Does salvage logging make things better or worse?

When a serious wildfire rips through a forest, it has a tendency to kill nearly all the trees in its path. Then come the logging companies. On one hand, to log a burned forest makes a good deal of sense. Some of the timber is still useful, and it’s a way to derive some economic benefit from a landscape that’s otherwise devastated. The process, called “salvage” logging, typically operates in two phases. In the first phase, machines called “feller-bunchers” come through, cut down the dead trees, and pile them into bunches. In the second phase, machines called “skidders” are brought in. Their function is to take those piles of felled trees and cart them back down the mountain.

The problem with salvage logging is that its ecological impact is uncertain. The literature is full of contradictory conclusions, and that’s in part because there are so many variables at play, and no landscape is quite the same as another. In addition, it’s hard to disentangle the effects of the post-fire salvage logging from the effects of the fire itself.

In general, it’s thought that post-fire salvage logging can lead to increased runoff and increased erosion. But those alterations are, as Michigan Technological University research Joseph Wagenbrenner writes in the journal Forest Ecology and Management, “superimposed on a system that already has been highly altered by fire.” Sometimes logging could exacerbate the underlying problem; other times, theoretically, it could counteract it.

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For example, some researchers have written that salvage logging could be beneficial, because it disturbs the soil surface, breaking up the water repellant layer that forms in a fire, and increases the soil’s ability to soak up water. On the other hand, the heavy machinery itself has a tendency to even further compact the soil. Others have argued that salvage logging could be beneficial thanks to the “slash” (the detritus left behind from the logging activity) which increases surface cover and, theoretically at least, decreases erosion. But nobody really knows whether logging increases or decreases surface cover following a fire.

As the frequency and severity of wildfires are increasing – particularly in the western United States – Wagenbrenner and his colleagues brought a decade of scientific observation of unlogged, burned forest ecosystems as well as logged, burned forests to bear on the question. His research sites are scattered through Montana, Colorado, and Washington.

They discovered that the amount of sediment in water runoff increased when a burned forest was logged, at least at smaller scales. Increased sedimentation can lead to flooding, if it disrupts the natural flow of rivers and streams. It can also raise water temperature and alter the aquatic food web, making survival challenging for freshwater fish. And water treatment plants have a more difficult time treating the resulting water.

In addition, the heavy machinery caused soil in burned forests to become significantly more compact than it would have been if left unlogged. Compacted soil is less able to soak up water, which not only influences runoff but also increases the likelihood of erosion.

Finally, the changes in runoff and erosion combined to influence the speed of vegetative regrowth, with unlogged forests recovering faster than those subjected to salvage logging.

Together, this makes it seem like salvage logging is altogether bad news. There is, however, a silver lining. For example, the researchers discovered that leaving slash behind does reduce the severity of the problem, by increasing surface cover. They write that more research needs to be done to determine whether this is because of increased surface texture or because of reduced rainsplash.

The researchers also recommend that “water bars” be formed across the long tracks made by feller-bunchers and skidders. These are mounds of dirt that are meant to divert water runoff and the sediment within it, into areas with higher surface roughness and with higher infiltration rates. They say that adding slash to the ends of the water bars would make them even more effective. They further suggest that logging be done in dry weather or while the ground is covered in snow to reduce the amount of soil compaction that occurs. If that’s not possible, they say that logging companies ought to de-compact the soil after they’re finished, or to apply a layer of mulch.

None of this is to say that burned forests ought to be logged. Instead, Wagenbrenner and colleagues have simply taken a realistic approach. They implicitly acknowledge the infinitesimal probability that logging companies would voluntarily opt to leave burned forests unlogged, offering mitigation strategies instead. – Jason G. Goldman | 14 January 2015

Source: Joseph W. Wagenbrenner, Lee H. MacDonald, Robert N. Coats, Peter R. Robichaud & Robert E. Brown (2015). Effects of post-fire salvage logging and a skid trail treatment on ground cover, soils, and sediment production in the interior western United States, Forest Ecology and Management, 335 176-193. DOI:

Header image:; Chart via Wagenbrenner et al/Forest Ecology and Management