Spatial interactions among short‐interval fires reshape forest landscapes

Abstract

Aim Ecological disturbances are increasing as climate warms, and how multiple disturbances interact spatially to drive landscape change is poorly understood. We quantified burn severity across fire regimes in reburned forest landscapes to ask how spatial patterns of high‐severity fire differ between sequential overlapping fires and how landscape heterogeneity is shaped by cumulative disturbance patterns. We also characterized the amount and configuration of an emerging phenomenon: areas burned as high‐severity fire twice in successive fires. Location Northwest USA. Time period 1984–2020. Major taxa studied Forests of western continental USA. Methods We used a field‐calibrated atlas of satellite‐measured burn severity across diverse fire regimes (more than three decades, >200 short‐interval fires) to quantify landscape metrics of high‐severity (>75% tree mortality) fire in sequential overlapping short‐interval fires. We used generalized linear models to test differences in individual and cumulative landscape patterns of burn severity following the first and second fires. Results The amount of severe wildfire and patch size/configuration were generally similar between successive overlapping fires and across fire regimes. However, overlapping individual fires produced cumulative landscape patterns of recent high‐severity fire that were consistently more homogeneous after two fires, with greater distances to remaining mature forest. Additionally, 19–25% of landscapes affected by short‐interval fires burned at high severity in both fires, highlighting the spatial extent of repeatedly and severely disturbed forests. Main conclusions When two individually heterogeneous fires overlap, burn mosaics can fit together like puzzle pieces, whereby twice‐burned landscapes are composed of large and simple‐shaped patches of cumulative recent high‐severity fire interspersed with small patches of mature/old forest. These cumulative spatial outcomes of interacting disturbances can be mechanisms of shifting ecosystem dynamics as global change unfolds and reburns continue.

Few large or many small fires: Using spatial scaling of severe fire to quantify effects of fire‐size distribution shifts

“…Our calculation of RdNBR included an offset term to account for phenological differences between pre‐ and postfire imagery and to facilitate comparison of RdNBR between fire events (Parks, Holsinger, Voss, et al, 2018b). Following Harvey et al (2023), we used statistical models calibrated to northwestern United States field plots (Saberi & Harvey, 2023) to identify a threshold of RdNBR (RdNBR ≥ 542) corresponding to ≥75% tree basal area mortality. We then used this threshold to categorize each burn‐severity map into high (RdNBR ≥ 542) and low‐to‐moderate (RdNBR < 542) burn‐severity classes.…”

Less fuel for the next fire? Short‐interval fire delays forest recovery and interacting drivers amplify effects

“…Stand‐level percent serotiny of lodgepole pine is highest at lower elevations (up to ~2300–2400 m) and ranges widely (0 to more than 85% of trees with serotinous cones; Schoennagel et al, 2003; Tinker et al, 1994). Approximately one‐third of 1984–2010 area burned in United States Northern Rocky Mountains subalpine forests was stand‐replacing (Harvey et al, 2016a), and 19%–25% of 1984–2020 short‐interval area burned in Northwest United States forests was stand‐replacing in both the initial and subsequent fire (Harvey et al, 2023). Mean aboveground biomass in lodgepole pine‐dominated forests averages 139 Mg ha −1 (live tree) and 98 Mg ha −1 (dead woody) across a 300‐year chronosequence, and stand density stabilizes to approximately 1200 stems ha −1 after 200 years of stand development (Kashian et al, 2013; Kashian, Turner, & Romme, 2005).…”

“…Independent effects of short-interval reburns and long distances to live forest on post-fire conifer densities are well documented (Stevens-Rumann & Morgan, 2019), and here we found amplifying interactions. This is particularly concerning in western United States forests, where current trends in area burned and stand-replacing fire indicate that these two conditions will co-occur with increasing likelihood (Buma et al, 2020;Harvey et al, 2016aHarvey et al, , 2023Westerling, 2016). Climate-driven increases in other agents of tree mortality, such as bark beetles, wind, drought, and pathogens (Anderegg et al, 2020;Seidl et al, 2017), could further reduce seed source availability throughout forest landscapes and exacerbate impacts on post-fire regeneration (Coop et al, 2020).…”

Consistent spatial scaling of high‐severity wildfire can inform expected future patterns of burn severity

“…We included an offset term in our calculation of RdNBR to account for phenological differences between pre‐ and postfire imagery (Parks, Holsinger, Voss, et al, 2018). Following Harvey et al (2023), we used statistical models calibrated to Northwest United States field plots (Saberi & Harvey, 2023) to identify a threshold of RdNBR (RdNBR ≥542) corresponding to ≥75% tree basal area mortality. We then used this threshold to categorize each burn severity map into high (RdNBR ≥542) and low‐to‐moderate (RdNBR <542) burn severity classes.…”