Why Thinning Is Necessary
Oak savannas require open canopy conditions — typically less than 60 percent tree and shrub cover — to maintain the light-demanding ground flora and the young oak regeneration that characterize the system. In the absence of fire or other disturbance, most Great Lakes oak woodlands have moved well beyond this threshold. Mid-successional shrubs, young trees of fire-sensitive species, and invasive shrubs like buckthorn now form dense understory layers that intercept light before it reaches the ground.
Manual and mechanical thinning removes this competing woody biomass, reopening the canopy and creating conditions in which prescribed burning, native seeding, and natural processes can begin to restore the historical community structure.
What Gets Removed
Understory thinning in oak savanna restoration is selective. The goal is not to clear all vegetation, but to remove species and individuals that are incompatible with the target structure. In Ontario sites, the primary removal targets include:
- Common buckthorn (Rhamnus cathartica) and glossy buckthorn (Frangula alnus) — invasive shrubs that shade out the ground layer
- Shade-tolerant native trees encroaching from adjacent forests, particularly sugar maple (Acer saccharum) and red maple (Acer rubrum) in early growth stages
- Dense stands of native shrubs at densities that preclude a functioning ground layer, such as wild grape thickets or hawthorn groves that have spread beyond historical levels
- Non-oak tree species in the sub-canopy that are overtopping or suppressing retained oaks
Retained plants typically include mature oaks of all sizes, native shrubs at lower densities, and any native herbaceous ground cover that has persisted under the dense canopy.
Target Canopy Cover
Restoration plans for Great Lakes oak savannas often set target canopy cover ranges based on historical reference conditions or community classification. A typical target for an oak opening is 20–50% canopy cover, measured by crown projection. In practice, initial thinning tends to bring cover to the lower end of this range to allow maximum light penetration for ground-layer recovery.
Prioritizing Work on a Site
Large restoration sites cannot usually be thinned in a single season. Practitioners typically prioritize areas based on residual oak quality and abundance. Areas with the highest density of mature oaks in good condition are treated first — these are the areas with the most restoration potential and the most to lose from continued competition pressure.
Secondary priority is often given to areas where native ground-layer species have persisted at low density under the canopy. These remnant populations can expand more quickly once light conditions improve, reducing the need for reseeding.
Areas dominated by non-oak species with few or no native remnants are typically treated later, and often paired with active reseeding or planting of native species immediately after clearing.
Timing of Thinning Work
Most understory thinning in Ontario is conducted between late autumn and early spring, when vegetation is dormant. This timing offers practical advantages: visibility into the canopy is better without leaves, access to some sites is easier when the ground is frozen, and cut stumps of buckthorn can be treated with herbicide without risk to nearby non-target vegetation in leaf.
Some practitioners prefer to work just before or during leaf-out on buckthorn — approximately two to three weeks before native oaks leaf out — when the bright green colour of buckthorn foliage against bare native canopy makes identification unambiguous. This narrow window allows efficient targeted removal with lower risk of misidentifying native shrubs.
Stump Treatment After Cutting
Cut stumps of invasive shrubs, particularly buckthorn, resprout vigorously from the root crown unless treated. The standard approach used in Ontario is direct application of an approved herbicide concentrate to the freshly cut stump surface immediately after cutting. The timing is critical — treatment is most effective when applied within a few minutes of the cut.
For larger stumps, the outer ring of exposed wood (the cambial tissue just inside the bark) is the primary target for herbicide uptake. Central wood tissue in larger-diameter stems does not effectively conduct herbicide to the roots.
Mechanical alternatives to chemical stump treatment include cutting below the root crown where practical, or repeated re-cutting of resprouts over several seasons. These approaches work but require more follow-up effort per plant than a single well-timed cut-stump treatment.
Follow-Up and Monitoring
A single thinning pass is not a complete restoration treatment. Follow-up visits in the growing seasons after initial clearing are needed to address:
- Resprouts from cut stumps that were missed or where treatment did not fully take effect
- New seedling cohorts germinating from the persistent soil seed bank
- Encroachment from surrounding untreated areas
- Opportunistic weedy species that colonize disturbed ground after thinning
Monitoring typically involves periodic transect surveys or photo-point comparisons to track canopy cover, buckthorn density, and ground-layer composition changes over time. Results from monitoring inform the timing and intensity of subsequent management interventions.
Integration with Prescribed Fire
Where feasible, understory thinning is most effective when followed by prescribed burning. Fire does several things that thinning alone cannot: it kills resprouts and seedlings across the entire treated area rather than plant by plant; it reduces accumulated woody debris and litter; and it temporarily mineralizes nutrients in a way that can stimulate native grasses and forbs.
The typical sequence at Ontario restoration sites that use both tools is: initial thinning to reduce fuel load and create accessible conditions, followed by prescribed burns once the site meets safety and regulatory requirements for burning. The combination tends to produce faster ground-layer recovery than either approach used in isolation.