Thinning, when combined with an appropriate strategy of handling cut plant material (see chapters on Managing Woody Material, Beneficial Fire, and Pile Burning), can reduce the risk of high-severity wildfire by separating fuels and live trees, which in turn reduces the temperature and severity of subsequent fire so it doesn’t kill desired plants or escape control lines. Overly dense forests can lead to catastrophic wildfires which are more likely to damage the mature trees we value and can make tree regeneration difficult. 

Specialized thinning along roads (see Shaded Fuel Breaks) makes emergency access roads safer for firefighters and residents. Once regular beneficial fire is returned to the land, manual or mechanical thinning can be greatly reduced or may no longer be necessary. 

Thinning can be beneficial if there are signs of overcrowding or poor health. Giving plants room to grow to their full potential and mature size creates forest health. Appropriate thinning promotes vigor among the remaining plants, increases their size and resilience to droughts, wildfires, pests and diseases, promotes greater seed production, and increases overall ecosystem health. Vegetation communities with optimal spacing compete less for resources such as light, water, nutrients, and fresh air. When plants are less crowded, more moisture is retained in plants and soil later into the dry season, reducing flammability. Some plant pathogens such as Sudden Oak Death rely on wet conditions and are less transmittable when air circulation is strong. 

Thinning for forest health creates beneficial “mosaics” of denser and less dense areas. These foster diverse habitats by increasing light at the forest floor, which in turn promotes a variety of plants that can be used for shelter and forage for animals. Mosaics can also reduce severity and size of fire; because openings in the landscape have less fuel, a wildfire traveling through areas of mixed vegetation tends to be less severe.

Plants are mostly carbon, so thinning affects carbon storage and sequestration. Globally, this matters enormously, because excess carbon and other gasses in our atmosphere are cooking our planet. But it’s complicated; see Carbon in Your Goals for more information. 

Thinning can increase the amount of water in the soil or in local streams, at least for some period of time, because thinning reduces the number of trees competing for water.

Thinning makes it easier for both humans and animals to move through natural areas. Thinning is done differently depending on land management goals. When setting goals, expert guidance from an arborist, forester, ecologist, Indigenous practitioner, or land management professional is highly recommended. (See Resources for Implementing.) A common rule of thumb is that trees larger than 11 inches in diameter (measured at breast height) are more appropriate for professionals to handle.

Thinning combined with Limbing, Managing Woody Materials, and Beneficial Fire can produce dramatic results. This aerial photography shows how different treatments moderated the impact of Oregon’s 2021 Bootleg Wildfire.

For at least 10,000 years prior to European contact, Indigenous peoples of what is now California tended the land with thinning and fire. Their complex and sophisticated land stewardship practices promoted biodiversity and reduced the severity of fires.

Fire suppression and exclusion of Indigenous stewardship from the land, among other factors, have substantially changed the composition of Sonoma County’s woodlands in the last 200 years. A 2023 study found that in some parts of the county, the extent of Douglas fir forest has increased between ten to 80 times. Many scientists find overcrowding throughout the West and link these changes to hotter and larger wildfires. One study found that severe fires, in which a conflagration kills 75 to 100 percent of the trees, are larger and occur more frequently in California than at any time in recorded history.

Thinning is called for where the stems, trunks or resprouts of woody species are too close together to allow the plants to grow to their full size, or when they create high-severity wildfire conditions such as contiguous flammable material (i.e. an oak woodland where Douglas Firs are encroaching).

Thinning can be beneficial if there are signs of overcrowding or poor health, including: 

• Only 30 percent or less of a tree’s crown is alive,
• The area has many dead trees,
• The shapes of trees are much different (usually skinnier) than a free-growing tree of the same species,
• The tree and shrub density makes travel for wildlife such as deer or bear difficult. 

Thinning is called for in many vegetation communities (such as mixed conifer stands, oak woodlands, mixed hardwood forests, or chaparral) that have not been thinned, limbed, or burned for ecological or fire resiliency goals in the recent past. (Approximately, “the recent past” means the number of years in that vegetation community’s pre-European fire return interval). 

There are exceptions: some less-common vegetation types, such as Bishop pine or Sargent cypress forests, do not reproduce except after high-severity, stand-replacing fire, and they naturally grow in very dense same-age stands. While there may be other reasons to thin such stands, it is unnecessary for forest health.

Vegetation management including thinning when combined with Beneficial fire can reduce the severity of subsequent wildfire. The right side of the photo is an area burned intentionally in 2019 at Monan’s Rill in northeastern Sonoma County. Surface fuels were consumed by the burn. The left side has just burned in the 2020 Glass Fire. The Glass Fire stopped at the edge of the prescribed burn, because there was not enough fuel to carry it. Photo: Chris Schaefer.

Preparing for thinning follows the same thought process regardless of whether you will do your own work or hire contractors:

• Thinning begins with deciding which trees to retain, and marking the trees you want to remove to achieve your land management goals. For example, if promoting wildlife is important, you will retain more unique, damaged, and some dead, standing trees (snags), because they provide increased habitat for animals and insects. Consider retaining trees with unique structures and trees with multiple trunks that can often grow together, limbs leaning on other limbs, snags, or dead tops–as these can offer great habitat for animals.
• Generally, thinning for any purpose should retain the largest and healthiest trees and shrubs. Large, healthy trees are typically most fire resilient and provide the most ecosystem services, such as clean air, habitat, or carbon sequestration. If cut, it could take decades or centuries to recover or replace the services a mature tree provides.
• Consider favoring oak trees for retention, if your site has them. Ecologists consider oak trees keystone species, because they provide more food and shelter to animals and insects than any other tree. (See Retention and Creation of Habitat Features to learn more.) Evergreen oak species like coast live oak cast dense shade all winter, quite different from deciduous oak species, which may affect your priorities.
• Consider retaining fruit-and nut-bearing plants such as oaks, madrones, bay laurels and manzanitas, among others, because they provide food for wildlife and cultural resources for people. This is the practice of many Indigenous communities in California. 
• As a forest or shrubland ages, the plants need to get bigger, so the spacing needs to increase. Visualize the full extent of the canopy of the longest-lived trees or shrubs on your site, at maturity, to help understand the ultimate stem density your site could attain. If plants are crowding or over-topping your desired retained trees or shrubs, consider removing them. For instance, California bay trees over-topping oaks or other hardwoods can be removed to reduce Sudden Oak Death (which they carry) and bring in more light.

Thinning is often ongoing, unless beneficial fire or another low to moderate intensity disturbance such as targeted grazing can be introduced and repeated regularly. A site with favorable growing conditions (plentiful water, ideal sunlight, rich soil) will need more frequent maintenance.

Once cut, many woody species, other than conifers such as Douglas fir, are likely to resprout. How resprouts are managed is defined by the intended outcome: Do you want this plant in this location? 

• If no, then remove all resprouts. Where manual or mechanical maintenance is not feasible, for very vigorous woody resprouters like eucalyptus, acacia, mature broom, or California bay, you may consider dabbing a small amount of herbicide on fresh-cut stumps, without causing wider harm.
• If yes (the plant is acceptable in that location(, then ask…
  • Do dense resprouts pose a problem or risk for this particular location and warrant thinning? 
    • If no, then no action is needed.
    • If yes, then for trees remove all but 2 or 3 of the tallest or most vigorous sprouts to accelerate upward growth. For shrubs, generally retaining all resprouts is fine.