Formation and ecology of oak shrubby layers during long‐term oak savanna restoration

Woody plant encroachment can eliminate open‐structured habitats and commonly triggers ecological restoration. However, the long‐term dynamics and formation processes of encroaching woody layers are often unclear, including after initial restoration has reestablished open habitats, making sustainability of restoration gains in open habitats uncertain. To examine the multi‐decade change and formation processes of encroaching oak shrubby layers in midwestern U.S. savannas and woodlands undergoing restoration, we measured: (1) oak shrubby layers (primarily Quercus velutina ) for 24–34 years on 24 restoration sites receiving 2–17 prescribed fires, (2) demographics of 379 shrubby oak shoots, and (3) age structure to determine recruitment history of oak shoots. The oak shrubby layer did not form until the second or third decade of restoration management. It was maximized when sites had cumulatively received ≥4 fires, but none within 3 years, and overstories were open. Most (91%) small‐diameter (<2.5 cm at 1.4 m) oak shoots were top‐killed by fire if incurring ≥15% scorch, but 99% resprouted. Moreover, the greater the scorch, the faster resprouts subsequently grew. Repeated establishment of the shrubby layer occurred through synchronized, even‐aged pulses the year after fires. Results suggest that the oak shrubby layer, and its resilience, expands decades into restoration management and represents a conundrum. Some oak regeneration must occur to maintain savanna‐woodland structure, and depauperate oak regeneration threatens oak ecosystems in many regions. Yet for restoring savannas, continual restoration inputs were required to perpetually reduce encroachment by oak, and the inputs needed increased—not decreased—as restoration sites aged.