Still standing: Recent patterns of post-fire conifer refugia in ponderosa pine-dominated forests of the Colorado Front Range

Abstract

Forested fire refugia (trees that survive fires) are important disturbance legacies that provide seed sources for post-fire regeneration. Conifer regeneration has been limited following some recent western fires, particularly in ponderosa pine (Pinus ponderosa) forests. However, the extent, characteristics, and predictability of ponderosa pine fire refugia are largely unknown. Within 23 fires in ponderosa pine-dominated forests of the Colorado Front Range (1996-2013), we evaluated the spatial characteristics and predictability of refugia: first using Monitoring Trends in Burn Severity (MTBS) burn severity metrics, then using landscape variables (topography, weather, anthropogenic factors, and pre-fire forest cover). Using 1-m resolution aerial imagery, we created a binary variable of post-fire conifer presence ('Conifer Refugia') and absence ('Conifer Absence') within 30-m grid cells. We found that maximum patch size of Conifer Absence was positively correlated with fire size, and 38% of the burned area was � 50m from a conifer seed source, revealing a management challenge as fire sizes increase with warming further limiting conifer recovery. In predicting Conifer Refugia with two MTBS-produced databases, thematic burn severity classes (TBSC) and continuous Relative differenced Normalized Burn Ratio (RdNBR) values, Conifer Absence was high in previously forested areas of Low and Moderate burn severity classes in TBSC. RdNBR more accurately identified post-fire conifer survivorship. In predicting Conifer Refugia with landscape variables, Conifer Refugia were less likely during burn days with high maximum temperatures: while Conifer Refugia were more likely on moister soils and closer to higher order streams, homes, and roads; and on less rugged, valley topography. Importantly, pre-fire forest canopy cover was not strongly associated with Conifer Refugia. This study further informs forest management by mapping post-fire patches lacking conifer seed sources, validating the use of RdNBR for fire refugia, and detecting abiotic and topographic variables that may promote conifer refugia.

A changing climate is snuffing out post‐fire recovery in montane forests

“…Notably, we found that HLI and TPI may help to identify topographic conditions that moderate the influence of broad-scale climate on tree seedlings. Similar terrain variables also help to predict the locations in which trees are most likely to survive fire (Chapman et al 2020;Meigs et al, 2020 Across a range of GCMs and emissions scenarios, we projected that further declines in post-fire seedling densities are likely across much of the SRME. It has been suggested that future warming will reduce conifer seedling establishment and density in many areas, but that increasing temperatures could benefit regeneration at the leading edge (Kemp et al, 2019;Petrie et al, 2017).…”

“…Across 23 recent fires in the montane zone of the SRME, 38% of the total area burned is distant from any surviving trees (Chapman et al, 2020). A lack of mature trees also leads to increased daytime temperatures and greater diurnal fluctuations (Davis et al, 2018), with potential impacts on tree seedlings.…”

Influence of topography and fuels on fire refugia probability under varying fire weather conditions in forests of the Pacific Northwest, USA

“…Quantifying burn severity with satellite imagery presents multiple challenges, including spatial variability (e.g., subpixel fire effects), temporal variability (e.g., delayed tree mortality), and the inherent disconnect between remote and ground-based metrics of burn severity (Morgan et al 2014;Dunn and Bailey 2016;Harvey et al 2019). Nevertheless, Landsat-based RdNBR mapping is valuable as a relative indicator of fire-induced change across numerous fire events spanning heterogeneous conditions, particularly when interpreted in the context of field-measured fire effects such as tree mortality (Reilly et al 2017;Chapman et al 2020). We recognize that the fire refugia threshold of 10% BA mortality is subjective, and future studies could test other refugia thresholds or leverage additional spectral information in Landsat imagery (Meddens et al 2016;Collins et al 2019), as well as finer-resolution satellite and aerial imagery (Walker et al 2019;Chapman et al 2020).…”

“…Nevertheless, Landsat-based RdNBR mapping is valuable as a relative indicator of fire-induced change across numerous fire events spanning heterogeneous conditions, particularly when interpreted in the context of field-measured fire effects such as tree mortality (Reilly et al 2017;Chapman et al 2020). We recognize that the fire refugia threshold of 10% BA mortality is subjective, and future studies could test other refugia thresholds or leverage additional spectral information in Landsat imagery (Meddens et al 2016;Collins et al 2019), as well as finer-resolution satellite and aerial imagery (Walker et al 2019;Chapman et al 2020). Future studies could also integrate field observations to distinguish low-severity from truly unburned refugia (Meddens et al 2016) and quantify the distinctive composition and structure in old forests, particularly in the West Cascades where large, fireresistant Douglas-fir trees are prevalent.…”

“…Recent studies have mapped fire refugia patterns and quantified drivers with large geospatial databases and innovative quantitative approaches. Mapping studies have typically employed Landsat imagery to identify fire refugia within recent fire events as locations including both unburned and low-severity fire effects where fire resulted in low mortality to dominant trees (e.g., Krawchuk et al 2016;Meddens et al 2016;Haire et al 2017;Meigs and Krawchuk 2018;Collins et al 2019;Walker et al 2019;Chapman et al 2020). Functionally, these remote sensing approaches identify fire refugia as locations exhibiting minimal spectral change relative to the broader burn mosaic.…”