Thinning Speeds Recovery to Old-Growth

Ecological Basis for Old-growth Redwood Forest Restoration: 25 year assessment of redwood ecosystem response to restorative thinning

Mill Creek. Photo by Evan Johnson
Mill Creek. Photo by Evan Johnson

Dr. Christopher Keyes and Andrew Chittick have found that thinning—removing select trees in a second-growth coast redwood forest—speeds up the forest’s development of old-growth characteristics, which include tall and bulky trees, small gaps in the canopy through which sunlight can penetrate, trees of varying heights, thicker tree branches, understory shrubs and ferns, and healthy young saplings.

After a redwood forest has been clear cut, sunlight pours down on what used to be a relatively dark forest floor. New saplings, getting their starts at roughly the same time, initially don’t have to compete with their neighbors for sun. But as they grow taller and wider, they begin to shade one another and block almost all light from reaching the forest floor. Due to overcrowdedness, some trees grow very slowly and others die; understory plants struggle to survive. If a forest in this light-suppressed condition is left untouched, it can take hundreds of years for it to even start developing old-growth characteristics.


In an old-growth forest, natural disturbances in the form of fire, lightening, and wind create and maintain gaps in the canopy. The gaps, allowing light to hit the ground, give young saplings the chance to shoot up and provide variety in the age and physical structure of a forest’s trees. Thinning in a second growth forest is an attempt to mimic natural disturbance. It relieves the forest’s unnatural, uniform growth created by the clear cut.

Twenty five years ago, park managers thinned groups of trees in the Holter Ridge area of Redwood National Park and predicted that their careful removal of some trees would accelerate the development of old-growth characteristics. Keyes and Chittick took advantage of a rare opportunity to observe the effects of restoration-based thinning. They’ve been watching the forest develop for the last twenty five years.

Based on their observations, they suggest that best results come from thinning second-growth forests at an early age. Relative to comparable unthinned groups of trees, the thinned areas supported tree diameters nearly twice as wide, a higher percentage of living tree crowns, more understory vegetation, including shrubs, ferns, and redwood saplings, a higher diversity of understory vegetation after five years, larger redwood branches, and a more varied mix of redwood, Douglas-fir, and tanoak trees in the canopy. The added sunlight in the thinned stands should allow dominant trees to reach greater heights more quickly. This is especially important for forest organisms, such as insects, birds, salamanders, and mammals, which more frequently use large trees for their associated habitats—large snags, fallen wood, and bark fissures.

Keyes and Chittick note that if forest managers create the right initial forest structure early on and allow nature to freely disturb stands, old-growth characteristics will self-perpetuate sooner than if the stands were left alone.

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