USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science

Forest vegetation

A graphic representation of the different vegetation layers. The tallest layer in the middle with understory vegetation, midstory vegetation on the left and young vegetation on the right.
A graphic representation of the levels of forest vegetation.

Forests can have many vegetation layers, which are often described by their vertical position in the forest. The adult trees in a forest form the canopy layer (also known as the dominant or co-dominant trees), while smaller saplings and young adults, along with some tall shrubs, form a mid-story layer.

At the understory layer, vegetation includes low shrubs as well as non-woody plants like herbs and grasses. Many forests are adapted to tolerate, and even promote, frequent understory fires. When fires burn frequently in forest understories, they often do not burn into the canopy layer except in small patches, because the midstory layer of forest vegetation is often absent and/or the forest is discontinuous with numerous small openings with no trees.

Repeated fires can kill small trees that would otherwise form the midstory layer. When these fires encounter occasional dense patches that escaped the previous fire, the fire sometimes reaches the canopy layer, killing all the trees and creating small openings in the forest. The large trees that form the canopy layer, such as Ponderosa Pine and Sugar Pine, often have adaptations that help them survive these understory fires, including thick bark and self-pruning of their lower branches to keep fire from climbing up them.

These frequent fires spread through fuels composed primarily of leaf litter from the canopy trees as well as understory vegetation, and most often burn in a patchy fashion, leaving portions of the stands untouched, while burning at higher intensity in other areas, creating gaps that shrubs and small trees often grow into.

After an extended fire-free period, the midstory vegetation layer can become very dense, with the understory vegetation layer ventually replaced by thick layers of canopy litter and dead wood. When this happens, forests may lose their resilience to wildfire because the large canopy trees can be killed by fire reaching the tree tops and/or fuel heating up their roots leading to death, sometimes years later.

While these forests may grow back eventually, the vegetation that comes in right away is often a dense understory shrub layer that can persist for decades or even centuries. To restore resilience to these forests, it is critical to create conditions conducive to patchy fire spread primarily through the understory layer. This may involve "fuel treatments" that remove the midstory layer plus fire to remove the fuel from the forest floor and understory layer.

Understory vegetation often responds positively to fuel treatments, and as long as fire is periodically used to keep the fuels below critical thresholds, flame lengths will continue to be low, thinning out small tree seedlings without killing large overstory trees and maintaining a diverse understory layer.

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