Underthinning

Underthinning is a forestry term used to describe thinning that removes fewer trees than recommended or required to meet stand management objectives. In practice, it leaves the stand relatively dense, with less release of growth or improvement in stand structure than intended. The concept is often discussed in contrast to overthinning, where too many trees are removed.

Causes of underthinning can include conservative risk management, economic or logistical constraints, misinterpretation of silvicultural objectives, or delays in planned entries. Indicators include residual basal area and density remaining higher than target after thinning, limited growth response in crop trees, minimal improvement in crown class distribution, and persistent competition among residual trees.

The consequences of underthinning can be slower diameter growth and height growth for crop trees, poorer stem form and timber quality due to ongoing competition, and increased vulnerability to pests, disease, or weather stress from high stand density. In some ecosystems, sustained density can also elevate flame length and fuel ladders, affecting fire risk management.

Management implications emphasize aligning thinning prescriptions with objectives and re-entry planning. If underthinning is detected, subsequent entries should increase thinning intensity or adjust the distribution of removals to favor the release of dominant and co-dominant trees, improve crown illumination, and reduce competition in key size or age classes. Quantitative assessment using metrics such as residual basal area, density, and growth response helps determine whether a stand has been underthinned and what corrective step is needed.