Influence of topography, vegetation, weather, and climate on Big-cone Douglas-Fir fire refugia and high fire-induced mortality after two large mixed-severity wildfires

Abstract

Big-cone Douglas-Fir (Pseudotsuga macrocarpa, hereafter BCDF) is an endemic, fire-adapted conifer found throughout the mountains of southern California. Because recent large high intensity wildfires have resulted in loss of BCDF, understanding how environmental factors, such as topography, fuels, climate, and weather, impact BCDF survivorship is important for informing restoration and conservation efforts. Here, we used randomForest (RF) and accumulated local effects (ALE) plots to examine how environmental variables contribute to the occurrence of both fire refugia and high fire-induced mortality of BCDF stands during two large wildfires. Additionally, we explored how the influence of these variables changed between the use of two different response variables: (1) visually-assessed mortality evaluated through estimation of canopy survival using Google Earth imagery and (2) RdNBR. This comparison allows us to evaluate the potential that RdNBR overestimates BCDF mortality because it is highly indicative of understory conditions post-fire, rather than direct changes to BCDF trees. We found that pre-fire fuel was one of the most influential variables contributing to both fire refugia and high mortality; sparse and oak dominant understories contributed to fire refugia, while chaparral contributed to high mortality. We also found that the role of certain variables was not consistent across the two fires. For example, areas of the landscape with hotter temperature and higher vapor pressure deficit (VPD) during the fire experienced high BCDF mortality in the Zaca Fire, but had the inverse effect in the Thomas Fire. Lastly, we found that our two metrics of response resulted in significantly different classification of BCDF stands: RdNBR resulted in more stands being classified as high intensity and fewer low severity/unburned areas, supporting our concern that it can overestimate high severity impact in some ecosystems. However, the two model types resulted in relatively similar explanatory environmental variable selections, although different rankings.

Restoring bigcone Douglas-fir post-fire in drought-stricken Southern California: Assessing the effects of site choice and outplanting strategies

“…Hence restoration in those habitats makes little sense: conditions might appear to be more favorable initially due to somewhat more mesic conditions, but the ultimate fate of bigcone in chaparral vegetation is very poor due to the inevitability of stand-killing fires. By contrast, Parkinson et al (2022) found that association with Q. chrysolepis and with more open understory increased the likelihood of at least partial stand survival during large scale fires. Stands that suffered some mortality during fire but are in these refugia (open understory, or associations with oaks) could be selected as targets for postfire population enhancement through outplanting as done here.…”

“…This study was initiated in response to the Zaca Fire that burned over 97,124 hectares in southern California from July to September 2007 including > 4,000 stands of bigcone Douglasfir (Parkinson et al, 2022). Selection of study sites was confined to the Zaca Fire perimeter.…”

“…Across its range it occurs from within mixed conifer forests to mesic chaparral, and in some cases xeric open shrubland habitats. In more mesic chaparral sites, where chaparral is adjacent to or comprises the understory of bigcone stands, mature bigcones suffer very high mortality during fire (Parkinson et al, 2022). Hence restoration in those habitats makes little sense: conditions might appear to be more favorable initially due to somewhat more mesic conditions, but the ultimate fate of bigcone in chaparral vegetation is very poor due to the inevitability of stand-killing fires.…”