High-intensity fire and long-term destocking causes top kill but fails to reverse encroachment and restore biodiversity.

Woody encroachment is a pervasive threat in grasslands worldwide. Encroachment has proven resilient to many nature-based interventions, including frequent burning and destocking grazers. A potential solution is high-intensity fires-or "extreme" fires-which combine high fuel loads with burning under dry windy conditions. Large-scale tests of high-intensity fire are limited and have had mixed results. We compared two catchments of tallgrass prairie heavily encroached by resprouting shrubs, with similar land-use histories, including long-term grazer destocking. In 2021, one catchment burned under typical low-intensity conditions. The other burned on a dry, windy day, producing a high-intensity fire with approximately 10-m flame lengths. Based on remote-sensed and on-the-ground measurements, low-intensity fire decreased woody encroachment initially, with nearly full recovery of shrub height/cover after 2 years. High-intensity fire caused top-kill across almost the entire catchment, but vigorous resprouting resulted in nearly full height recovery after 3 years and an increase in woody cover. After the high-intensity fire, diversity metrics and forb cover increased, then fell to prefire values 1 year later. Our results suggest that high-intensity fires may help constrain low-severity encroachment. However, reversing encroachment will require a long-term approach and novel combinations of interventions, especially where resprouting shrubs are expanding.