"Fire exclusion is thought to be one factor in longer term decline of aspen throughout the western U.S. Mature aspen stems produce a hormone, auxin, which prevents the root system from producing new suckers. Fire acts to promote aspen regeneration by removing the mature aspen stems, thus removing the hormonal influence and promoting the growth of new aspen suckers."