The effect of sudden aspen decline on understory microclimate and vegetation in southwestern Colorado

Sudden aspen decline (SAD), present in many parts of North America, is the sudden dieback of branches, crown loss, and rapid mortality of aspen (Populus tremuloides Michx.). We surveyed 21 plots in southwestern Colorado and categorized each plot by the mean percentage of recent crown loss (RCL) into three SAD levels: low SAD (0%-25% RCL), moderate SAD (25.1%-50% RCL), and high SAD (50.1%-100% RCL). Our research quantified the effects of SAD on microclimate and understory vegetation at the individual species and community level. Mean day surface, day subsurface, and night subsurface temperatures were warmer in high SAD stands than in low ones. High SAD stands had lower soil moisture, lower litter and duff depth, higher bare soil cover, higher photosynthetically active radiation, higher arbuscular mycorrhizal propagule densities, and higher grass biomass. Indicator plant species were uniquely associated with low and high SAD. Our study illustrates that SAD has multiple ecological effects on aspen understories, including a potential positive feedback in which warmer temperatures and decreased soil moisture, consequences of SAD, may lead to increased branch dieback and tree mortality, which would alter microclimate-making conditions more favorable to SAD and escalate the effects of SAD on understory vegetation.