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

Teresa B Chapman,Thomas T Veblen,Kyle C Rodman,Tania Schoennagel,Angela Marie White

doi:10.1371/journal.pone.0226926

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.

Highlights

The number of large wildfires in the western US has increased in recent decades [1]
Post-fire recovery in the northern Rockies might not be impacted by recent increases in fire severity due to the consistent proportion of fire refugia within wildfires [34], we found that in the CFR, the spatial arrangement of forested fire refugia is a key component to evaluating the potential resilience to fire for ponderosa pine
We found that post-fire conifer refugia are not consistently detected by Monitoring Trends in Burn Severity (MTBS) thematic burn severity classes (TBSC) and that these classes generally under-estimated conifer loss in the ponderosa pine cover type in our region

Summary

Introduction

The number of large wildfires in the western US has increased in recent decades [1]. Weather conducive to large wildfires in forests has become more extreme and more common over the past few decades due to anthropogenic climate change [2], and a trend toward larger, more severe fires is expected with further warming [3,4]. Larger fires tend to have larger patches of high-severity fire, leaving contiguous expanses without surviving trees [4,5] In combination with these trends, many dry coniferous forests of the western US with a historically low-severity, high-frequency fire regime show a marked increase in tree density over the 20th century, increasing the potential for high-severity fires [6,7,8,9,10,11]. Identifying indicators of potential shifts in states is critical to understanding forest resilience, defined as the system’s ability to absorb a disturbance and not fundamentally shift to another state governed by a different set of processes [16] Disturbance legacies, such as individuals that survive and persist in the landscape following a disturbance, leave valuable material like seeds and microsites for disturbance recovery [17]. A better understanding of conditions that influence disturbance legacies is necessary to evaluate potential forest resilience to wildfire

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