[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 June 14, 2010]

Article reviewed: Radial growth responses to gap creation in large, old Sequoiadendron giganteum

By R.A. York, D. Fuchs, J.J. Battles, and S.L. Stephens published in Applied Vegetation Science In Press, full text available at the bottom of this page.

The plot line: Large, old giant sequoia trees in a native grove of the southern Sierra Nevada were measured to see if they increased in growth (i.e. to see if they “released”) following harvesting of smaller adjacent trees. The researchers found that giant sequoia trees adjacent to harvested openings grew more than similar trees that were not adjacent to harvested openings. They conclude that, even when giant sequoia trees are massive and very old, they maintain their capacity to increase their growth rates when nearby competing vegetation is removed. The authors suggest that moderate-severity disturbances (i.e. when patches of trees die from fires or harvesting) can increase growth rates (and possibly the overall vigor) of old giant sequoia trees that persist through the disturbance.

Relevant quote: “Despite their great age and massive size, old S. giganteum responded positively and with surprising sensitivity to the creation of adjacent canopy gaps. The response occurred quickly and was sustained for the decade following gap creation.

Relevance to landowners and stakeholders:

Large giant sequoia trees are a sight to behold. Even the most utilitarian of souls can appreciate the beauty of the largest trees on earth. In a state known for its eccentricity, it is fitting that California’s iconic native tree should also be an ecological outlier. It is an outlier not only because of its huge size, but also because of the unique way in which it survives (i.e. it’s “life history strategy”). Tree species can usually either grow very fast OR live a very long time, but not both (think baseball: a player can focus on hitting home runs OR hitting for average, but usually not both). Like with baseball, however, there are a handful of exceptions. Giant sequoia is an extreme exception. Its potential growth rate is much higher than the tree species it lives with, and its potential longevity is much greater by a long shot. It is like a baseball player who is a man among boys, hitting for both power and average. In other words,

Giant sequoia is the Babe Ruth of tree species.

It is no wonder, therefore, that people pay a lot of attention to management activities that influence the iconic giant sequoia (there’s no Giant Sequoia candy bar, but there is an SUV named after it). In this article, the researchers documented that even very large giant sequoia trees respond positively in terms of stem radial growth when competing vegetation is removed. Recently, a lot of attention has been given to the capacity of large trees to grow very fast because large and fast growing trees can have several benefits that are relevant for today’s forest management objectives. Such trees can be more resistant to fire, they can pack on lots of carbon, and they are important for wildlife habitat.

Relevance to managers:

Removing surrounding vegetation (via thinning or burning, for example) in order to release trees is nothing new. But often managers tend to think of tree release in the context of trees in regenerating or young stands. Removing competing shrubs and thinning trees are common ways to make individual trees grow faster when they are young. What seems to be surprising about this research is the fact that even very old and huge trees released much like a sapling or young tree would release. It is sometimes assumed that very old trees are “decadent” and therefore do not have much capacity to release. But in the case of giant sequoia and many other long-lived trees, they remain young at heart and can grow faster when surrounding trees are removed. While this study observed release following harvesting, similar release events have been observed following fires that remove adjacent vegetation.

The authors speculate that the reason for the increased growth is increased water and/or nitrogen availability. The implication is that the root systems of the large giant sequoia overlap with the root systems of the surrounding, smaller trees. Giant sequoia is known to have a two-tiered root system, with the capacity to suck water from both deep sources and widely-spread shallow sources.

The harvests in this case were group selection harvests, where up to ¾ acre areas were cleared of all vegetation. The large giant sequoia right on the edges of the openings were the trees that were measured. It is unclear whether lower intensity thinning would cause the same release, but it would be reasonable to predict that lower-intensity thinning would result in lower-degree release.

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

The study was done in one location. If the study had been done in other native grove locations, especially where conditions were wetter, results may have been different.

It would have helped if the authors calculated what the growth response meant in terms of increased carbon gain for the trees. The actual increase in radial growth was on the order of 2 millimeters a year. What does this correspond to in terms of total carbon per year on the whole tree?

It is fair to question whether or not these increased radial growth increases actually correspond with increased vigor. It is true that faster growing trees tend to have a lower likelihood of dying. It could therefore be the case that the releasing trees have greater vigor. On the other hand, the authors bring up the possibility that the trees are simply changing their growth pattern to grow more in stem girth instead of growing taller (as a response to being exposed to wind, perhaps). In other words, it is possible that the trees are re-allocating carbon differently and not increasing the total amount of carbon that is allotted.

[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.

[originally posted at www.foreststeward.com]

Article reviewed: Responses of oaks and tanoaks to the sudden oak death pathogen after 8 years of monitoring in two coastal California forests

By B.A. McPherson, S.R. Mori, D.L. Wood, M. Kelly, A.J. Storer, P. Svihra, and R.B. Standiford, published in Forest Ecology and Management 2010 Vol 259 pp 2248-2255

The plot line: The researchers closely monitored the progression of Sudden Oak Death (SOD) over 8 years, tracking the rate of mortality in coast live oaks, California black oaks, and tanoaks. They compared SOD-caused mortality with mortality not related to SOD (i.e., the “background level” of mortality). Over the monitoring period, they observed a steady increase in SOD infections (bad news for oaks) coupled with a steady decrease in trees without SOD infections (also bad news for oaks). It was much more common for trees to die from SOD infections than for reasons not related to SOD, by a factor of 7 to 9 (very bad news for oaks).

Relevant quote: “Under the pressure of this aggressive pathogen, the presence and propagation of resistant genotypes among the host oaks and tanoaks may provide the best chance for sustainable wildland populations of these species and for management of these forests.”

Relevance to landowners and stakeholders:

When SOD was first identified as the cause of widespread oak mortality in 2000, there was a huge amount of attention drawn to it. Whoever gave it the name “Sudden Oak Death” was a genius if they were trying to draw media attention to the disease. In addition to the name being inherently catchy, “Sudden Infant Death” was in the news at the same time so SOD got some bonus hype via phonetic-association. The disease deserved the attention it received (although I wonder what kind of attention and funding it would have received had it occurred somewhere other than coastal California). At the time, scientists and managers were wondering if SOD would march through all of the western US, leaving billions of dead trees in its destructive path.

Permanent monitoring plots that are revisited year after year are a good way (perhaps the best way) of tracking how forests change. It appears that the range of the SOD impact on native forests has not expanded in the last decade, but the disease continues to have a profound influence on the forests where it has been established (central and northern coastal forests of California). Mature tanoak, California black oak, and coast live oak trees are dieing at a much faster rate than they otherwise would. If the mature trees are not replaced with resistant trees, the decline of these species’ populations in SOD-impacted areas will continue.

Relevance to managers:

The relevant metric in this case is the comparison of SOD-caused mortality rate to background mortality rate. The SOD-caused mortality rate for live oak was 3.1% per year. It was 5.4% per year for tanoak. This may not sound like a lot, but it is indeed a very high mortality rate, especially when compared against the background rate of 0.33% per year for live oak and 0.75% per year for tanoak.

Besides the relevant quote given above, no explicit relevance is provided for managers in this paper. My inference is that this paper provides documentation that SOD-related mortality rates continue to be very high and that managers should be anticipating very significant change in SOD-affected forests. Managers should expect species composition shifts, and therefore shifts in processes such as nutrient cycling and fire behavior as well.

As was the case when this disease was first discovered, a lot of attention and funding is still being given to SOD research and outreach programs. More information is available at here.

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

For an article in Forest Ecology and Management, it has very little content in terms of explicit management implications. The one sentence referring to management suggests that propagation of resistant genotypes could be a worthwhile management response. This idea could have been developed more. How much do we know about current levels of resistance? Are there resistant breeding programs in place? Not that it couldn’t be done, but it would seem to take a lot of work to create an operational nursery program and infrastructure for oak species that currently are not widely planted.

It was unclear to me whether or not IN-GROWTH was measured in these plots. If it was just tracking individual trees over time without also tracking in-growth of new trees, then we are only getting one side of the equation in terms of knowing long-term population trends. I am assuming that in-growth was indeed measured, but the authors did not explicitly say that this was the case so I’m not sure.

It also would have been very helpful to report the overall change in basal area over time. Given the rapid mortality rates and the fact that mature trees are infected readily, I would expect that basal area declined substantially over the monitoring period. Knowing the change in basal area over time gives more information about how competition-related stand dynamics might be interacting with the pathogen. Since they did not report change in basal area, it makes me think that perhaps in-growth was not measured?