Investigations of compound disturbances that alter forest resilience (i.e., recovery time or trajectory) have escalated over the past two decades. We used a systematic approach to identify and describe the ecological consequences of discrete forest disturbance events involved in compound interactions. Fire, wind disturbance, and salvage logging were the most common disturbance types investigated. Most compound interactions occurred in North America and involved five or fewer years between disturbances. Common and interrelated disturbance legacies that facilitated compound interactions included reduced seed source availability, deadwood deposition and extraction, and increased light and growing space availability. Forest recovery was assessed with a diversity of metrics including woody and herbaceous plants, soil properties, and carbon stocks, which sometimes determined whether and what kind of compound interaction was detected. Distinctions between recovery time and trajectory, forest succession and development, and species-specific and community-level responses also influenced the detection and direction of compound interactions. Moving forward, we advocate a more holistic approach to quantify ecosystem recovery that considers multiple response variables. Other opportunities to improve compound disturbance ecology include increased emphasis on understudied disturbance types, regions, and forest types. We also encourage more research on buffering interactions that increase forest resilience, which were underrepresented in this review.

The classification of discrete forest disturbance events is usually based on the spatial extent, magnitude, and frequency of the disturbance. Based on these characteristics, disturbances are placed into one of three broad categories, gap-scale, intermediate-severity, or catastrophic disturbance, along the disturbance classification gradient. We contend that our understanding of disturbance processes near the endpoints of the disturbance classification gradient far exceeds that of intermediate-severity events. We hypothesize that intermediate-severity disturbances are more common, and that they are more important drivers of forest ecosystem change than is commonly recognized. Here, we provide a review of intermediate-severity disturbances that includes proposed criteria for categorizing disturbances on the classification gradient. We propose that the canopy opening diameter to height ratio (D:H) be used to delineate gap-scale from intermediate-severity events and that the threshold between intermediate and catastrophic events be based on the influence of residual trees on the composition of the regeneration layer. We also provide examples of intermediate-severity disturbance agents, return intervals for these events, and recommendations for incorporating natural intermediate-severity disturbance patterns in silvicultural systems.

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