- 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 Jan 15, 2011]
Article Reviewed: Giant Sequoia (Sequoiadendron giganteum) Regeneration in Experimental Canopy Gaps
By R.A. York, J.J. Battles, A.K. Eschtruth, and F.G. Schurr. Published in Restoration Ecology Vol. 19, 1 (2011) pp. 14-23. Available for open-access.
Plot line: These researchers created canopy gaps within a giant sequoia forest and then planted seedlings of giant sequoia within the gaps. They wanted to see how giant sequoia survived and then grew in different sized gaps (ranging from 1/8 to 1 acre). They also measured how seedlings grew in different positions within the gaps, some positions being shady (near gap edges) and other positions being sunny (near gap centers). They found that, while seedlings usually died if they were not underneath any canopy gap, they only needed the smallest size gap to survive at the same rate as larger sized gaps. Seedlings grew a lot more when gap size was increased to about ½ acre in size, but did not increase when gaps were greater than ½ acre. When planted in an ash substrate following burning, seedlings grew twice as much compared to seedlings planted in unburned soil. They conclude that canopy gaps are necessary for restoring giant sequoia regeneration, and that increasing canopy gap size up to ½ acre can benefit growth (but not necessarily survival).
Relevant Quote: “For giant sequoia and other long-lived species around the world, locally-severe disturbances are an important factor of their persistence and hence restoration.”
Relevance to landowners/stakeholders
Over the past century or so, there have been far fewer giant sequoias reproducing than what would be expected from a self-sustaining population. Fortunately, the primary reason is obvious and in theory should be addressable with restoration programs. The culprit is fire suppression. Giant sequoias regenerate following disturbances (like fire) that kill or remove several trees that are big enough to create a discernable gap in the forest canopy (I like to think of a “gap” as being big enough to allow new trees to regenerate, but not so big that the center of the gap is uninfluenced by the shading and roots of the surrounding trees). A reduction in fires (which used to occur every 12 years or so in the Sierra Nevada) has led to fewer canopy gaps, thus erasing the conditions needed for giant sequoia regeneration.
Restoring fires in order to promote giant sequoia regeneration, however, can be a formidable task. This study suggests that canopy gaps need to be “sweet-spots” of both light and soil moisture in order for giant sequoia to regenerate and then grow well. 1/8 or ½ acre doesn’t sound very big, but creating gaps this big with a fire takes a pretty hot fire- one that might be “out of prescription” if burning near sensitive areas where escapes are unacceptable. On the other hand, there are a lot of good “burn bosses” out there (especially on federal land where giant sequoia are) who seem to be able to conduct fires that are patchy in nature and that can indeed create canopy gaps that are of sufficient size.
Of course, one could create gaps artificially (as was done in this study) with mechanical treatments that remove trees. But this option is often not available because of other competing objectives. On the other hand, fires may not be an option if near areas that are sensitive to smoke or if areas can not accept any risk of escape. Some type of disturbance that creates distinct canopy gaps via the death or removal of several trees is a prerequisite for giant sequoia regeneration. Since giant sequoias live thousands of years, restoration projects should easily be able to “replace” the seedlings that have not been regenerating over the past century.
Relevance to managers
The relationship between gap size and growth is asymptotic. As gap size increases, so does giant sequoia seedling growth. But the benefit of larger gap size diminishes and then levels off. In this study, it didn’t benefit giant sequoia seedling growth to have gaps larger than about ½ acre. This relationship could change in the future as the seedlings grow into the canopy, but a related study of giant sequoia by the same authors have tracked the same asymptotic relationship through 12 years and counting.
Maximizing growth may not be an objective even within the context of restoring giant sequoia seedlings. Survival may be more important than growth if the seedlings will eventually recruit into the canopy. In this study there was no relationship between survival and growth. So seedlings survived as well in the big gaps as they did in the small gaps. But gap presence was necessary for survival. Virtually all seedlings planted underneath the dense canopy died.
The differences between seedlings planted in ash and bare soil are striking. The ash-planted seedlings appear to be men among boys. They are bigger and their color is much better (this can only be appreciated by those managers who are used to looking at giant sequoia seedlings). Another study reviewed earlier found similar results and suggests increased nutrient availability as the reason for the increased growth within ash substrates.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
Although the results are sometimes discussed within the context of using fire to restore giant sequoia regeneration, it should be noted that fires were not used in this study to create the canopy gaps. The ash substrate was created by placing harvest debris into piles and then burning them. While the experimental treatment attempted to mimic the disturbance severity that might be achieved with fire, there will undoubtedly be some differences in giant sequoia regeneration when fires are used to create the gaps. Most notable is that a much higher amount of seed production is expected following an intense fire.
There was only one site used in this study. The results may have been different if the study had been done closer to the edge of the species’ range, or on a different aspect or elevation.
The graph that summarizes the growth response to light and soil moisture is pretty (below), but it should be noted that a surrogate for soil moisture was used. They didn’t measure soil moisture directly, but instead used distance from edge as a proxy. They have other data in the study that suggests this is a reasonable thing to do, but it would have been even more powerful had they measured soil moisture directly.
- 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>