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Seedlings? The unexpected elders of understory trees - Final Report to the Joint Fire Science Program

Author(s): Zoe Schapira, Camille Stevens-Rumann
Year Published: 2020

Over the past several decades, the impacts of climate change have threatened the health and functioning of forested ecosystems on a global scale. Warming and drying trends have altered disturbance regimes and have created significant uncertainty about tree regeneration and postdisturbance recovery in subalpine ecosystems. Aging seedlings is an important mechanism for ascertaining establishment dates of tree seedlings in disturbed stands, 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, including height class and terminal bud scar count, and seedling age to assess the accuracy of age proxies in predicting true age of seedlings from disturbed stands. We also compare climatic conditions in years of widespread tree establishment to years with no establishment 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 taller and older seedlings, and seedlings from beetle-affected stands that exhibited suppressed growth. Growing season minimum temperatures and precipitation, and annual vapor pressure and climatic water deficits were significantly correlated with spruce-fir establishment. Height and terminal bud scar counts do not accurately predict precise ages of subalpine tree establishment from beetle-affected stands; however, terminal bud scar counts provide more accuracy in postfire tree establishment. Average climate may provide suitable conditions for low-levels of semi-continuous tree establishment in beetle-affected stands. However, large spruce-fir establishment pulses occur in cooler and wetter growing years compared to the long-term average, posing significant uncertainty about post-disturbance subalpine recovery with continued drying and warming trends.

Citation: Schapira, Zoe; Stevens-Rumann, Camille. 2020. Seedlings? The unexpected elders of understory trees - Final Report to the Joint Fire Science Program. JFSP Project No. 19-1-01-59. Fort Collins, CO: Colorado State University. 17 p.
Topic(s): Fire Ecology, Fire Effects, Fire & Climate
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 22546
FRAMES RCS number: 62664
Record updated: Feb 2, 2021