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The recent impacts of climate change have threatened the health and functioning of forested ecosystems on a global scale. Widespread tree mortality from altered disturbance regimes creates significant uncertainty about tree recovery in subalpine ecosystems. Aging seedlings is an important mechanism for understanding tree regeneration, and when paired with long-term climate, can provide critical information on climatic drivers of subalpine tree establishment. In this study, we destructively sampled and aged 229 Picea engelmannii (Engelmann spruce) and Abies lasiocarpa (subalpine fir) seedlings from beetle-affected and postfire subalpine stands in northern Colorado and southern Wyoming. We modeled the relationship between nondestructive aging methods and seedling age to assess the accuracy of age proxies in predicting true age of seedlings. We compared climatic conditions between years with high tree seedling establishment and non-establishment years to ascertain regional drivers of subalpine tree seedling recruitment. Both height and terminal bud scar counts were significant predictors of seedling age, although correlations were weaker in older seedlings that exhibited suppressed growth. Growing season precipitation had a significant positive relationship with spruce-fir establishment while minimum temperatures, annual vapor pressure and climatic water deficits had significant negative correlations with subalpine tree establishment. Height and terminal bud scar counts do not accurately predict precise ages of subalpine tree establishment from beetle-affected stands but provide more accuracy in postfire tree establishment. Average climate conditions compared to long-term climate may provide suitable conditions for low-levels of tree establishment in subalpine stands. However, large spruce-fir establishment pulses occur in cooler and wetter growing years compared to the long-term average, posing significant uncertainty about new seedling recruitment in a warming world.

Due to the shifting global climate, the frequency and severity of disturbances are increasing, inevitably causing an increase in disturbances overlapping in time and space. Bark beetle epidemics and wildfires have historically shaped the disturbance regimes of Western North American forests. Their interactive effects on stand dynamics and recovery are inadequately studied in Picea engelmannii (Engelmann spruce)-Abies lasiocarpa (subalpine fir) dominant forests; understanding these interactions is imperative to the management and health of forested ecosystems. This study focuses on the effects of epidemic Dendroctonus rufipennis (spruce beetle) outbreaks, high-severity fires, and the subsequent species and structural diversity of subalpine forest regeneration and structure in Northern Colorado and Southern Wyoming. We compared tree seedling densities and species composition, surface fuel loading, and stand structure characteristics across 80 sites that experienced either high tree mortality from epidemic spruce beetle outbreaks (>50% affected basal area), high-severity wildfire, post-outbreak high-severity wildfire (1–3 years post-outbreak), or no disturbance (control). The beetle outbreak sites span multiple years post-outbreak from 1996 to 2017, ultimately comprising a chronosequence of beetle-affected stands. Analyses indicate a significant increase in fuel loading over time-since-outbreak, as aerial fuels are transferred to the forest floor following high tree mortality. Tree seedling densities among outbreak and control sites differ significantly from burned areas, indicating that wildfires override the effects of repeated disturbances on regeneration. There was consistent Engelmann spruce seedling survival following beetle outbreaks, providing evidence for stable forest recovery following a single disturbance. However, fire was a dominate force in determining post-disturbance species composition, indicating continued prevalence of high severity fire may prove detrimental for the persistence of spruce-fir species, while promoting shifts toward more drought and fire tolerant tree species (e.g., Pinus contorta). It is critical to understand post-disturbance fuel dynamics and stand recovery to identify hazards for subsequent fire suppression, implement treatments to enhance forest resilience, and to understand the potential consequences of climate-induced shifts in disturbance regimes on forest health.