- Author: Rob York
Article reviewed: Stand-replacing patches within a ‘mixed severity’ fire regime: quantitative characterization using recent fires in a long-established natural fire area
By B.M. Collins and S.L. Stephens. Published in the journal Landscape Ecology and available for download.
The plot line: This study used an area within Yosemite National Park where wildfires had been allowed to burn over the past ~30 years. They looked for patterns in how often fires created large gaps (holes in the canopy) versus small ones. In other words, they measured how often fires killed a lot of trees versus just a few. They also attempted (with pretty good success) to explain the reasons why some gaps were large (fire more severe) while others were small. They found that, while most gaps were small (less than about 5 acres), there were also a few very large gaps that were created by fire- up to 230 acres! Overall, the portion of the burned areas that actually created gaps (as opposed to the fires remaining on the surface and not killing lots of trees), was about 15% over a 30-year time period. For previous fire occurrence to reduce the chance of another high severity fire occurring, the fire had to occur recently (within about 30years). This is what I call the Janet Jackson effect… “what have you done for me lately?” They conclude that, while the high severity fires that create gaps were not the dominant type of fire behavior that occurred in this case, they had a significant contribution to the mix of fire severity that occurred.
Relevant quote: “While high-severity fire represents a fairly low proportion of the total burned area (15%) stand replacing patches should be considered an important component shaping these forests.”
Relevance to landowners and stakeholders:
Studies like these that attempt to measure how disturbances shape forests are important because debates about forest management often come down to debates about what types of disturbances are “more natural” than others. If a certain treatment “mimics” a natural disturbance then it might be considered better. For example, doing clearcuts or allowing high severity fires to occur may be preferred because they are thought to be a more natural type of disturbance. On the other hand, doing light thins or only allowing low severity fires may be thought of as more natural. Mimicking a disturbance regime might be the primary objective of management, as was discussed in this post about using disturbances as a guide for management.
With respect to fire, most people think that the “most natural” regime for the Sierra Nevadas is one referred to as mixed-severity. As the authors point out, this term is difficult to define. Very broadly defined, it simply means that when fires occur, they are diverse in terms of having some areas where lots of trees are killed but also having areas where no or very few trees are killed.
The study points out the confusion of this term, however, when it is defined more precisely. From the results, one could conclude that these fires were not of mixed severity at all because most of the canopy gaps were relatively small (i.e. it was a low severity regime). On the other hand, the portion of total area in canopy gaps was dominated by a few very large gaps (i.e. a high severity regime). Putting this fire regime into the context of other types of regimes that we see in different forest types around the world, however, I think that it is safe to say that the researchers found these fires to be of mixed severity.
Relevance to managers:
Figure 4 in this paper is very useful. Perhaps not as a broad guide for management across the Sierras, but more as a demonstration of how one could think of disturbances as a guide for management:
The graph shows that, in this case, most of the gaps that were created by the fires were relatively small. In general, there was a downward trend in the frequency of larger gaps. One could related this to management, for example, by allocating forests to either even-aged or uneven aged management in order to also achieve a downward trend in gap size. I think the total patch size area would be tremendously variable from fire to fire, so that part of the graph is less relevant. And if they had looked for smaller gap sizes (their minimum was 1.2 acres), the dots on the graph may have ended up looking more U-shaped.
Taking it another step, one could even set a rotation age based on this graph. Over a 30-year time period, 15% of the total area that burned was converted to gaps (i.e. regenerated to new trees). If one were to mimic this conversion rate into the future, it would lead to an approximate 200 year rotation age. Again, I don’t think this is useful as a broad guide until more studies like this are done, but the method may be useful for those how have an objective of mimicking what they think is a natural disturbance regime.
Critique (I always have one, no matter how good the article is):
I really like what these researchers did, and I’ve been waiting for something like this to be done for mixed conifer forests. Similar studies have been done in other forest types, but it is more difficult with mixed-severity fire regimes so there are some limitations.
In terms of being useful for management, it would have been much better to have a smaller minimum mapping unit that 1.2 acres. What we consider regeneration of a distinct cohort can occur at much smaller scales. Ideally, we would go down all the way to the scale of a single canopy tree dying. It makes perfect sense why they used 1.2 acres- it was because of technological limitations of remote sensing data. But I think it would have been useful to do some kind of sensitivity analysis. In other words, how would the results have changed if the MMU was smaller? Bigger?
While this study uses an area that is probably as good as we can find when it comes to areas where fire has been allowed to burn, the reality is that it still has not been very long. This area has only had two fires, and as this study points out, fires are tremendously variable so more will be needed in terms of having broad implications. The authors know this and point it out, but it is worth noting as a limitation- we’ll get better information as more time passes and more fires burn in these areas. It will take a while for us to overcome the 60+ years of fire suppression, a period of time that were the dark ages of fire ecology where we learned nothing! Fiat flamma!
- Posted By: Rob York
- Written by: Reproduced from the website, www.foreststeward.com
Article reviewed: Interacting disturbances: wildfire severity affected by stage of forest disease invasion
By M.R. Metz, K.M. Frangioso, R.K. Meentemeyer, and D.M. Rizzo, published in the journal Ecological Applications, vol. 21: 313-320
The plot line: These researchers evaluated the influence of Sudden Oak Death (SOD) on wildfire severity. They were able to do so because they had measured forests that were infected to various degrees (ranging from no infection to very advanced infection) by SOD prior to a wildfire (the Basin Complex fire) occurring. They found that, where SOD had recently infected forests and caused lots of standing dead trees, fire severity was greater but SOD infection was not the primary determinant of fire severity. Burn severity was very patchy and influenced by many other factors besides whether or not the area had been infested with SOD. In areas where SOD infection was advanced (i.e. several years since first infections), there was greater burn severity at the forest floor but again SOD infestation was not a major determinant of fire severity. They suggest that management efforts may be more effective if targeted in areas where SOD is still in the initial stages of infestation (i.e. where there are lots of standing dead trees with dead leaves and branches).
Relevant quote: “Our results indicate that the timing of fire relative to disease progression is an important predictor of burn severity in infested areas because differences among fuel types were more important indicators of damage than pathogen presence alone.”
Relevance to landowners and stakeholders:
Wildfire risk is often grossly over-simplified. During this time of year, for example, we begin to hear reports on the news that this year’s fire danger will be “especially high.” If it is a wet spring, they say that fire danger will be “especially high” because of all of the growth of fuels (vegetation) that is occurring. If it is a dry spring, they say that fire danger will be “especially high” because of the dry fuel conditions. And if it is an average year, they usually wait until we have a hot and dry week and then claim that it is actually a very dry year and, you guessed it, fire danger will be “especially high.” The reality is that fire behavior is a result of a huge complexity of factors that include how much fuel is available to burn, the structure of the fuel, the local topography, and the weather conditions at the time of the fire.
Historically, fires in central coast forests of California have not occurred as frequently as they have up in the Sierra Nevada. But as a landowner, I am actually more comfortable in the Sierra Nevada than in central coastal forests when it comes to altering the behavior of fire when occurs. Fires in the coastal forests appear to be more weather-driven and less fuel-driven than those in the Sierra Nevada. And as a forester, I know that I can alter fuels on my land to manage fire behavior. Weather? Not so much. This research suggests that treating areas of recent SOD infestation might lower fire severity a little, but that other factors will accumulate to have more of an influence on fire behavior. For forestland owners in the central coastal forests of California, you should hope that insurance companies don’t read this article.
Relevance to managers:
While infestation occurs as a gradual process, there appears to be 4 logical stages of SOD infection:
1. Initial infection- trees lose vigor and slowly decline over about 6 years.
2. Crown mortality- over 1 or 2 more years, leaves and small branches die and slowly shed off of the dead trees
3. Snag decomposition- snags either gradually crumble apart of fall over
4. Log buildup- Logs are on the ground and gradually decompose
As discussed in a previous post, Sudden Oak Death is anything but sudden. As the authors of this article point out, SOD truly is a “chronic and progressive stress” rather than a sudden one.
It is during stage 2 above where the authors of this research seem to be recommending that managers focus on in terms of reducing fire risk. This could mean prioritizing fuel-reduction treatments to occur in areas that have high densities of standing dead trees with lots of dead biomass still in the crowns.
Fire severity may be just as high or higher in stage 3, but this study did not measure fine surface fuels so it is unknown. But it would make some intuitive sense to me that a buildup of litter and debris from SOD may increase fire severity. As the authors mention, this needs further study. See a related post on the interaction of bark beetles and fire severity in lodgepole pine forests.
Of course, the most effective management would be to try to stop SOD in the first place, but this is obviously difficult. I heard one of the authors of this article give a talk about management options with respect to lowering SOD infection. He mentioned two things that I recall:
1. Thinning + burning might be effective (presumably by increasing individual tree vigor and by reducing future fire severity)
2. A no-host buffer around critical areas (e.g. removing host species around a park core area, for example) would be very difficult because most hosts sprout.
Critique (I always have one, no matter how good the article is):
The primary limitation is the fact that only large logs were measured as surface fuel prior to the fires. Obviously if the researchers had known that a fire was going to happen, they would have been more comprehensive in measuring surface fuels. But just measuring “1000 hour fuels” leaves a lot of the surface fuel equation unaccounted for.
- Author: Rob York
[originally posted at www.foreststeward.com on May 28, 2010]
Article reviewed: Fuel buildup and potential fire behavior after stand-replacing fires, logging fire-killed trees and herbicide shrub removal in Sierra Nevada Forests
By T.W. McGinnis, J.E. Keeley, S.L. Stephens, and G.B. Roller, published in Forest Ecology and Management 2010 Vol 260 pp 23-35
The plot line: Four areas that burned with intense wildfires in the Sierra Nevada were examined in order to explore salvage logging and herbicide spraying effects on species composition and predicted future fire behavior. The researchers conclude that logging had small effects on species composition and fire behavior, especially when compared to the effects of spraying shrubs with herbicides. As would be expected, herbicide-treated areas had lower amounts of shrubs present and greater amounts of grasses and forbs (including some exotic grasses and forbs). Herbicide-treated areas had lower predicted flame lengths and rates of fire spread, but mortality to small trees was still expected to be high in herbicide-treated areas. In the case of the four fires used in this study, it was post-fire management treatments such as shrub removal, thinning, and pruning (and not salvage logging) that most influenced forest change and future fire behavior following wildfires.
Relevant quote: “Ultimately, the amount of fuel remaining in any given stand after logging was under the control of individual Forest Service managers…”
Relevance to landowners and stakeholders:
The debate continues. Should we do salvage logging after wildfires? This study looks at the issue with respect to the effect of logging on forest structure and composition, but there are of course many other effects that could be considered.
Although this study is limited by a lack of experimental control (they found areas that happened to be treated differently, rather than controlling and assigning treatments experimentally), the stark difference between the effects of logging versus herbicide treatments seemed convincing. It was not the logging, per se, that influenced what plant species were present or how vulnerable the forest was to fire. It was the actions that occurred after the logging that made the difference. In central and southern Sierra Nevada forests, shrub communities profoundly influence how a forest develops following disturbances. It therefore makes sense that management treatments which influence the shrub community (like spraying herbicide) would influence forest development.
There is a need to improve upon this study and conduct a variety of treatments (including controls where nothing is done) in an experimental fashion following wildfires in the Sierra Nevada. Rather than doing nothing because there is uncertainty in what the effects of active management are (after all, there is plenty of uncertainty in the outcome of doing nothing), different alternatives can be tested in order to hone in on preferred treatments for meeting given objectives. This is the essence of active adaptive management.
Relevance to managers:
Disturbances of moderate or high intensities in Sierra Nevada mixed conifer forests tend to initiate a “shrub response.” Shrubs can germinate from dormant seeds or sprout from existing plants to quickly occupy a site and its plentiful resources (light, water, and nutrients). Shrubs can dominate a site for decades to centuries to indefinitely. Shrub removal has been a common and effective treatment for managers aiming to ensure or accelerate the time it takes for the site to be dominated by trees, but there is of course biological and social baggage associated with using herbicides. Rapid tree dominance following fires may not always be an objective, but where it is an objective, it is hard to beat herbicides in terms of treatment effectiveness in meeting that objective. In this study, it was not surprising that spraying shrubs with herbicides reduced shrubs (duh), or that there were more exotics (because there are more of ALL species when resources are plentiful, not just exotics). The more relevant results were the effects of herbicides on the fuel structure.
Having a lot of shrubs creates a certain fuel structure that facilitates a certain type of fire (often a canopy fire), while trading shrubs for trees and grass/forbes (via spraying herbicide) creates a different type of fire (often a surface fire). The researchers predicted that either structure would promote a fire behavior that would kill many of the trees while the trees are small. But eventually big trees will become established (if they aren’t killed by fire) and become more resistant. And the time it takes to grow big trees is shorter when shrubs are controlled. Again, this assumes that tree dominance (as opposed to shrub dominance) is an objective.
For a manager wanting to greatly reduce the probability that a young stand of trees is lost to wildfire, the modeling done in this study actually implies that a relatively intense host of treatments might be necessary to reduce risk to a minimal level. Assuming unlimited resources (impossible, I know), a manager really trying to reduce risk of loss in a young stand of trees might do the following:
- Maintain, via thinning, wide spacing to maximize individual tree growth (and target smaller trees for removal when thinning)
- Reduce or maintain low surface fuels by whole tree harvesting when thinning or by burning (prescribed or piles)
- Reduce exotic and grass understory biomass via either prescribed burns or herbicide application
- Prune up trees as high and as frequently as feasible while avoiding loss of growth from pruning too much
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
The primary limitation is the lack of experimental control. For example, two of the controls had higher pre-fire basal area than the corresponding treated areas. This means that the areas that were logged (treatment areas) had higher tree densities than the areas not logged (i.e. the controls). The authors state the problems with the controls, but then never explain why this was OK in their opinion for the various inferences made or what it might mean for limiting the scope of the study (the area for which they are making inferences appears to be the entire Sierra Nevada).
It is definitely worthwhile to do studies like this that create experiments retrospectively (case studies, in other words), but they are inherently limited when compared to experiments designed before treatments are applied.
