In the absence of fire, intensive mechanical treatments needed to keep ponderosa pine forests healthy

Forest Service scientists say they’ve quantified the relationship between the density of ponderosa pine stands and disease. The study shows that — no surprise — increased density, mainly due to the agency’s long-term fire suppression policies, makes the trees more susceptible to bark beetles and diseases. Competition for soil moisture, nutrients, and sunlight in dense stands weakens trees and therefore also contributes to fuel buildup. The study, led by Dr. Jianwei Zhang, research forester at the U.S. Forest Service’s Pacific Southwest Research Station, considered if the onset of risk could be determined. The findings are based on an analysis of 109 long-term research plots established on even-aged natural stands and plantations from 1944 to 1988. Given the desire to keep fire out of some forest ecosystems where human developments are in harms way, forest logging and thinning projects can help keep trees healthy. The research also confirmed the added value of such long-term study sites which allow new questions to be addressed that were not included in the original studies. Other research from this group of scientists shows that thinning forest stands to a lower density reduces fuel buildup significantly, and enhances its economic value by increasing growth of residual trees. Specifically, stand basal area, which is the cross sectional area of all trees in a stand measured at breast height, is not affected by thinning ponderosa pine stands to half the normal basal area of a specific site quality. If the stand has experienced high mortality caused by bark beetles, it can be thinned more heavily without sacrificing timber, biomass, or volume increment and plant diversity. In addition, results from these long-term studies show that early shrub removal and tree density control are the most effective and efficient ways to reduce fuel buildup. Under Mediterranean climatic conditions, shrubs reduce overstory tree growth and keep tree crowns in contact with the shrub canopy. In turn, this growing fuel ladder can carry a ground fire into the crowns of the overstory trees...more