Rapid mortality of Populus tremuloides in southwestern Colorado, USA

Abstract

Concentrated patches of recent trembling aspen ( Populus tremuloides) mortality covered 56,091 ha of Colorado forests in 2006. Mortality has progressed rapidly. Area affected increased 58% between 2005 and 2006 on the Mancos-Dolores Ranger District, San Juan National Forest, where it equaled nearly 10% of the aspen cover type. In four stands that were measured twice, incidence of mortality increased from 7–9% in 2002/2003 to 31–60% in 2006. Mortality generally decreased with increasing elevation over the primary elevation range of aspen and occurred on less steep slopes than healthy aspen. Slope-weighted mean aspects of aspen cover type were northern at low elevations and generally southern at high elevations. Relative frequency of mortality was generally highest on southern to western aspects. In 31 stands measured in detail, mortality ranged from 0 to 100% (mean 32%) and was negatively correlated with stand density ( P = 0.033). Size of trees affected was strongly correlated with amount of current mortality ( P < 0.001), and current mortality was skewed toward larger diameter classes. Density of regeneration was in a low range typical of undisturbed stands and did not increase with overstory mortality. Agents that typically kill mature trees in aspen stands were unimportant in this mortality. Instead, a group of interchangeable, usually secondary agents was most commonly associated with mortality, including Cytospora canker (usually caused by Valsa sordida), aspen bark beetles ( Trypophloeus populi and Procryphalus mucronatus), poplar borer ( Saperda calcarata), and bronze poplar borer ( Agrilus liragus). The rapidity of mortality, mortality agents involved, and probably other causal factors distinguish this phenomenon from the long-term loss of aspen cover usually attributed to successional processes operating in an altered disturbance regime (and often exacerbated by ungulate browsing). Our data are consistent with a hypothesis that (a) predisposing factors include stand maturation, low density, southern aspects and low elevations; (b) a major inciting factor was the recent, acute drought accompanied by high temperatures, and; (c) contributing factors and proximate agents of mortality are the common biotic agents observed. On sites with poor regeneration and weak root systems, clones may die, resulting in the long-term loss of aspen forest cover.