July 11, 2009

Thinning on the mountain: A medium-term comparison of burning and thinning in ponderosa pine forests

Article reviewed: Developing resilient ponderosa pine forests with mechanical thinning and prescribed fire in central Oregon’s pumice region

By M.D. Busse, P.H. Cochran, W.E. Hopkins, W.H. Johnson, G.M. Riegel, G.O. Fiddler, A.W. Ratcliff, and C.J. Shestack. Published in the journal Canadian Journal of Forest Research, Volume 39 pp. 1171-1185

The plot line: This was an experiment involving 16 different forest restoration treatment options, tracked over 2 decades. They tried out combinations of mechanical thinning, prescribed burning, and fertilizing in order to compare thinning and burning as a means for achieving management objectives. They conclude that, while no treatment is perfect in achieving all objectives, thinning alone was effective at achieving fuel reduction while not having adverse impacts on productivity. Following-up thinning with burning did not add significant additional benefits, but did result in short-term reductions of shrub cover (which may or may not be a management objective).

Relevant quote: “…the thinning treatment…was a suitable practice for restoring several ecological characteristics of presettlement pine forests and served as an important first step for restoration of these sites. Following thinning, decisions about reintroducing fire or alternative fuel-reduction treatments can be made on a site-specific basis with knowledge of fuel loads, ladder fuel accumulation, wildlife habitat needs, soil fertility, and public concern.”

Relevance to landowners:

Landowners and stakeholders have recently echoed the central question that this study asks- is mechanical thinning an effective surrogate to fire for restoring fire-adapted forests? The authors bluntly answer the question with: “the thinning treatment served as an effective surrogate to fire for managing central Oregon forest vegetation.” Although this and other studies still need to be conducted over even longer time periods to more appropriately answer the question, this study does indeed provide some good guidance. In general, comparisons of thinning and burning go something like this when trying to measure up their relative utility in restoring fire-adapted forests:


·        Easily controls post-treatment species composition of trees

·        Precisely reduces density to desired level

·        Easily targets ladder fuels or smaller trees of poorer vigor for removal

·        Can offset costs by harvesting some trees of commercial value


·        Prepares a bare substrate for seed germination

·        Creates a non-uniform pattern of residual trees

·        May lead to a more fire-adapted composition of understory vegetation if burning is repeated

·        Typically removes smaller and poorer vigor trees preferentially

In this particular study (in no means representative of all forests), the burning treatment did not provide many of these benefits when compared to the thinning treatment or when added to the thinning treatment. The thinning treatment alone also did not achieve all the conditions that may be expected after repeatedly burning (e.g. non-uniform patterns of tree), but the thinning could have been designed to achieve many of these conditions (one big exception is the promotion of fire-adapted understory plants). The relevant quote given above seems to be on target: thinning can be effective, but this does not mean that burning should be dropped all together.

Relevance to managers:

Long-term studies like these that try out different management alternatives and track their impacts over time are vital for inductively improving our management decisions. To set up the context for how to compare thinning and burning, the authors use 4 basic principles of fuel reduction treatments that are provided in a very readable article by Agee and Skinner. The principles for focusing fuel-reduction treatments are:

1.     Reduction of surface fuels

2.      Reduction of ladder fuels

3.      Reduction of canopy fuels (i.e. thinning)

4.      Retaining large, vigorous trees that are fire adapted (e.g. thick bark, fire-resistant foliage).

In this particular case, the method of thinning whole trees was clearly the most effective fuel treatment if the objective was to achieve these 4 principles. At the same time, there was no detection of the potential negative effects sometimes associated with thinning (no effect on nutrient loss or productivity).

Critique (I always have one, no matter how good the article is) for the pedants:

The authors briefly point out that these particular fires were not very hot. I think this could have used some more discussion. A fire ecologist or manager might argue that the prescribed burning treatments would have stacked up much better if they had been hotter. The authors also did not address the fact that the burns were done in early summer. What if they had been done in late summer or early fall, when fires typically used to occur? This is an especially relevant question for the restoration of fire-adapted understory plants.

Maybe they are planning on doing this in another paper, but they could have modeled fire behavior in these different stands to see what the treatment effects were on predicted fire intensity.