Cumulative effects of forest fuel reduction and restoration treatment regimes on horizontal and vertical structure in the Sierra Nevada

In forests adapted to frequent fire, fuel treatments aim to restore resilience by disrupting the horizontal and vertical fuel continuity that drives catastrophic crown fires. Although foundational, traditional plot-scale measurements cannot capture the continuous structural patterns that influence fire behavior at stand or landscape spatial scales. Airborne laser scanning (ALS) can be a useful tool for characterizing these patterns beyond the plot scale. In this study, we evaluated the stand-scale effects of mechanical thinning (Mech), prescribed fire (Fire), and their combined application (Mech + Fire) at the Blodgett Forest Fire and Fire Surrogate (FFS) site in the Sierra Nevada, California, USA. Using wall-to-wall ALS metrics, we quantified vertical fuel continuity, canopy height distributions, spatial tree clustering, canopy gap extent, and internal gap structure (regeneration height and vertical complexity). Relative to untreated controls, all active treatment regimes reduced vertical fuel continuity. Fire and Mech + Fire treatments produced broad reductions, while Mech reduced overall load but retained localized high-continuity pockets. Mech + Fire uniquely expanded canopy gap extent and reduced gap height while preserving dominant overstory height. Fire increased within-gap heterogeneity but reduced maximum canopy height by > 3 m. Mech did not significantly increase gap extent or meaningfully alter canopy conditions. Tree clustering patterns remained similar to Control across all treatments. These results provide new stand-scale information that extends decades of plot-based FFS findings, demonstrating that combining fire with mechanical harvest treatments most effectively disrupts vertical and horizontal continuity. • All treatments reduced vertical fuel continuity relative to untreated controls. • Mech + fire increased canopy gap extent and within-gap structural heterogeneity. • Fire treatments reduced maximum canopy height by more than three meters. • Canopy tree clustering patterns were broadly consistent across treatments.