DOI: 10.5061/dryad.9s4mw6mhj
title: Conifer seedling demography reveals mechanisms of initial forest resilience to wildfires in the northern Rocky Mountains

description: Climate warming and an increased frequency and severity of wildfires are
 expected to transform forest ecosystems, in part through altered post-fire
 vegetation trajectories. Such a loss of forest resilience to wildfires
 arises due to a failure to pass through one or more critical demographic
 stages, or “filters,” including seed availability, germination,
 establishment, and survival. Here we quantify the relative influence of
 microclimate and microsite conditions on key stages of post-fire seedling
 demography in two large, lightning-ignited wildfires from the regionally
 extensive fire season of 2017 in the northern Rocky Mountains, U.S.A. We
 tracked conifer seedling density, survival, and growth in the first three
 years post-fire in 69 plots spanning gradients in fire severity,
 topography, and climate; all plots were limited to within 100 m of a seed
 source to assure seed availability. Microclimate conditions were inferred
 based on measurements in a subset of 46 plots. We found abundant post-fire
 conifer regeneration, with a median of 2,633 seedlings per hectare after
 three years, highlighting early resilience to wildfire. This robust
 regeneration was due in part to moderate post-fire climate conditions,
 supporting high survivorship (&gt;50% on average) of all seedlings
 tracked over the study period (n = 763). A statistical model based on
 variables describing potential seed availability, microclimate, fire
 severity, understory vegetation, and soil nitrogen availability explained
 75% of the variability in seedling density among plots. This analysis
 highlights the overarching importance of fine-scale heterogeneity in fire
 effects, which determine microclimate conditions and create diverse
 microsites for seedlings, ultimately facilitating post-fire tree
 regeneration. Our study elucidates mechanisms of forest resilience to
 wildfires and demonstrates the utility of a demographic perspective for
 anticipating forest responses to future wildfires under changing
 environmental conditions.
description: These data were collected in 69 stratified-random field plots in
 two recent wildfires in the northern Rocky Mountains, USA over the first
 three growing seasons post-fire. Plots spanned gradients in fire severity
 (unburned, moderate, high) and biophysical setting (via elevation and
 aspect), and were limited to within 100 m of a live tree to ensure seed
 availability. Variable-width belt transects were used to sample post-fire
 conifer seedling regeneration, and permanent
 1-m<sup>2</sup> subplots were established to monitor the
 vertical growth and survival of individual seedlings. Plots were revisited
 annually over the first three years post-fire to measure seedling density
 within transects and monitor individual seedlings in subplots. Additional
 plot measurements were collected, including: overstory tree density and
 basal area, ground cover and canopy cover, and soil nitrogen content and
 availability. In addition, near-ground microclimate was monitored in a
 subset of sites using data loggers and used to develop statistical models
 to predict microclimate conditions across all sites and sample years. The
 attached files include both raw data and derived data (e.g., plot
 averages, modeled microclimate estimates); see README file for more
 information. 
description: See "README_for_ClarkWolf_et_al_2022_Data.docx" for
 metadata information.