Spatial interactions among short-interval fires reshape forest landscapes

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.