- 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: 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.