[originally posted on www.foreststeward.com on Aug. 2, 2010]

Article reviewed: Fire regimes, forest change, and self-organization in an old-growth mixed-conifer forest, Yosemite National Park, USA

By A.E. Scholl and A.H. Taylor, published in Ecological Applications, Vol. 20 pp. 362-380, available for download here.

The plot line: The researchers went to a forest in Yosemite National Park that had no evidence of recent disturbance (what one might refer to as “old growth”). By measuring the annual growth rings of trees and by estimating when dead trees had originated, they reconstructed what the forest looked like prior to 1899 (when Euro-American settlement and fire suppression started changing forests). They confirmed that accuracy was in the right ball-park with survey data that were collected from the same area in 1911. Like many other studies, they found convincing evidence that the forest of the past was a lot different than it is today. The forest-past had far fewer trees, more ponderosa pine, and less white fir and incense cedar. They deduce that patchy, low severity fires burning less than 10 years apart functioned to “maintain” forest structure by killing individual or groups of trees and by creating conditions amenable to seedling establishment of several tree species.

Relevant quote: “Multiple re-burns at relatively short intervals (5–10 yr) will need to be applied for a sustained period to reduce surface fuels and thin the canopy… application of high-severity prescribed fire would create novel conditions compared to fire effects over the last four hundred years.”

Relevance to landowners and stakeholders:

1905 was a dark year in the natural history of the western US. It is when the policy of fire suppression was implemented (and Bambi hadn’t even come out in theaters yet). Since then, forests have marched in a slow and circuitous fashion farther and farther away from their past condition (a condition largely maintained by Native Americans). In recent decades, researchers have been focusing on quantifying what those pre-fire suppression conditions were. How many trees were there? How big were they? What species were there? These are important questions for landowners and stakeholders who have restoration as an objective.

There is growing realization, however, that those pre-settlement conditions can never actually be restored. The environment, both physical and social, is totally different than it was then. Even if we could know exactly what the forest looked like and were then able to reconstruct it, we would not re-create the forest of the past since it would then change under novel environmental and social conditions. Reconstruction studies like this one that quantify past forest structure are critical for land managers because they help inform restoration treatments in a very general way (i.e. they don't provide "hard targets," but rather set the stage or range of possible targets. Some generalities highlighted by this study include:

• Fire suppression has led to homogenization of forest structure. Variability in structure at several scales is a worthwhile restoration objective.

• Fire: what have you done for me lately? Perhaps Janet Jackson sang this because she knew that fire was much more likely to occur in areas that had not recently burned (within one or two decades).

• Low severity fires rule. There is not a consistent definition of what makes a fire low- versus moderate-severity. In this study, they conclude that low severity fires were the norm and that they should be used in restoration treatments. These “low severity” fires, however, would include locally intense flare ups that killed individual or groups of mature trees that would create canopy gaps up to 4 or 5 acres in size (Personally, I would tend to call this type of fire “moderate severity.”)

Relevance to managers:

For managers hoping to use prescribed fire as a restoration tool in forests similar to the one used in this study, there are several applications that are implied from the study: 

• Repeated low-severity fires at high frequency may be preferable over one high-severity fire. Canopy gaps for shade intolerant species can be developed by the repeated burns and patchy tree mortality (your bound to get some hot spots after several burns).

• At the ~5000 acre scale, there is not much evidence from this study to suggest that south facing slopes should be burned more frequently than north facing slopes. Although from a fire hazard or tactical stand point, there might be.

• To get closer to the forest structure that was present before fire suppression, one would reduce density to roughly 1/3 the present density and basal area would be cut roughly in half. Trees of all size classes would be reduced in density, with a more dramatic reduction in smaller trees. Avoid hard-target upper diameter limits (such as, "thou shalt not kill a tree greater than 24" dbh!").

• Species composition could be restored by having higher mortality in shade-tolerant species, although it may be necessary to actively recruit ponderosa pine in order to achieve it’s past composition.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

I do not prefer the term “self organization” because it hints at the misconception that forests somehow come into perfect harmony if they are left alone. Or it suggests that, prior to fire suppression, the forest was in perfect balance. The authors clearly do not have this connotation in mind, since they discuss the fact that climatic conditions in the past were different than they are now. But the term brings to mind an outdated way of thinking about forests as achieving a “steady state” environment, when actually they are constantly changing and interacting with disturbances and climatic trends. Again, I am sure that the authors are not trying to imply this connotation, but perhaps a different phrase could have been used.

There are lots of sources of uncertainty when it comes to reconstruction studies. There are missing data (trees that decomposed away), inaccuracies in decomposition rates, assuming dead trees grew at similar rates as live trees, assuming that all sudden growth releases/suppressions were caused by fire and not insects or other physical damage. The authors discuss these and state the need for caution in interpreting the results. But in this case, the authors had the unique opportunity to use actual data that was collected in the study area in 1911 as a way to judge the accuracy of their reconstruction. 1911 was shortly after fire suppression began, but is still close enough to be a great opportunity to validate the reconstruction methods.

It is therefore puzzling why they did not reconstruct their forest back to the same exact year as the survey (1911). Instead, they compare their 1899 reconstructed forest with the 1911 measured forest. Why not use the same year? The forest could have changed considerably between 1899 and 1911. From a graph in the paper, it appears that the fire with the largest extent in the last 400 years occurred in 1900. This could have changed the structure throughout the study area considerably. They found that the 1899 reconstructed forest was no different 1911 forest, but perhaps it was different in 1911. Using the same year for comparison may have provided useful information on the accuracy of the reconstruction method.

[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.