Gord McNickle and I are pleased to announce that our review/idea paper on game theory and plant ecology is now officially published in Ecology Letters. I’m proud of the whole thing, start to finish, but if you read only one section, start with “Game theory essentials for the plant ecologist.” It may just change your life! …or at least your perspective on plant ecology:)
A lovely paper was published today! Authored by my good friend and colleague Caroline Farrior (with some help from me and a couple of literal wise guys), it presents a tractable model of competitive trees under conditions of water limitation. Although we use some different parameter names and combinations, it is really the sibling of our earlier paper that looked at competitive trees under conditions of nitrogen limitation. It’s chock full of savory ecological goodness (dig in!), but I’ll highlight only the most surprising result here.
Briefly, elevated CO2 improves plants’ leaf-level photosynthetic efficiency via two mechanisms. First, elevated CO2 increases the photosynthetic metabolism involved with carbon (which the plant turns into food) and decreases the photosynthetic metabolism involved with oxogen (which is a wasteful process and an evolutionary artifact that looks like a colossal mistake under current conditions). Second, elevated CO2 increases the net photosynthetic rate when a plant is water limited because more CO2 diffuses into the leaf for a given amount of water lost from the leaf. I’ll only focus on the second mechanism here (i.e. everything that follows pertains only to the second mechanism!).
You might expect elevated CO2 to allow plants to store greater amounts of carbon under water limited conditions because of that enhanced water-limited photosynthesis, and you’d be in good company. However, our results suggest that competitive plants will divert all of the net increase in photosynthate to fine roots. Of course, the plants could maintain competitive parity by keeping their original fine root investment and putting the additional photosynthate elsewhere. Similarly, a group of body builders could maintain competitive parity by agreeing not to take an undetectable steroid. In both cases however, “cheaters” would be at a such huge advantage that the only rational alternative is for everyone to take the steroids/ build up a greater fine root armament. But of course, after doing so, everyone is back where they started, at competitive parity! It’s a tragedy of the commons.
Because fine roots store carbon over vastly shorter timescales than wood, the net effect of elevated CO2 under water-limited conditions is a negligible change in carbon storage. While this is perhaps sad news for the fate of our changing climate, we have other papers in the works that suggest that in many other important scenarios (other than water limitation), competitive plants should respond to elevated CO2 in ways that tend to reduce atmospheric CO2.
I’m not generally a grumpy guy, and years ago I made peace with the popular misuse of the phrase “begs the question.” (See? I’m easy-going, through and through!) But when I read Robert Krulwich’s NPR blog entry “New Superhero, 3,200 Years Old, Turns Air Into Wood Superfast : Krulwich Wonders… : NPR” I blew up in a self-righeous huff. As I endeavor to impress upon my audiences whenever I give a public talk, our default caricature of how forests work leads otherwise good scientists to make bad assumptions. Krulwich summarized that bad caricature nicely:
“Young trees, we thought, suck in great gulps of CO2 and then, with a series of chemical tricks, turn that air into wood, adding bulk to the trunk, thickening the branches, spitting out the oxygen, and they keep at it for years, fighting for sunshine, until, if they’re lucky, they end up taller than their neighbors, so at last they can relax, and eventually, slow their growth rate, weaken and die. That’s what we’d call a normal tree cycle. But giant sequoias, apparently, do it differently.”
Yes, giant sequoias do it differently… just like every other tree. The race is never over, and a canopy tree continues to jockey for position, gaining or losing space to its (adversarial) neighbors until the day disease or wind finally ends its life. Height growth is always paramount to a forest tree, no matter its stage in life. They never “relax.”
How did the caricature of a static canopy tree come to be? I think there are three ingredients. First, we have ground-level familiarity with myriad herbaceous species that do have a somewhat fixed maximum height. Second, we’re short and so we can’t have the same first-hand knowledge of the goings on in the canopy. Third, a tree’s height growth scales as the square root of its diameter, which means that as trees grow very large, their height growth – though still paramount – becomes harder and harder to perceive. Ancillary to the third point is the observation that disturbance regimes are somewhat regular for any given eco-region, which tends to maintain a characteristic canopy height across a landscape as a balance between young and old forests.
In summer 2011, we enjoyed a wicked storm several times a week and a brilliant sunset almost every evening. I kept wishing I had the means to make nice time-lapse movies. That wish came true last winter, but this past summer was remarkably devoid of sky-shattering storms and stunning sunsets:(
I did manage to get one excellent movie of a very distant storm (~150 miles away) while I was in northern Wisconsin. Ironically, I only had the camera and none of my time-lapse gear… so it’s a little shaky. (I like to think it adds character:) It’s worth realizing that the thunderhead I captured is several miles high…impressive stuff.
Ah, the first counterfactual adaptation: branch razors. Why shouldn’t the most distal branches on a tree have little razor-sharp appendages out in front that would tear up neighboring trees’ leaves and branches? This seems like a more direct route to maintaining a light-harvesting territory than the one employed by extant trees: ceaseless height growth and overtopping…
Allow me to introduce a new “category” for my posts: Counterfactual adaptations. In order to describe what a “counterfactual adaptation” is, I first have some give you some preamble. Roughgarden, May, and Levin wrote in their introduction to Perspectives in Ecological Theory:
Whether or not in mathematical form, theoretical ideas serve many purposes. They suggest observational protocols and manipulative experiments both in the field and in the laboratory. They provide frameworks around which curricula can be organized, so that the body of facts can be given coherence rather than presented as a jumble. Most important, perhaps, theory can imagine and explore a wider range of worlds than the unique one we inhabit, and by so doing can lead to fresh perceptions and new questions about why our actual world came to be as it is…
I love this quote, and as much as I appreciate ecological theory’s ability to explain why the world is the way it is, I appreciate even more its ability to explain why the world isn’t some other way. When I stare at the analytical results of a mathematical model, there is often parameter space that bears no resemblance to the unique world that we inhabit. This is part of the “wider range of worlds” of which Roughgarden, May, and Levin wrote. Commonly, consideration of parameter values or of other results from the model suggest why that parameter space is biologically unrealistic. These insights are deeply satisfying to me.
In my day to day musings, I sometimes imagine an adaptation that seems like a “good idea,”* but that doesn’t exist in our world. It is a counterfactual adaptation and is part of the “wider range of worlds than the unique one we inhabit.” Perhaps there is no evolutionary path from our current biota to a particular counterfactual adaptation. But the more exiting possibility is that there is a good, but non-obvious reason why a particular counterfactual adaptation would in fact be a “bad idea.”* Relevant theory should permit the counterfactual adaptation, but then explain why it is biologically unrealistic.
My plan is to collect my counterfactual adaptations here. I’d love to hear yours!
*yes, yes… adaptations are not “ideas”:)
A few months ago, I placed this geologic timeline on my desktop. I wanted to see it again and again and again, so that the majesty of its arc would sink into the pit of my brain.
I think the mission has been successful, and now I’m excited to create my own version of a geologic timeline. In addition to the information contained here, I’d like to include graphs that depict changes in the atmosphere, percentage of land, ice cover, etc. Look for it as soon as I can catch my breath from all these job applications and revisions:)
In addition to adding new information, I’ll make mine linear. There’s something strange about the circular depiction above, with the zero hour and now both sitting at twelve o’clock… it doesn’t provide any room to imagine the future. I had been planning to leave about five billion years of blank space (at which point my high school education had taught me the sun will go red giant), but then I learned from Wikipedia (that’s right) that the sun is expected to heat up within the next one billion years such that no liquid water will exist on Earth.
As adaptable as our flavor of carbon-based life seems, I’m just not optimistic that it can persist without liquid water. So it seems that life on Earth is in the autumn of its life, so to speak. Like an island that rises from the sea, hosts eons of riotous life, and then abandons that life as it sinks back into the sea, so too is life’s finite flourishing on island Earth.
A little while back, I reviewed a paper that had a fundamental flaw. I won’t go into details, but suffice it to say that the flaw was incontrovertible; the claim they were trying to make did not follow from their results. I detailed this in my review and even suggested some ways that they might correct the mistake …or I suggested that they could at least qualify their claim. Based on my review and another not-so-glowing review, the paper was rejected.
Now, after all that effort, I am dismayed to discover that the authors simply resubmitted the exact same manuscript [EXACT SAME!!!] to another journal and that a random throw of the reviewer dice evidently turned up a pair of “accepts” this time. The paper is now published, fundamental flaw and all.
And the irony is that I discovered this as I was working hard to address reviewer comments from my own rejected paper before I resubmit elsewhere! Yeeeeesh……
Think of the most wicked storm you experienced this past summer… think about the power and fury of the gust front that announced that storm. For me, it was a storm a few weeks ago where the winds in Chicago where I live were knocking everything around with a violence that truly scared me as I walked home. The previous summer a tree had gone down a block away under similar circumstances, stopping a traveling minivan in its tracks (not the minivan on the left, but the one hiding on the right). Miraculously, its massive branches fell just in front and just behind the vehicle… a moment sooner or later and the driver would have surely been killed. But I digress…
As awe-inspiring as it is to witness a massive tree felled by the wind, it is perhaps more awe-inspiring that most trees stand strong against such tremendous forces! Thus, it is undoubtedly true that at any given moment, a tree’s investment in wood far (far!) outstrips the demands placed upon it by gravity and the gentle breezes that prevail most of the time. Here, the relevant selection pressure for wood construction and investment occurs with great infrequency.
So here’s a thought experiment: Imagine for a moment that humans only live for a few days, but that we still possess technological innovation, cultural evolution, and scientific curiosity (i.e. suspend your disbelieve and imagine that everything else about humans is the same:). How might the perspective of an ecologist studying the ecophysiology of wood in her few short days differ from ours? I suggest that although she knows that “wind events” happen and that they often kill trees, she is unlikely to ever experience one of these events in her short lifetime and that their importance will take a back seat to the sorts of things that she can measure and experience. I suggest that she may either be perplexed by an apparent over-investment in wood or, worse, she may convince herself of other reasons why that level of investment “makes sense.”
You see where I’m going with this… As humans who spend at best 100 years on this planet, we are unlikely to experience the sorts of cataclysms (multi-decadal droughts, glaciation, epidemics, periods of volcanism, etc.) that we know occur with some regularity, geologically-speaking, and that exert profound selection pressure on organisms. Might this cause ecologists to be perplexed by the phenomena they study or, worse, convince themselves of other reasons why those phenomena “make sense?”
Back by popular demand! As most anyone who has spoken with me knows, I get pretty excited when the conversation turns (as it inevitably does) to the topic of root territoriality. Whether and under what conditions roots of different individuals vie for the same belowground resources is vitally important for determining the traits of plants and the way that those traits scale up to ecosystem-level properties. If roots are territorial, they are evolutionarily-free, so to speak, to optimize their root investment to match the resource availability of their environment. If roots are not territorial, ecological and evolutionary dynamics effectively compel individuals to invest heavily in roots so as to preempt resources from neighbors (or, more to the point, to avoid being preempted themselves). I recently met Frances O’Donnell, an advanced grad student in the Civil and Environmental Engineering department at Princeton, and Frances has operated an Air-Spade. An Air-Spade is a device that fires supersonic air, and when it is aimed at dry soil, the air stream catches in the pockets and pits of the soil and literally explodes it away. When it hits a root, in contrast, it flows smoothy around. I’ve never used an Air-Spade, and Frances confirmed it is laborious and unpleasant to uncover a root system with an Air-Spade… but it is possible!
Here is a picture of a root system exposed and described in a paper by Nadezhda Nadezhdina and Jan Cermak in the Journal of Experimental Biology (2003). Notice that these roots, at least, don’t look very territorial. They criss-cross like crazy. All of the evidence that’s I’ve ever seen from forests or grasslands suggest that roots are not territorial, but Frances did suggest that a pair of trees that they exposed appeared to divide the area that their roots explored. Clearly, we need more Air-Spading:)
It is interesting to note that the idea that plants hold distinct territories, which appears to be the default assumption among folks who haven’t thought as deeply about roots as I have, may have its origins in two popular depictions of plant root systems. First, Weaver’s influential pictures always depict prairie plant root systems as spatially distinct:
But it seems clear to me that this is not meant to be an illustration of a community of plants (note that even the aboveground component does not look like a crowded prairie). Instead, I think Weaver was providing illustrations of the differences between the individual species, and he placed them side by side for comparison. Indeed, Frank and coauthors (2010 Ecology) used genetic techniques to identify the species of prairie roots in small soil cores and found over four species per core! Who knows how many individuals were represented?!
The second misleading popular depiction of root systems is artistic in origin. How often have you seen illustrations like this?
In truth, tree roots extend well beyond the “drip line” (the edge of the leaves) and shown on this landscaping illustration, intended to set landscaping customers straight: