Abella et al. 2025 (Restoration Ecology)
RESEARCH BRIEF #54
A summary based on the following publication:
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MANAGEMENT IMPLICATIONS
Low-severity fires may need to occur at least every three years to prevent the development of dense oak shrubby layers.
Black oak stems appeared to survive dormant season fires once they attained a DBH of 2.5 cm (1 in).
A variety of management activities may be necessary to limit oak shrubby-layer expansion in restored savannas and woodlands.
In this multi-decade study, authors examined oak shrubby-layer encroachment at sites undergoing oak savanna-woodland restoration in the midwestern United States. A clearer understanding of the conditions that promote shrub-layer development, particularly after initial restoration, is needed to guide more effective and sustainable long-term maintenance of savannas and woodlands.
The study was conducted in the Oak Openings Preserve, managed by Metroparks Toledo, in northwestern Ohio. Records from the early 1800s indicate that this region was historically dominated by oak woodlands and savannas. The overstories of both systems were primarily composed of black oak (Quercus velutina) and white oak (Q. alba) trees with ground layers supporting a diverse assemblage of forbs, graminoids, and shrubs. The absence of fire in the 1900s has resulted in a transition to closed-canopy forests with increased presence of fire intolerant tree species.
The authors posed four questions associated with oak shrubby-layer formation:
1) When during restoration management does oak shrubby encroachment occur?
2) How does overstory tree canopy and fire frequency associate with shrubby encroachment?
3) Does survival and growth of oak shoots after fire relate to stem size and degree of scorch?
4) What are the age structure and recruitment timing of encroaching oak shrubby layers?
Data were collected over multiple decades at sites where closed-canopy oak forests have undergone restoration to savanna and woodland. Sites were prioritized for restoration based on suitability for prescribed fire use, historical classification as savanna or woodland, and the presence of mature oaks 50-100+ years old in the overstory. Non-oak trees deemed too large to be top killed by fire were cut and removed. Dormant-season (March-April and November) prescribed burns were conducted with a goal of achieving at least three fires per decade in savanna plots and 1-2 fires per decade in woodland plots, with flame lengths typically 1-2 m (3.3-6.6 ft). Unburned, unmanaged oak forests were used for comparison with restoration management.
In study plots, the diameters of all tree stems were measured at a height of 1.4 m (4.5 ft – diameter at breast height, DBH). Stems ≥ 1 and < 10 cm (≥ 0.4 and < 4 in) DBH were categorized as the “sub-canopy layer” and those ≥ 10 cm (4 in) DBH as “canopy trees.” In a subset of plots, individual oak stems ≥ 1 cm (0.4 in) DBH were monitored for fate and condition; new stems meeting this threshold were surveyed and measured for height and DBH; stem scorch was recorded as percent scorch up to 1 m (3.3 ft) and maximum scorch height was measured; and basal cross-sections from black oaks in the shrub layer were collected and aged via tree-ring count. Prescribed fire reports were used to examine relationships between fire history and the age structure and recruitment patterns of the shrub layers.
Questions 1 & 2 – Study outcomes demonstrated that oak shrubby layers formed only in restored savanna–woodland plots, decades after treatment, and were absent from unmanaged forest plots. Dense oak shrubby layers were promoted by several conditions: low overstory tree basal area; older, repeatedly burned restorations; and no fire within the previous three years. Oak shrubby layers were maximized in older restoration plots that had undergone at least four burns, provided that no fires had occurred within the preceding three years. Oak shrubby layers were found to be regulated if at least one fire had occurred within the previous three years.
Question 3 – Results (see figure this page) showed that nearly all (93%) sub-canopy oaks survived when stem scorch was ≤ 15%. When scorch exceeded 15% and DBH was ≤ 2.5 cm (1 in), 91% of stems were top killed, but 99% resprouted from the base. In contrast, stems with DBH > 2.5 cm (1 in) retained live crowns in 96% of cases, even when scorch exceeded 15%. Complete mortality was rare and was exceeded by new recruitment. Top-killed stems that resprouted exhibited greater growth with increasing percent scorch and scorch height.
Question 4 –The study found that, following prescribed fires, recruitment occurred in even-aged pulses of oak sprouts or seedlings that consistently established during the first growing season after autumn or spring burns.
This study highlights key challenges in restoring oak forests to savannas or woodlands. Although oak regeneration and recruitment are essential for maintaining savanna-woodland structure, continued disturbance treatments are necessary to prevent excessive oak shrubby-layer encroachment that could undermine restoration outcomes. Sustaining open-habitat conditions and surface vegetation will depend on site-specific management that maintains an appropriate disturbance regime and, where needed, incorporates additional approaches such as spatial rotation among savanna, woodland, and forest areas or more intensive treatments (e.g., targeted herbicide applications and high-intensity prescribed fires).
