Northern Arizona University, M.S.
Crown dieback and mortality of quaking or trembling aspen (Populus tremuloides) were extensive within pine-oak and mixed conifer forest types of the Williams Ranger District, Kaibab National Forest in northern Arizona. I collected data from 48 aspen sites to determine if predisposing site and stand factors and contributing damaging agents were associated with aspen crown dieback and mortality. Overstory aspen mortality averaged 50% by stems per hectare and 44% by basal area. Based upon univariate relationships, elevation was the most significant site factor related to both overstory aspen crown dieback (R2 = 0.15, P = 0.0069) and overstory aspen mortality (R2 = 0.24, P = 0.0004). The most significant stand factor related to crown dieback was live aspen density (R2 = 0.18, P = 0.0029), while percent conifer (R2 = 0.45, P < 0.0001) was the most significant stand factor related to mortality. Canker diseases, wood-boring insects, and animal damages were common in the overstory size class. The significant damaging agents in relation to both overstory crown dieback and mortality were canker diseases (R2 = 0.13, P = 0.0123; R2 = 0.18, P = 0.0028, respectively) and wood-boring insects (R2 = 0.24, P = 0.0005; R2 = 0.56, P < 0.0001, respectively). Sapling and tall sucker aspen mortality were high (> 80 and 70%, respectively), while short sucker mortality was low (16%). Many sites did not have live aspen regeneration, therefore, sample sizes were low, and relationships were often inconclusive or weak. Animal damages and canker diseases were common in the sapling and tall sucker size classes. Only animal damages were common in the short sucker size class. Among damaging agents and regeneration size classes, the only significant univariate relationship found was between animal damages and short sucker aspen mortality (R2 = 0.15, P = 0.0198). Based on a negative exponential diameter distribution, there was lack of aspen recruitment in saplings and small diameter overstory stems. If high mortality and low recruitment continues, aspen stands will be replaced by conifer after larger, and presumably older, overstory aspen stems die. The multivariate relationships of overstory aspen crown dieback, overstory aspen mortality, and short sucker aspen mortality among site, stand, and damaging agent factors were explored using step-wise multiple regression. The significant multivariate associations with overstory aspen crown dieback were elevation (F1,44 = 16.38, P = 0.0002) and incidence of canker diseases (F1,44 = 15.02, P = 0.0004). The significant factors explaining the variation in overstory aspen mortality were forest type (F1,43 = 5.92, P = 0.0192), overstory percent conifer (F1,43 = 8.24, P = 0.0063), and incidence of canker diseases (F1,43 = 33.05, P < 0.0001), and wood-boring insects (F1,43 = 33.29, P < 0.0001). The significant factors explaining the variation in short sucker aspen mortality were slope (F1,31 = 4.90, P = 0.0344), short sucker percent conifer (F1,31 = 5.00, P = 0.0327), and incidence of animal damages (F1,31 = 6.85, P = 0.0136). According to previous research, ungulate herbivores contribute to aspen decline in northern Arizona by causing damage to aspen regeneration. Ungulate damages were common in all size classes (between 49 and 66%), but significant relationships were limited to short sucker aspen mortality. No data were collected from within ungulate exclosures in this study. Controlled experiments inside and outside of ungulate exclosures are needed to determine the impact of ungulates.
Zegler, Thomas J. 2011. Aspen crown dieback and mortality on the Williams Ranger District, Kaibab National Forest, Arizona. Master of Science Thesis. Northern Arizona University, Flagstaff