- Author: Rob York
Article reviewed: Arbuscular mycorrhizal colonization of giant sequoia (Sequoiadendron giganteum) in response to restoration practices
By C. Fahey, R.A. York, and T.E. Pawlowska. 2012. Published in the journal Mycologia, 104: 988-007. DOI: 10.3852/11-289
The plot line: This study looked at the way that roots of giant sequoia seedlings interact with a fungus (together forming what is known as mycorrhizae). They found that when they planted giant sequoia seedlings, beneficial fungi would attach on to the seedling’s roots mainly when the seedlings were planted in open sunny conditions. While it was hypothesized that the fungi would not be as common on roots in areas that had been burned, there was no difference between burned and unburned locations. Also interestingly, the beneficial fungi actually seemed to outcompete the harmful fungi, thus possibly helping seedlings to avoid other diseases. They make the inference that this mycorrizal interaction between tree and fungus is a potentially important process in giant sequoia growing fast as a seedling and may be a key ingredient in how it eventually becomes the world’s largest organism.
Relevant quote: “This [rapid seedling growth in sunny locations] suggests that the symbiosis as a whole has improved function at the centers of gaps because both partners have improved growth.”
Relevance to landowners and stakeholders:
Nature is dominated by individualistic, chaotic, and brutal selfishness. Organisms are hard wired to have a primary goal- to reproduce. Often, plants achieve this goal at the expense of other organisms via a fierce competition for the triple-crown of resources: light, water, and nutrients (it’s a baseball theme today). But sometimes it is in an organism’s best interest to be of assistance to another. Such is the case with mycorrhizae, which is a combination of plant roots and fungi attached to each other (“myco” = fungi; “rhizae” = roots).
Giant sequoia is an interesting species because it is so different than any other in so many ways. The most obvious difference that people know about is its tremendous size- larger than any other tree on earth. But the way that it reaches this size, and in fact its entire “life history strategy” is somewhat of an outlier when you compare it to other tree species. It’s mycorrizal interactions are no different. It forms what are known as “arbuscular mycorrizae,” which is uncommon in conifer trees. Beyond that, not much is known about this plant-fungus interaction in giant sequoia, but this study offers a little insight.
The primary relevance to landowners and stakeholders might be that this paper reminds us that planting a tree and getting it to survive and grow is a complex, ecological process. Planting is something we might be doing a lot more of in forests, as climate change and wildfires become forces that hinder natural regeneration. Successfully planting a tree, where the measure of success is getting the tree to complete its life cycle, involves much more than planting a tree and walking away. It involves understanding the resource requirements for that species, and how that particular tree will be able to make its way up into the canopy to become mature. For giant sequoia, and most other trees, the mutualistic interaction that seedlings will have with root colonizing fungi is key information. This study suggests that planted giant sequoia seedlings have the best chance of success when they are placed in distinct canopy openings in sunny conditions, in part because this is where the mutualistic relationship with fungi can benefit giant sequoia growing quickly into the tall canopy above.
By the way, I think most green campaigns that ask you to pay a little extra so that you can sponsor the planting of tree seedings are scams. I would not advise believing or certainly not paying for such “plant-a-tree campaigns” unless you knew the species that was being planted, the location, and the method used for tracking survival.
Relevance to managers:
OK, here’s where the baseball analogy suggested by the title finally comes into play. Stay with me here…
Giant sequoia is a base runner, where rounding third means going home, which in terms of a tree is equivalent to reaching the canopy and reproducing (and for a person on a date, this is of course equivalent to something similar).
The fungus that forms the mycorrhizae is the third-base coach, hoping to be of some assistance to the base runner but hoping to get something in return (a job).
A base runner doesn’t really need the third base coach, but the third base coach definitely needs the base runner to have a job and make a living. Often the third base coach can be helpful to the runner, but only when things are already going pretty well for the runner. When they are rounding third base, the runner is in pretty good position to score, and the third base coach can help them score. Sometimes, however, the third base can be a hindrance if they get in the way or if they give the runner some bad advice. But usually they are a help. And of course no championship team (such as the Giants) would be without a third base coach.
Get it? Giant sequoia seedlings are happy to have this relationship with fungi, but only when things are already going well. Mycorrhizae were more common on seedlings when they were planted in the open, so there was plenty of carbon for the seedling to spare. It is carbon that is the currency paid by the tree, in return for nutrients like Phosphorous from the fungus. And fungus can also keep the plant out of trouble by fighting off pathogenic fungi, kind of how a third base coach can tell the runner to get back when the pitcher tries to pick them off.
Implications? If you plant giant sequoia, do so in distinct canopy openings and pay attention to how the nursery either sterilized or inoculated the soil. In this case, the nursery had sterilized the soil so the mycorrhizae developed on roots after the seedlings were planted in the field. When you plant far away from a mature forest edge, don’t worry about it taking a long time for fungus to colonize the area- they are probably already there because of lateral roots from surrounding trees.
Critique (I always have one, no matter how good the article is):
The authors set this study up as a hypothesis-testing experiment, but there is so little known about mycorrhizae in giant sequoia that doing so sets up an easy claim of “surprising” or “unexpected results.” In fact, some information in the discussion that is presented would actually suggest that the hypothesis should have been the opposite of the one proposed in the introduction. It’s not a big deal with this study, but more of a critique of studies in general that tend to set themselves up so that they can easily say that they got a “surprising result…”
- Author: Rob York
Article Reviewed: Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests?
This review provided by the Battles lab of UC Berkeley
By M. Simard, W.H. Romme, J.M. Griffin, and M.G. Turner. Online preprint. Ecological Monographs. Availability: http://esa.org/papers/
Plot line: Bark beetle outbreaks have caused extensive mortality of pine forests across western North America. In the aftermath, these forests with many dead and dying pines are widely considered to be at extreme risk of catastrophic wildfire. The goal of this study was to evaluate this conventional wisdom. Specifically they focused on the potential for the combined effects of these two disturbances, beetle kill and fire¸ to alter the structure and function of the Greater Yellowstone Ecosystem. Their approach was to quantify the impact of beetle-induced mortality on fire behavior in lodgepole pine forests. They measured forests fuels in an extensive network of plots that spanned a gradient in time since bark-beetle mortality. In other words, they used a chronosequence approach. This chronosequence included currently undisturbed stands (no bark-beetle mortality) to stands where bark beetles killed all the pines in the canopy 36 years ago. They then used this empirical data to inform fire behavior models. These models were used to simulate the impact of beetle damage on fire behavior.
The results of the simulations convincingly showed that mountain pine beetle outbreaks in the lodgepole pine forests of Greater Yellowstone do not increase the risk of active crown fires, the most destructive type of wildfire. Instead, their results suggest that beetle kill may actually decrease the risk of crown fire. Their explanation is that the main fuel for crown fires is tree needles. Once killed by mountain pine beetles, these needles are retained in the canopy for only brief period (1-2 years post outbreak). Thus the exposure to the risk of catastrophic crown fires is short. In the longer term (25-35 years post outbreak), the accelerated growth of understory trees may increase the potential for passive crown fires. However these fires are less intense and spread more slowly than active crown fires. In general, the authors concluded that weather conditions may have a greater influence on wildfire behavior than fuel characteristics affected by bark-beetle mortality.
Relevant Quote: “Our results suggest that mountain pine beetle outbreaks in Greater Yellowstone may reduce the probability of active crown fire in the short term by thinning lodgepole pine canopies.”
Relevance to landowners/stakeholders
The public perception of the ongoing bark-beetle outbreak in western North America is that the resulting forest of dead red and grey trees is an environmental disaster. Chief among the concerns is the risk of catastrophic wildfires like those experienced in 2007 in southern California (primarily San Diego county). This study, one of the few informed by evidence, suggests that the perceived risk of wildfire is exaggerated for the lodgepole pine forests in the Yellowstone area. However this study does not address other forest health concerns related to the widespread tree mortality nor does it address the future dynamics of these forests.
Relevance to managers
The abundance of dead and dying in trees in the western North America causes great concern among private and public forest managers. These managers struggle with the question of what to do about the “aftermath” forests. One option is salvage harvesting. The arguments for and against such an intervention are complex but a common reason to intervene is to reduce the risk of catastrophic wildfires. The results from this research cast doubt on this rationale. Based on their data and simulations, beetle kill does not increase the risk or hazard of wildfire in these forests.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
This research relies on two well-established methodologies in forest science – chronosequences to acquire temporal data and simulation models to predict ecosystem behavior. Chronosequences assume that the only difference between research sites is time since last disturbance (in this case, the disturbance is the bark beetle outbreak). The authors do a good job supporting this assumption but still no space-for-time substitution is perfect. Fire science must rely on models since direct tests at the appropriate scale are impossible. Nobody is going to set an experimental fire at a landscape level. The fire behavior simulator used in this paper is a well-established approach. The physics of fire are well understood. However our ability to accurately represent the three dimensional distribution of fuels in a forest is limited. For example, the surface fuel loads and their interaction with fire are summarized in fuel models. While the authors did measure the surface fuel loads, they did not explain how they translated these results into synoptic fuel models. Nor do they specify which fuel models were used in their simulations. Also a key to their argument is the potential for crown fires in the aftermath forests but the current fire models (like the NEXUS model used in the paper) struggle to capture the complex dynamics of crown fires. This challenge is particularly relevant to question posed in the title since mountain pine beetle outbreaks kill trees while they are still standing. There is a well-documented progression from recently dead trees where all the leaves die and turn red but remain on the tree (red stage) to the later stage where these dead leaves have fallen off the tree. Leafless standing dead trees are referred to as the grey stage. The transition from live green tree to red stage to grey stage also represents a change in canopy fuel loads. The authors account for these losses in foliage by assigning discount rates but these assignments are made in an ad hoc manner without support or evidence to justify the discounts. Since predicting crown fire behavior is essential to assessing the hazard related to beetle-induced mortality, it seems that a sensitivity analysis is needed to demonstrate that the results are not particularly dependent on the discount rate assignments.
- Author: Rob York
[originally posted on www.foreststeward.com on Oct. 20, 2010]
-> This post graciously provided by the Battles lab at UC Berkeley <-
Article reviewed: Interactive effects of historical logging and fire exclusion on ponderosa pine forest structure in the northern Rockies
By C. Naficy, A. Sala, E. G. Keeling, J. Graham, and T. H. Deluca. Published in Ecological Applications, Vol. 20, No. 7, pp. 1851-1864. Full article available.
The plot line: This study evaluated the contribution of historical logging to the widespread increases in stand density and the abundance of fire-intolerant tree species that are often attributed solely to fire exclusion. The research was conducted in the ponderosa pine/Douglas-fir forests of the northern Rocky Mountains. The study paired 23 historically logged, fire-excluded sites with 23 unlogged, fire-excluded sites. In addition, the study compared stand structure and composition in unlogged, fire-excluded and logged, fire-excluded stands to contemporary unlogged, fire-maintained stands in order to provide a baseline to quantify management-induced changes in forest characteristics.
The authors conclude that historically logged, fire-excluded ponderosa pine forests of the northern Rocky Mountains have much higher average stand density, greater homogeneity of stand structure, more standing dead trees and increased abundance of fire-intolerant trees than similar unlogged, fire-excluded forests. Further, the study found that the interactive effect of fire exclusion and historical logging significantly exceeded the effects of fire exclusion alone. Based on these findings, the authors conclude that historically logged sites are more prone to severe wildfires and insect outbreaks than unlogged, fire-excluded forests and suggest that unique restoration approaches may be required to manage ponderosa pine forests with these distinct management histories.
Relevant quote: “To the extent that modern wildfires are driven by vegetation and fuel characteristics, historically logged stands are likely more prone to severe, stand-replacing wildfires than unlogged, fire-excluded stands.”
Relevance to landowners and stakeholders:
This article challenges the common assumption that fire suppression is the sole cause of the increased forest density that is associated with severe, contemporary wildfires and insect outbreaks in many fire-prone forests of the western United States. The article highlights the lack of data on the long-term effects of various modern silvicultural practices. The authors suggest that where allowance of natural fires is not feasible, the potential negative impacts of alternative fuels treatments requires more careful consideration as part of a longer term approach to fuels management.
Relevance to managers:
For managers attempting to reduce the risk of severe wildfires in ponderosa pine/Douglas fir forests in the northern Rocky Mountains, this study makes several suggestions:
- The current forest structure and composition in historically logged ponderosa pine forests suggests that these forests should be primary targets for fuels reduction efforts.
- While previously logged, fire-excluded forests may benefit from significant mechanical stand manipulations before fire can be safely introduced, unlogged, fire-excluded forests may require much less invasive treatments.
- The authors also conclude that the potential long-term risks associated with mechanical treatments, particularly in unlogged forests, should receive greater attention because the extent to which modern mechanical treatments could have similar long-term counterproductive effects remains largely unknown.
The authors note that the effects of the interaction between historical logging and fire exclusion are likely to vary across broad geographic regions and that long-term responses to timber harvest are likely sensitive to differences in the specific nature and intensity of past silvicultural treatments.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
While we accepted the general conclusions of this article, we did have some methodological/analytical concerns. For instance, the article claims to be novel in its consideration of the interactive effects of historical logging and fire exclusion. However, this interaction was not actually tested with the statistical approach used in the study. The conclusions regarding the interaction of these disturbances actually examined the additive effects of these disturbances and simply implied the interaction. Many of the potential methodological problems in this study were difficult to avoid given the nature of the data. For instance, the lack of detailed historical data on logging history and the large variation in average fire return interval. We discussed the potential problems of the close proximity of the “paired” stands and the methodological problems with the treatment of the logged and unlogged sites as paired data. The statistical approach used (i.e., paired t-tests) was inappropriate given the selection strategy for the paired stands and was perhaps too simplistic of an approach to test some of the stated objectives, such as the interaction between historical logging and fire exclusion. Finally, one of the final conclusions of this paper was that historical logging had increased forest homogeneity. However, the impact on the homogenization of stand structure was never actually tested.
/span>- Author: Rob York
[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.
/span>- 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.
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