- 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: 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.
