Populus tremuloides mortality near the southwestern edge of its range

Mortality and crown dieback of quaking aspen (Populus tremuloides) were extensive on the Williams Ranger District, Kaibab National Forest in northern Arizona. We collected data from a random sample of 48 aspen sites to determine the relationship of predisposing site and stand factors and contributing agents to ramet mortality. Mortality of overstory (≥10.1. cm DBH) aspen stems averaged 50% (44% by basal area). Pine-oak type aspen stands suffered greater basal area mortality (57%) than stands in mixed conifer type (38%). An average of 48% of live overstory aspen stems had. >33% crown dieback, and mortality significantly increased with increasing crown dieback. Based upon univariate relationships, elevation was the most significant site factor and relative conifer basal area was the most significant stand factor related to overstory mortality. Canker diseases and wood-boring insects were significantly related to overstory mortality. Sapling (≥5.1-10.1. cm DBH) and tall regeneration (<5.1. cm DBH) aspen mortality were high (>80% and 70%, respectively), while short regeneration (<1.37. m tall) mortality was low (16%). Many sites did not have live aspen sapling or tall regeneration stems; therefore, relationships were often inconclusive or weak. Based on a null size-density model, there was a lack of tall regeneration, sapling, and 10.1-15. cm DBH overstory aspen recruitment. Multiple regression was used to explore multivariate relationships among aspen mortality and site, stand, and damaging agent factors. Forest type, relative conifer basal area, and incidence of canker diseases and wood-boring insects were significantly associated with overstory mortality. Slope, relative conifer density, and incidence of animal damages were significantly associated with short regeneration mortality. Ungulate damages to aspen stems were common across all size classes, but significant relationships were limited to short regeneration mortality. The Southwest is forecasted to transition to a more arid climate, and aspen in pine-oak sites are already experiencing a population crash. If high mortality and low recruitment continues, conifer will replace aspen stands after overstory aspen stems die.