Ponderosa Pine

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From A Short Guide for Developing CFRP Restoration Prescriptions

Ponderosa pine forests in the Southwest have become too dense with small trees over the last century because of overgrazing by livestock, suppression of cool surface fires, and logging of the largest trees. Meadows that in the past were scattered throughout the forest have become filled with young trees that have moved in from the forest edge. The forest floor is now covered with a layer of dead pine needles instead of grasses and other plants. Dead standing trees, especially large ones, are not as numerous as they once were. 

Some recommendations for developing an ecological prescription for ponderosa pine forests are as follows:

  • Remove small trees and keep large trees. The overwhelming number of trees in ponderosa pine forests of today are small. Most stands do not have many trees larger than 16 inches in diameter at breast height. For many stands, you might cut all trees smaller than 12 inches in diameter, or even all trees smaller than 9 inches in diameter, to produce the desired density. Such small trees often make up 90% of the trees in a stand. In stands with dense mid-sized trees, it may be necessary to cut some larger trees to get a desired density.
  • Incorporate prescribed fire into the prescription.Cool surface fires were part of the history of almost all ponderosa pine forests in the region. Without these fires a thinned forest will soon revert to a dense, crown-fire-prone stand, perhaps in less than ten years. In the best case, lightning will start natural cool fires often enough to keep fuel load down. These natural ignitions will benefit the forest if they are carefully managed. Otherwise, land managers will need to burn the forest periodically with a prescribed fire.
  • Develop density and basal area targets that reflect local site history. For example, adult tree density from 40 to 100 trees per acre is probably appropriate on most sites in the Southwest. You can typically vary density across a project site, with lower density on ridges or near roads, and higher density in areas of wildlife concern, such as goshawk nesting habitat.
  • Create clumps of trees—perhaps 6 to 12 mature trees together—throughout the site to benefit wildlife. Tree crowns should touch or nearly touch within clumps, but be isolated from tree crowns outside the clump. Aim for two clumps per acre. Tree clumps are at less risk of crown fire if surrounded by areas of lower tree density.
  • Foster the growth of an understory. For the most part, the understory will recover on its own, when there is more sunlight and water available after thinning. Leaving some of the branches from thinning treatments strewn on the ground will help understory plants reestablish. Spreading slash on the ground before a prescribed burn rather than piling and burning may also help. Intense heat from burning piled slash can kill seeds in the soil beneath slash piles.
  • Avoid reseeding. Even native seed mixes are usually contaminated by weedy species. Most sites have enough native plants to naturally reseed the site.
  • Reduce surface fuels. The majority of fuels in most ponderosa pine stands are small, living trees. Thinning treatments will produce large amounts of dangerous fuels from these trees, which must be removed from the site to prevent a hot fire. Downed wood may be removed for wood products, firewood, or composting materials. Every last bit of slash need not be removed; leaving some slash is valuable for protecting soils, fostering understory, and providing wildlife habitat. But enough must be removed to allow a cool burn that does not rise to the canopy. Removing slash greater than four inches in diameter also reduces the likelihood of attracting bark beetles to the stand.
  • Rake away needles from base of large old trees. Pulling thick pine needle mats and branches three feet away from the base of tree trunks can protect trees from mortality. In general, you probably need to do this only before the first fire.
  • When possible, close small unused dirt roads that are eroding. Many of these roads will erode into gullies without an understory cover. Eroding roads may need rehabilitation, such as trenches to guide storm water off the road, or branches strewn on the surface to assist revegetation.
  • Preserve large snags for wildlife use. Lightning, insect mortality, and prescribed burns will create new snags in the future.