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…”

[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.

[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.

[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.