• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Effects of Thinning and Prescribed Burning on Tree Physiology and Predicted Fire Behavior in an Old Growth Ponderosa Pine/Western Larch Stand

This study was designed to evaluate alternative restoration treatments for reducing the potential for high intensity crown fire in an old growth stand and to test and compare treatment effects on old tree physiological performance. It was conducted in a remnant 330-year old ponderosa pine/western larch stand on the Lolo National Forest, 5 miles north of Missoula, Montana. Historical fire return intervals averaged 20 years until 1919, followed by an 80-year fire absence. During this time the stand density increased from 60 trees/acre of pine and larch to 500 trees/acre of mostly Douglas-fir. Three treatments reported here are understory removal with slash burning, understory removal and overstory thinning with slash burning, and a control. Slash burning included prescribed fire and pile burning. The treatments were designed to produce a condition that resembled the historical stand.

Six years after treatment, stand characteristics and surface fuels were re-measured and these data were used in current fire behavior models to predict potential wildfire characteristics. Under severe wildfire weather conditions, the model predicted an almost certain high intensity crown fire in the untreated, control stand. This was primarily due to high levels of ladder fuels. In both treated stand conditions, the initiation and movement of crown fire was predicted to be highly unlikely, even under the worst fire weather conditions.

Physiological performance of the old trees in the treated stands was found to be greatly improved over the control trees for five years after treatments. Removal of several hundred understory trees/acre resulted in increased soil moisture in the treated stands which was measured as increased water use by the old larch. This was at least patially responsible for increased foliar biomass and radial growth over the control trees. The 300+ year old ponderosa pine also responded with increased needle biomass, bud size, and radial growth, all indicators of improved health and resistance to forest pests.

The results give resource managers clear evidence that fuel reduction/restoration treatrments in old growth, fire-adapted ecosystems can have several benefits: (1) reduced probability of uncharacteristically severe wildfire behavior and effects, even under severe fire weather; (2) improved general health of old trees by reducing competition for soil moisture and nutrients, allowing for greater resistance to insects and disease; (3) allows a greater opportunity for regeneration of pine and larch in the created open understory with exposed mineral soil.

More detailed information on this study can be found in the Final Report to RJVA #RMRS-99563, "Physiological responses of old growth ponderosa pine and western larch to restoration cutting and burning treatments", on file with the Fire Ecology and Fuels research work unit, Missoula, MT.