This is another post about forest ecology that may be boring for some folks but is necessary background for future observations about Rocky Mountain forests. I am including a lot of photos, hopefully that will keep people’s attention…
Good forest management has to recognize that forests are very different. Some forests want to get quite old—some don’t. I grew up in the Oregon Coast Range, a very moist region dominated by Douglas fir/western hemlock forests. This forest type experiences stand replacing fire quite infrequently and individual trees often grow to be quite old—Douglas fir is capable of growing to be 700-800 years old or even older (the oldest age cohort in the Coast Range tends to be around 400-500 years old).
The Forest Service used to refer to these old growth stands as “decadent”—biological wastelands of rotting trees that needed to be “regenerated” (clearcut) to make room for fast growing young Douglas fir.
More recently the Forest Service has recognized that “decadence”—copious rotting logs and broken off trees—creates a unique and increasingly rare habitat. I know of no silvicultural treatment that makes much ecological sense in these classic moist old-growth Douglas fir/western hemlock forest. Regenerating the stand can mimic the effects of stand replacing fire… but there’s already way more than enough young even-aged stands in western Oregon.
Other forest types don’t like to get very old. A classic example is aspen, which I’ll discuss in a future post.
We’re frequently told that forests in the West are too dense, in danger of burning up in unnaturally severe fires, and require thinning to restore more open forest conditions. This is true, but only in some forest types. There are many fine examples of ponderosa pine forests in the Rockies, found between 6,000–8,000 feet in Colorado where I’ve been for several weeks. In some parts of the Rockies, especially in southern Colorado, Arizona and New Mexico, ponderosa pine grows in stands that were once quite open, the open conditions maintained by frequent, low intensity fire that killed understory plants but left the widely spaced overstory trees. Thinning to remove understory trees and restore open forest conditions makes a lot of sense in these stands. The open forest provides unique habitat for different species, and there is compelling evidence that these forests are not resilient (see 7/10 post about forest change and resilience) and that severe forest fires will convert these ponderosa stands into brush fields.
Open, park like ponderosa stands were less common in other parts of the Rockies. Dendrochronological (tree ring dating) research in the Front Range in northwestern Colorado indicates that ponderosa pine forests there might have grown to be quite dense and experienced severe fire at times. It is less clear that thinning is appropriate in this forest type—at least to achieve strictly ecological objectives.
Lodgepole pine and spruce and fir forests grow at higher elevations sites in the Rockies between 9,000–11,000 feet. These sites are colder, receive more precipitation and are snow-bound for longer than lower elevation sites. As a result, they don’t burn as frequently as lower elevation forests and are often quite dense. In general, there is little or no ecological rationale for thinning these forests—they have always burned in infrequent, high severity fires.
Many of these high elevation types are experiencing significant insect outbreaks that are killing enormous swaths of trees. Spruce are being killed by the spruce beetle (see 7/13 post about spruce). The mountain pine beetle has killed more than 750,000 acres of lodgepole pine in the northern Rockies. Neither epidemic is without historic analog—these sorts of massive die-offs have happened before and are not “unnatural.”
The factors that contribute to big die-offs interact with one another in complex ways. The biggest culprit is probably drought, which stresses trees and makes them much more susceptible to attack by bugs. The other major contributing factor is stand age and structure. Many contemporary lodgepole stands in Colorado date to large fires in the 1850s (many of them associated with early land development, especially mining—see 7/16 post). Today, these stands are relatively old for lodgepole. They are mostly even aged, closely spaced stands—the stand structure that is most vulnerable to bark beetles. Thinning is generally ineffective in preventing these outbreaks and I am not aware of a particular reason why we should prevent these outbreaks. Some people don’t like the aesthetic of dead trees. I think they look cool.
A number of researchers and managers I’ve spoken to have told me that they think that the fires in the lodgepole type—like 2002’s 138,000-acre Hayman Fire—are somewhat larger than they’ve been in the past. These folks have suggested that the most appropriate ecological outcome would be patchy fires of 1,000–5,000 acres that create a more diverse mosaic of age classes on the landscape and break up fuel continuity.
“Fire is the solution, not the problem,” one Forest Service line officer told me two weeks ago. I’ll have a lot more to say about fire later.
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