Fire in the Dry Forest
Along the east slope of the Cascades range in Oregon and Washington and into parts of Idaho and Montana low elevation forests are populated with ponderosa pine and Douglas-fir trees. Both ponderosa pine and Douglas-fir can live in places with minimal precipitation if given adequate space. As low elevations experienced prolonged dry periods, historically, fires happened frequently. A study in the Mud Creek drainage on the Entiat Ranger District of the Okanogan-Wenatchee National Forest came up with a fire return interval of seven to fifteen years. At the Sinlahekin Wildlife Area in Okanogan County, WA fires occurred as frequently as two years apart. The Sinlahekin would have been a travel corridor for tribal people. Native Americans intentionally lit fires for many purposes.
Seedling trees of any species succumb easily to fire, but by the time young ponderosa pine trees are a few inches in diameter the bark is thick enough to provide some protection. As trees age, both ponderosa pine and Douglas-fir develop very thick bark, which insulate the living tissues under the bark from intense heat. Ponderosa pine also has a loose branching structure, and needle arrangement that allows heat to rise and dissipate. The interruption of natural fire cycles has allowed these once very fire resistant forests to become vulnerable.
U.S. Forest Service entomologist Connie Mehmel looks for evidence of red turpentine beetle in a stand of ponderosa pine, burned in the Byrd Canyon Fire of 2012. As seen here, most of the sapling trees are badly scorched, but few of the medium to large size trees have been seriously damaged. Periodic fire keeps ponderosa stands open. Entiat Ranger District, Okanogan-Wenatchee National Forest 2012.
- The lower branches of these ponderosa pines were scorched in the Lime Belt Fire of 2015. They will eventually fall off, and not grow back. Canopies will be higher off of the ground and the trees less less vulnerable to future flames. Sinlahekin Wildlife Area, (Washington Dept. of Fish & Wildlife) Okanogan Co. WA. 2016
- Tyee Fire stayed on the ground as it passed through this stand of mature ponderosa pines. The open character of the stand and absence of ladder fuels was the key to the low fire severity. Entiat Ranger District, Okanogan-Wenatchee National Forest, WA 1994.
- Two years have passed since Tyee Fire. The ponderosa pine trees show no signs of distress. The grasses are thriving from the nutrient flush that goes with a low intensity fire. Entiat Ranger District, Okanogan-Wenatchee National Forest, WA 1996
- In this scene from Tyee Fire, at Pine Flats Campground on the Mad River, the larger trees have escaped the flames while the brush has been top-killed. Several sapling Douglas-fir trees next to the larger trees did not survive. Entiat Ranger District Okanogan-Wenatchee National Forest, WA. 1994
- Eight years have passed since Tyee Fire, few losses of the trees have occurred and the broad-leaf species including ocean spray, wild rose, lewis mock orange, big-leaf maple, and red-stem ceanothus have rebounded from roots which lived through the fire. Entiat Ranger District, Okanogan-Wenatchee National Forest 2002.
- Trees that have been repeatedly scarred by fires exhibit what is popularly known as a "cat face". The tree exudes pitch to cover the wound caused by the fire. The pitch readily burns in future fires causing more scarring. Eventually after many fires, the tree trunk may be so reduced that the tree falls over. Roaring Ridge, Entiat Ranger District, Okanogan-Wenatchee National Forest WA.
- A history of fire is etched by black lines inside of a ponderosa pine tree. Scientists have studied fire history by taking "tree cookies". Not every tree is scarred by every fire, so there likely were even more fires than shown. Specimen courtesy of Wenatchee Forestry Sciences Lab.
Occasionally, even mature trees with thick bark succumb to low intensity fires. Seen here a Douglas-fir tree is being burned through, to where it simply fell over. Previous fire scarring, pitch, and rotting heartwood contributed to it's demise. Sometimes trees succumb because fire burns the roots. Tyee Fire 1994, Navarre Coulee, Chelan Ranger District, Okanogan-Wenatchee National Forest, WA.