Douglas-fir Encroachment and the Loss of Open Pine Stands

What is Douglas-fir encroachment?

It is the gradual replacement of open, fire-maintained stands of ponderosa pine and western larch by shade-tolerant Douglas-fir. The Forest Service's Fire Effects Information System (FEIS) review of Rocky Mountain Douglas-fir describes the species as a shade-tolerant climax tree in dry to moist low and middle elevation forests that, in the absence of disturbance, tends to replace ponderosa pine, lodgepole pine and western larch in the northern Rockies.

Shade tolerance is the key. Ponderosa and larch seedlings need open, sunny ground to establish; Douglas-fir seedlings can establish and survive under a canopy. FEIS notes that Douglas-fir has also moved into sagebrush grasslands and quaking aspen stands where fire exclusion reduced clonal reproduction. It attributes the invasion and densification of pine stands over the last 100 to 150 years to changes in fire regimes, climatic variation, selective logging and their interactions.

Why did fire once keep Douglas-fir in check?

Because young Douglas-fir burns more easily than young pine or larch. FEIS reports that mature Douglas-fir is generally more fire resistant than spruce, true firs and lodgepole pine and only slightly less resistant than ponderosa pine and western larch, but that in the sapling and pole stages it is slower growing and much less fire resistant than either. Its bark becomes thick and corky at about age 40 in the northern Rockies, about 1 inch thick on a 12-inch tree.

With fires every decade or so, most young fir on dry sites never reached that size. FEIS puts it directly: during presettlement times frequent fire often maintained ponderosa pine rather than Douglas-fir on drier sites, because the fir did not reach fire-resistant size before the next fire. On three Bitterroot National Forest study sites in the Douglas-fir/bluebunch wheatgrass habitat type, which was dominated by ponderosa pine before 1900, FEIS reports mean fire intervals of 6, 10 and 11 years. Montana DNRC's landowner guide gives a regional figure for forests more dominated by Douglas-fir: low to moderate severity fires about every 45 years, which maintained healthy spacing between trees.

How much has the forest changed in the Bitterroot?

The best local numbers come from the west side of the valley. FEIS cites a study of Bass and Blodgett creeks in the Bitterroot Mountains: between 4,500 and 5,800 feet, ponderosa pine fell from 52 percent of stand basal area in 1900 to 26 percent in 1995, while Douglas-fir rose from 19 to 55 percent. At higher elevations the fir share held steady or fell; the increase was concentrated in what had been the ponderosa pine zone.

On the Lolo National Forest around Missoula, FEIS reports that Douglas-fir basal area was ten times greater in 1994 than before 1900, while total stand basal area about doubled. And in chronosequence studies of ponderosa pine and Douglas-fir forests on the Lolo and Bitterroot national forests, Douglas-fir basal area, shrub cover and forest floor thickness all increased with time since the last fire. The FEIS ponderosa review dates effective fire exclusion to the 1930s.

Why do Douglas-fir thickets make wildfire worse?

They connect the ground to the crowns. FEIS says it plainly: fire suppression has allowed dense Douglas-fir thickets to form in some areas, providing continuous ladder fuels to the crowns of overstory trees, and fire exclusion has increased the potential for severe, stand-replacing fire. A surface fire that once would have crept under the old pines now climbs the young fir into them. How a fire makes that transition is explained in surface fire, crown fire and spotting.

Douglas-fir also holds its fuel. FEIS notes that the fire resistance of its thick bark is often offset by low-growing branches that may be retained even when shaded out and no longer green, and that trees infected with Douglas-fir dwarf mistletoe accumulate dense brooms that increase the likelihood of charring or torching. When those brooms fall, FEIS says, the fine debris burns hot and can girdle the trunk or torch the crown. DNRC's landowner guide links the same overcrowding to declining forest health, more insect and disease outbreaks and accumulating dead fuel.

Does encroachment make insect problems worse too?

Often, yes. DNRC's guide describes western spruce budworm feeding on Douglas-fir, true firs, spruce and larch, and notes that healthy mature trees can usually withstand repeated moderate defoliation but most understory trees do not have the reserves to survive it. Its mitigation list is a description of a restored stand: promote non-host species, thin to increase the vigor of remaining trees, or create single-storied canopies to interrupt caterpillar dispersal.

FEIS adds that low thinning and surface-fire prescriptions favoring ponderosa pine will likely reduce the frequency or duration of insect outbreaks, and that Douglas-fir beetle, wood borers, tussock moth and budworm cause significant postfire mortality. A multi-storied fir thicket is good budworm habitat and poor fire habitat at the same time. The individual pests are described in Douglas-fir problems in Montana.

Should all the Douglas-fir be removed?

No. Douglas-fir is native to these valleys and a mature one with thick bark is a reasonably fire-resistant tree, as FEIS notes. DNRC describes the region's dry-mesic forests as primarily western larch and ponderosa pine with Douglas-fir and grand fir throughout. The aim is to restore the proportion and structure, not to eliminate a species.

In practice that usually means keeping the large, well-formed trees of all three species, favoring pine and larch where there is a choice, and removing the small and pole-sized Douglas-fir crowded beneath them, along with fir that is heavily infected with dwarf mistletoe. Big old fir with low skirts of dead branches can usually stay once those branches are pruned off; see how high to limb up trees for fire. On sites where Douglas-fir is the only overstory tree, the work becomes spacing the best fir rather than converting the stand.

What does restoring an open stand look like on private land?

It looks like a forest with fewer, larger trees, visible distances between crowns and grass or low shrubs underneath instead of a wall of saplings. FEIS cites Arno's finding that ponderosa pine stands historically had trees spaced at least 25 feet apart. Around a house, Montana DNRC calls for at least 18 feet between trees or clumps in the zone 5 to 30 feet out; the zone-by-zone and slope numbers are in thinning conifers for defensible space.

DNRC's landowner guide recommends that management include thinning to reduce fuels, restoring an open forest structure and, where feasible, returning fire to the landscape. It notes that both mechanical treatments and prescribed fire are options, and that stands both mechanically thinned and burned have proven most resistant to high-severity fire. Where burning is realistic, prescribed fire on private land explains how it is arranged; DNRC suggests starting with your local service forester.

Will the fir just grow back?

It will, which is why restoration is a maintenance program rather than a one-time cut. FEIS describes a Washington study in which thinning and prescribed fire killed all the Douglas-fir and grand fir seedlings in a ponderosa understory, yet nine years later Douglas-fir seedlings averaged 51 per acre. It reports that in dry forests, prescribed fire generally needs repeating about every 8 to 10 years to keep dead fuel and shade-tolerant conifers from building up.

Without fire, the equivalent is cutting the new fir regeneration on a similar cycle while it is still small, which is quick work compared with the original thinning. Our guide to how often defensible space needs maintenance sets out a schedule.

Where HJ fits

Selective conifer thinning, ladder-fuel removal and limbing are the core of our fire mitigation service, and removing a fir understory under old pines is precisely that work. We cut on a schedule that avoids the spring beetle flight, chip or haul the slash so it is not left as new fuel (see slash disposal options), and leave the trees that the stand was built around. More on how wildfire behaves around Montana homes is in our Montana Wildfire Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. One of our owners, Justin Johnson, is an ISA Certified Arborist (RM-8889A), and we will walk the property with you before any work is quoted.

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