- Author: Rob York
Article reviewed: Density effects on giant sequoia (Sequoiadendron giganteum) growth through 22 years: Implications for restoration and plantation management
By R. York, K O’Hara, and J. Battles, published in Western Journal of Applied Forestry, vol 28: 30-36
The plot line: This study controlled the number of giant sequoia seedlings in a given area and measured the effect of the different densities on growth through 22 years. The researchers found that giant sequoia can grow very fast when density is low and that it grows very slow when density is high. This is a fairly typical result for most species, but giant sequoia had an exceptionally large difference in its growth under high and low density environments. The researchers relate the results to giant sequoia’s adaptation to growing in recently disturbed and open environments (i.e. it is a “pioneer species”), and make suggestions for managers desiring to alter the way that young giant sequoia forests grow. They conclude that giant sequoia can be “trained” to grow large quickly by thinning or prescribed burning early on and thinning to wide spacing compared to other species.
Relevant quote: “…large stem size can be achieved relatively quickly with low densities, producing large carbon reserves per tree (potentially the largest possible individual tree reserve on the planet) with relatively low risk of loss from fire or disease. Put simply, giant sequoia can be managed for a variety of objectives.”
Relevance to landowners and stakeholders:
This is a traditionally designed experiment applied to a very unique species. Experiments like this are usually designed for species that have commercial value because they can help understand the long-term effects of density management (i.e. planting and/or thinning) on timber production. While giant sequoia has potential to be an important commercial species, it is mostly known for its standing as the largest tree species in the world. Because humans have removed fire- the process that sustains giant sequoia, regeneration has declined within native groves. While some fire has been re-introduced, both the rate of re-introduction and the types of fire often fall short in terms of facilitating giant sequoia regeneration. For vigorous and dense stands of giant sequoia that actually have become established, this study can help inform decisions about whether to alter the development of the giant sequoia stands with further treatments such as thinning or burning.
The relevance for landowners and stakeholders is this paper’s reminder that giant sequoia is “disturbance dependent.” As discussed in previous entries, it needs a pretty large disturbance to the canopy in order to regenerate. Even the new forest made of small giant sequoia is adapted to further disturbances. In managed areas, this can mean thinning or prescribed fire. While giant sequoia is pretty good at competing with other species once established, its growth rate can be severely curtailed if left under high density.
Relevance to managers:
For managers who plant giant sequoia outside of groves and intend on managing it for large size (i.e. for timber, carbon, or assisted migration), the relevance is pretty clear: give it lots of room to grow. This means either planting at low density and controlling competing vegetation, or thinning relatively early. The researchers suggest that the widest spacing used in this study, 20 feet, was the best in terms of growing large trees without losing much in total stand volume. The optimal spacing may have been even wider had an even wider spacing been used. To sustain rapid growth in dense plantations, thinning would be applied around year 10 on a productive site. Sequoias seem to occupy the underground growing space very quickly. Even if crowns are not close to overlapping, it is likely that the roots of adjacent trees are competing heavily for water and nutrients.
For native grove managers, the relevance is to pay attention to the dense stands of giant sequoia that we do have. While more research is needed to find out the effects of burning frequency and severity on these young stands, I believe that fire does have an important role to play in their development (and if fire is not feasible, then thinning). Those who disagree would cite examples of dense giant sequoia stands developing just fine in pure, high-density conditions. But these stands may also be vulnerable to high severity fire and their capacity to be resilient in the face of climate change is uncertain at best. Some are also concerned with fires killing young giant sequoia that can be viewed of as precious given the past lack of regeneration following fire suppression. This and other studies show, however, that giant sequoias can release very quickly from disturbances that lower density. If there are so few giant sequoias present that a prescribed fire could endanger them all in a given area, then the density was probably too low to begin with. I have observed dense patches of giant sequoia surviving moderate intensity fires just fine, with the outer perimeter trees dying but acting like buffers and protecting those trees within the patch.
Another interesting note from this study is the incredible production of branches by giant sequoia. Branches are small but very dense, measured at an average of 17 branches per year. Compare this to ponderosa pine, which is more like 4 to 6 per year.
Critique (I always have one, no matter how good the article is):
It should be noted that the experiment did not include fire as a treatment. So the paper’s discussion of fire used to thin dense giant sequoia stands is speculative. The study also did not include a thinning treatment, so the discussion of thinning is also limited to the extent that planting density effects can be related to thinning. The experiment was also done on a productive site. The results probably would have been different on a lower productivity site.
This study was the brain child of Bob Heald, who I am sure understood that the more interesting results of the study would come along well after he retired. Such is the nature managing or studying forests. The legacy of a forester’s decision lives on well past the forester.
- Author: Rob York
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.
- Posted By: Rob York
- Written by: Rob York (view the full blog at www.foreststeward.com)
Article reviewed: Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests
By B.M. Collins, R.G. Everett, and S.L. Stephens. 2011. Published in Ecosphere, Volume 2(4):art51. doi:10.1890/ES11-00026.1
The plot line: These researchers found some rare vintage 1911 data that was collected in what is now Yosemite National Park. With what they think is reasonable confidence, they were able to relocate the 1911 areas and do a century-long re-measurement. The times of measurement in this case are especially relevant- 1911 conditions reflect what the forest looked like under an unaltered fire regime (i.e. before Euro-Americans came along and screwed things up). Because this particular forest has no history of harvesting and only recent reintroductions of fire, the re-measurement assesses long-term change as a result of fire suppression and evaluates the effectiveness of recent fires in re-establishing a forest structure that is similar to 1911. As many others have also found, they found that fire suppression dramatically altered forest structure over time, leading to much higher tree densities, and a decline in ponderosa pine traded for an increase in white fir. Areas that burned recently with moderate severity fires were much more similar in structure to 1911 conditions than areas that did not burn or that burned with low severity fires. They conclude that current restoration treatments are likely not creating forest structures that are similar to pre EuroAmerican times (they are still too dense), and that managers should consider the complex interaction of climate change with fire suppression when trying to create resilient forests.
Relevant quote: “While fires of lesser intensity likely will reduce surface fuels and understory trees which is important in reducing potential tree mortality from fire and possibly maintaining desired forest conditions once achieved initially, they may not be sufficient alone to achieve historical forest structure given the substantial tree establishment that occurred during the fire exclusion period.”
Relevance to landowners and stakeholders:
There are several ways to reconstruct what the forest looked like in the past (e.g. pictures, written accounts, backwards modeling, isotopes, pollen, etc.). All of the available methods have their problems with bias, but nothing beats raw data from measurements collected by actual people. This is assuming that the people took care in collecting the data objectively (i.e. they did not suffer from “majestic tree” bias by selecting areas to measure just because they had huge trees). The degree to which old data are useful is really a matter of how much you trust the original folks who did the measurements. In this case, the original folks did not have incentive to heavily bias their measurement, and it appears that they measured areas systematically, which should reduce bias. So as far as old data go, the data used in this study is pretty good. We are still not sure that they were totally unbiased because they were not doing research at the time, and therefore did not have the same level of quality assurance or precision checks that modern researchers do. While I am suspicious that these 1911 surveyors took as much care in measurements as I would expect from modern researchers, I nonetheless consider it likely that this old data is much better than the alternatives for reconstructing forest structure as of a century ago.
While reconstruction studies in the Sierra Nevada are all inexact, the overwhelming agreement among them overcomes their individual imprecision. This study agrees with what is now an abundance of work that has documented at least these very basic changes regarding the effects of fire suppression:
- Forests where only fire suppression has occurred are much denser than they were in the past (using any measure of density- canopy, # of trees per area, or basal area).
- There are lots more trees in the smaller and moderate size classes than there were in the past.
- White fir has increased in tree density, primarily because of increases in small and medium sized trees
- Ponderosa pine has decreased in relative density, primarily because of a lack of small and medium sized trees
Relevance to managers:
Managers will appreciate that the authors of this paper are very direct in providing management implications, yet they are also nuanced in describing how this research may inform decisions. They provide three implications:
- Treatments that attempt to recreate the forest structure of the past should avoid using average values for hard targets, and should instead consider recreating ranges of conditions or measures of variability
- The structural restoration targets that are currently being used are likely too conservative. If the objective of treatments are to recreate past structure, post treatment densities are too high.
- As managers move beyond the oversimplified approach of recreating past structure and instead incorporate the objective of building resilient forests, they will consider the effects of climate change along with the effects of fire suppression.
Areas that burned recently with moderate severity fires were closer to 1911 structure than areas that either had no fire or burned with low severity fire. Despite a lack of statistical significance, you can still see that even moderate severity fires did not reduce density to what the 1911 structure was. With higher sampling effort, they probably could have detected a difference (from the graph, it looks like areas burned with moderate severity fires had about 125 trees per hectare while 1911 density was about 80 trees per hectare). So for fire managers, this would indicate that fires following long periods of should be toward the moderate classification of severity if they are trying to recreate past structural conditions. Perhaps repeated low-severity fires will eventually lead to more similar conditions to the past, but we won’t know until we have a longer track record of conducting repeat burns.
This study further indicates to me that broadly-applied upper diameter limits (e.g. thou shalt not cut a tree greater than 24”) simply do not make sense. The authors point out that, while limiting tree removal to less than 12” diameter can make sense from the perspective of reducing fire severity, it can also mean that stand density in terms of basal area is much too high from either a structural restoration point of view or a forest resilience point of view.
Critique (I always have one, no matter how good the article is):
I was disappointed that they did not measure as many areas as they could have. They say they found 50 areas that were measured in 1911, but then only re-measure 30 of these areas because of “time and access constraints.” They make a big deal (appropriately) about how unique and important this data set is. If it is so important, why not spend the money and time to re-measure the whole thing? Further, they use the non-significance between 1911 forest and the moderately burned modern forest structure as evidence that the moderate severity fire re-created the 1911 structure. If you look at the data, however, the areas burned with moderate severity fire still had higher density (over 50% higher as far as I can tell from the graph). This difference may have been detectable with a few more plots measured. Finally, not measuring some of the areas makes me wonder if the more easy-to-access areas were measured first, thus introducing a possible bias.
I also wish they had reported basal area differences. Basal area is often a much more useful way of describing forest density, especially in the context of conducting restoration/resilience treatments.
All in all, however, it is a good study especially with respect to reconciling fuel reduction treatments (fire or thinning) with forest resilience treatments. And it is interesting to compare this reconstruction study with this one that I reviewed earlier.
- Author: Rob York
[originally posted on www.foreststeward.com on Dec. 10, 2010]
Article reviewed: Long-term vegetation responses to reintroduction and repeated use of fire in mixed-conifer forests of the Sierra Nevada
By K.M. Webster and C.B. Halpern. Published in Ecosphere, Vol. 1(5): 1-17. Available for full download here (in this new and OPEN ACCESS journal).
The plot line: Sequoia and Kings Canyon National Parks have the longest history of using prescribed fire in Sierra Nevada forests. The authors of this article analyzed data that were collected in the parks over time from sites that were either burned once, burned twice, or not burned at all. They looked for differences in how the treatments influenced understory species composition during the 10 to 20 years that followed burns. The relatively long-term nature of the monitoring allowed them to detect delayed effects of the burning that otherwise may not have been detected. The long-tenured burning program in the parks also allowed them to characterize effects of single versus follow-up second-entry burns on composition. Burning led to increases in the total number of species, especially beginning 5 years after the burns. Shrub species were especially responsive to the first-entry burns, and were then maintained with the second-entry burns. Ground cover made up of most types of plants tended to increase following burns, especially 10-20 years after burns. The authors suggest that prescribed burning programs can be very successful for reducing fuel while also achieving desired species compositions. The frequency of burns, their relative proximity to each other, and the severity of burns are discussed as critical management factors for burning programs.
Relevant quote: “If fire is to play an important role in restoration… it will need to be maintained as a frequent and spatially dynamic process on the landscape.”
Relevance to landowners and stakeholders:
Most people who have visited national or state parks in the Sierra Nevada have seen the signs and brochures that tout the important role that fire has had in shaping the forest. From an ecological perspective, the importance of fire is incontestable. It did indeed shape the forest. And now the forest has been forever altered because of fire suppression. We can never truly restore the forest conditions of the past, but using prescribed fire is one way that we can achieve modern goals of fuel reduction, species composition, and forest health.
Whether or not people who see the pro-fire signs in parks walk away as advocates for prescribed burning, however, depends a lot on their non-ecological perceptions of fire. One important factor is how sensitive their health is to smoke. In my neighborhood, I can talk to people endlessly about the benefits of fire, but all of those benefits are quickly forgotten when smoke from my prescribed fire creeps into their yard and starts to negatively impact their breathing. This is the great challenge for all of those pyro-foresters out there: How do you increase burning activity when the public's tolerance for smoke keeps declining?
This research suggests that increases in biodiversity following burning and then maintenance of diversity by repeat burns is one benefit that could be used to support fire (the more obvious one is the benefit of reducing high-severity fires that burn peoples’ houses down and put LOTS of smoke in the air, but that’s not really what this article was about). Biodiversity could even be put in the context of its importance for public health, as was demonstrated in last week’s post. Burning will likely remain a tough sell to anyone, however, who has asthma and who is already living in an area with high levels of air pollution (e.g. the Central Valley).
Relevance to managers:
- Burn when you can- many of us managers have far greater constraints than those within the parks. We work in the urban interface or have other logistical, legal, or risk-aversion challenges. While it is important to have objectives and clear plans about where/when to burn, often it is determined by weather and availability of personnel. So you end up burning when you can.
- Expect the unexpected- Fire is a blunt tool. In this study, a wide range of species composition responses resulted from patchiness in fire severity during burns. Other variable factors of species responses include the climate following the burn and availability of seed within soil banks or from nearby parent populations. Don’t expect to be able to predict exactly how the species composition will responds to fire. One thing that can be expected- continuing to suppress fire without doing anything else will decrease biodiversity until a high-severity fire occurs (which will, by the way, also increase biodiversity but not necessarily in a good way).
- The first burn is critical- It appeared from this study that the first burn after a long period of fire suppression was the critical one in influencing species diversity and cover over the next two decades. The second burn was important in maintaining composition, but did not appear to increase or decrease composition with anywhere near the same magnitude as the first burn. (the authors seemed to suggest that the second burns “enhanced” diversity, but I did not see that happening in the data or analysis that was given).
- The mechanical + burn option- This study did not include mechanical treatments that were followed by burns, but it makes me think of the mechanical+burn treatment as a potentially effective option for increasing biodiversity. A mechanical treatment that alters fuel structure in such a way that allows a hot yet manageable fire will likely see a distinct increase in richness and ground cover which can then perhaps be maintained by subsequent fires.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
This study compares three basic treatment options: burning once, burning twice, and not burning at all. It is not a comparison of burning with mechanical treatments, so it should not be interpreted as a recommendation of burning over mechanical treatments. It is more a demonstration (a very interesting and important one) of the benefits of burning versus not burning at all.
It is also worth noting that the second-entry burns did not appear to have been applied in an experimental fashion (e.g. they were not selected randomly). It makes me wonder if they were selected for second entry burn because they burned in a particular way during the first burn. It does appear from the graphs (Fig. 1A) that the second-entry burns may have been selected for a second burn because the first burn was particularly hot. The pre-treatment tree density prior to second burns looks lower than the tree density 10 years after the first entry burns. This could be just due to chance or not important, but it does make me wonder about how these areas were selected for burning or not burning.
Their repeated measures analysis seems to give a lot of leverage to the early responses since there were fewer measurements available for later responses. Normally a repeated measures analysis will only include plots that have data that span the entire time range being considered. But they seemed to use a non-traditional type of analysis that let them use all of the plots even if they didn’t have data across the entire period. This is probably completely justifiable, but they didn’t explain why they chose this type of analysis, which I bet most other researchers have never used. Typically a non-standard approach has more discussion of why it was used.
Their management recommendation that fires be done “asynchronously” with white fir seed production in order to avoid a pulse of white fir establishing after fire does not seem feasible. Most fires are done in the fall, after seeds have already been dispersed (white fir dispersal is usually in August or September). So tree seedlings establishing after a prescribed fire will come from seeds produced after the fire. White fir cones mature in one year, so we can’t tell what the cone crop will be like following the fire. A slightly more feasible (but still challenging) option might be to time higher severity fires with bumper crops of pine species. Pine cones take 2 years to mature, so it is more feasible to time the treatment with next year’s seed crop. This wouldn’t decrease white fir establishment necessarily, but it might increase the relative amount of pine establishment compared to white fir.
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