Long-term effects of fuel treatments, overstory structure, and wildfire on tree regeneration in dry forests of Central Washington

The long-term eectiveness of dry-forest fuels treatments (restoration thinning andprescribed burning) depends, in part, on the pace at which trees regenerate and recruit into theoverstory. Knowledge of the factors that shape post-treatment regeneration and growth is limitedby the short timeframes and simple disturbance histories of past research. Here, we present resultsof a 15-year fuels-reduction experiment in central Washington, including responses to plannedand unplanned disturbances.We explore the changing patterns of Douglas-fir regeneration in72 permanent plots (0.1 ha) varying in overstory abundance (a function of density and basalarea) and disturbance history—the latter including thinning, prescribed burning, and/or wildfire.Plots were measured before treatment (2000/2001), soon afterwards (2004/2005), and more thana decade later (2015). Thinning combined with burning enhanced sapling recruitment (ingrowth)into the overstory, although rates of ingrowth were consistently low and greatly exceeded bymortality. Relationships between seedling frequency (proportion of quadrats within a plot) andoverstory abundance shifted from weakly negative before treatment to positive after thinning,to neutral in the longer term. However, these relationships were overshadowed by more recent,higher-severity prescribed fire and wildfire that stimulated seedling establishment while killingadvanced regeneration and overstory trees. Our results highlight the dependence of regenerationresponses on the history of, and time since, fuels treatment and subsequent disturbance.Managers must be aware of this spatial and temporal complexity and plan for future disturbancesthat are inevitable but unpredictable in timing and severity.