Article reviewed: Fuel treatment longevity in a Sierra Nevada mixed conifer forest

By S. Stephens, B. Collins, and G. Roller. Published in the journal Forest Ecology and Management, 285: 204-212

The plot line: This study looks at how long fire hazard reduction treatments last. The researchers conducted 4 different approaches to reducing fire hazard: doing nothing, prescribed burning, mechanical thinning, and doing both a mechanical thin and a burn. They found that, while the mechanical treatment reduced fire hazard only modestly immediately following the treatment, the mechanical treatment turned out to be about as effective as the prescribed burning treatments were after seven years. All of the treatments were much better than doing nothing, which just got worse over time. They conclude that fire hazard reduction treatments could last quite a bit longer (and be more cost effective) if managers pay attention to the timing of treatments and possibly the combination of different fuel treatment methods over time.

Relevant quote: “The net effect is that fire hazard (indicated by predicted flame length and torching probability) noticeably decreased after 7 years and is similar to the two fire treatments, which was surprising.”   

Relevance to landowners and stakeholders:

It keeps getting worse…

The type of forest where this experiment was done is often called a “second-growth” forest, which in this case means that it was harvested with something similar to a clearcut about 110 years ago when we were building places like San Francisco. It took a lot of lumber to build all of these cities, so much of the Sierra Nevada was harvested at that time. One lingering result of those harvests that we are now faced with is that the forests are still growing as a response to those harvests (this is not true everywhere, but is often true in productive forests). As they grow without any disturbances such as fire or another harvest, fire hazard also tends to get worse and worse. The “control” in this experiment represents the choice that every a forest landowner has a right to make. That is, the choice to do nothing. There are serious risks that go along with making this choice, however, as this study shows. The risk of high severity fire and of the majority of the trees suddenly dying increases (what ecologists might call a “sudden and catastrophic” disturbance). This can even lead to a cycle of continued high severity fires and loss of trees as the dominant organism (i.e. no more forest).   

But there is hope…

These researchers are fire scientists. I don’t think that anyone can devote their career to studying fire unless they are, at least to some degree, pyromaniacle (can you believe that word is not in the spell-check dictionary?). So it is natural that a study like this would emphasize the benefits of using fire to manage forests. But these researchers also seem to have a healthy respect for practicality. They seem to realize that forests of the Sierra Nevada are in nearly-desperate need of treatments to reduce fire severity, and that it is not practical to be able to start burning everywhere (as much as they wish it was). Smaller landowners (who own about a third of the forests), especially have little opportunity or risk tolerance for burning. So it is important for studies like this to continue to identify the tradeoffs between the different treatment methods so that we can conduct treatments that work as soon as is possible, wherever possible.  

The finding here that mechanical treatments eventually were as effective as burning treatments (when using the limited metrics in the study) is significant on a practical level. It does not mean that any mechanical thinning or harvest will eventually reduce fire hazard. But it does mean that if a mechanical treatment is designed to reduce fire hazard then it can be beneficial from a fire hazard reduction perspective. Having options for meeting objectives is usually a good thing.

Relevance to managers:

The mechanical only treatment was characterized by the following 7-year changes:

• Little mortality
• Increased stand growth (as a response to the harvest)
• Eventual decomposition of the activity fuel that was created during the harvest

The fire only treatment was characterized by:

• Lots of immediate and delayed mortality, especially in small and medium sized trees
• A reduction in surface fuels that persisted
• A flat rate of stand growth (probably because of fire-caused mortality and possibly decreased vigor of individual trees).

The paper discusses the potential to realize the benefits of both mechanical and fire reduction treatments by doing a “staged treatment.” Specifically, they suggest conducting a mechanical treatment (including the removal or chipping of small trees in addition to medium sized trees) and then waiting for 10 years or so before conducting a prescribed fire as a way to maintain the low fire hazard.

Of course, if one likes the benefits of either mechanical treatments (the clear winner from a timber perspective), or if they like the benefits of burning (other organisms can surf in the ecological wake of a prescribed fire) then this study’s results also suggests that you can probably keep doing either thing repeatedly and get the particular benefits associated with that treatment, while also reducing fire hazard.

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

One of the surprising results is not explained in this paper. Somehow, the height to crown base decreased in the mechanical treatment between year 1 and year 7. It is not a big deal in terms of the results, because fire hazard decreased in the mechanical treatment in spite of this odd result. But I can’t think of a way in which crown base would have decreased unless there was either lots of ingrowth or epicormic sprouting. But tree density did not increase and canopy cover did increase, which suggests that there was not much ingrowth of smaller trees. And epicormic sprouting doesn’t make much sense either, since only white fir commonly has epicormic sprouts in this forest and I haven’t seen it occur much as a response to light thinning. The method of measuring height to crown base could have changed, which would be measurement error. Or, perhaps the way in which crown base was estimated with the FVS model had something to do with it. The reader is left to make their own speculation, since one was not provided by the authors.

There is some minor conflation between tree and stand growth in the discussion. They state that tree growth “stagnated” following the fire treatments, but mortality is not taken into account. And as far as I can tell, the fire only treatment actually caused a reduction in density (from fire-caused mortality) without a corresponding decrease in basal area. This would actually represent a growing stand in my mind (or at least not a “stagnating” one). Individual trees obviously aren’t all healthy since some are dying, but on balance the stand is growing. This would suggest that the survivors are growing. A more accurate analysis of stand or tree level growth would include a more detailed profiling of diameter distributions as well as ingrowth and mortality.

Some silvicultural terms used could have been chosen better. They say that the mechanical treatment was a thin from below followed by a crown thinning. This might suggest to some that the second thing done was a harvest of the largest trees. But in fact the second treatment was also a thin from below to a basal area threshold. It involved some medium sized trees, but it was never a large tree harvest.