Subalpine tree seedlings: Assessing aging methodology and drivers of establishment

Abstract 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.