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A networking resource devoted to biological soil crusts and the researchers who study them. We will provide a means for international scientists to communicate, share their research, share important news and announcements, ask questions and find collaborators. We will also provide a space for informal writing on research, opinion, and ideas.
My first impression upon hearing the hypothesis that Dickinsonia (Fig. 1) may be a lichen was that it does not look like any lichen I’ve ever seen. To be fair, the more normal interpretation that it is a marine animal, a segmented worm, sparks the same response…it does not look like any animal I’ve ever seen or heard of. What it actually resembles is a giant diatom, but that is pretty silly. This uncertainty is exactly what has been nagging paleontologists since the 1940’s….what are these things and what later life forms, if any, evolved from them? Dickinsonia and other Ediacaran Period (635-542 mya) fossils are enigmas. They left fossils in multiple locations world wide, then drop out of the fossil record before the Cambrian explosion in a mass extinction.
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A recent paper by Elbert et al. in Nature Geoscience estimates the global contribution of "cryptogamic covers" to nitrogen fixation and net primary productivity. One of the authors, Bettina Weber, recently presented this work at the Ecological Society of America meeting. The discussion about how carbon can be sequestered almost always revolves around carbon intense ecosystems, those which store a lot of carbon per unit area, or ecosystems which can produce a lot of biomass very quickly. Cryptogams -- a catchall referring to bryophytes, lichens, and cyanobacteria-- are small and have a reputation for being slow growers (sometimes they are, sometimes they aren't). Because they are small and because they are perceived as slow-growing, they get little attention as carbon repositories or players in the carbon cycle. This study investigates the global impacts of cryptogamic communities such as soil biocrusts, rock biocrusts, boreal moss & lichen "carpets", and epiphytic mosses and lichens growing in tree canopies (Figure 1).
Figure 1 (Elbert et al. 2012). a, Ground cover in the Namib lichen fields (Teloschistes capensis, Xanthoparmelia walteri, Ramalina spp.), Alexander Bay, South Africa. b, Soil crust with cyanobacteria (black) and chlorolichen (Psora decipiens), Nama Karoo semi-desert, Northern Cape, South Africa. c, Rock crust with chlorolichen (Rhizocarpon geographicum aggr.), Sadnig, Eastern Alps, Austria. d, Rock crust with chlorolichens (Chrysothrix chlorina, yellow, Leproloma membranaceum, whitish-grey) and mosses (Dicranum scoparium, Hypnum cupressiforme var. filiforme), Spessart, Germany. e, Plant cover with cyanolichen (Physma byrsaeum) on rainforest tree, northeast Queensland, Australia. f, Plant cover with chlorolichens (Evernia prunastri, Parmelia sulcata, P. subrudecta and others) and a bryophyte (Orthotrichum affine) on maple tree, Trier, Germany.
The very first terrestrial communities were likely communities of cryptogams. Today, to a large degree, these communities occupy the leftovers -- habitat not occupied by the vascular plants, or in some cases the actual surfaces of vascular plants. In some places it seems that the Earth is wearing a fuzzy cryptogram sweater....have you ever seem a temperate rainforest? If so you know what I mean. Can these forgotten botanical panhandlers, vagabonds and mendicants have any role to play in something so grand and large as the carbon and nitrogen cycles of Planet Earth? Yeah, of course (and stop calling them "lower" plants, it's just plain offensive).
Carbon-wise, it is the ground covers in temperate and boreal forests that are taking up the most carbon. Again, if you've ever been to one, you'll understand. A black spruce taiga up in Alaska might have half a meter of water-logged moss and lichen tissue accumulated on the ground. I notice in Figure 3 from the paper, that the deserts are showing some modest values due to soil and rock covers. Across the planet the authors estimate cryptogamic communities are comprising about 7% of the primary productivity.
Nitrogen-wise, the most intense fixation rate estimates are in desert cryptogamic soil and rock covers...a.k.a. biocrusts. Cryptogamic plant covers in extratropical forests are a distant second place. Across the planet the authors estimate cryptogamic communities are conducting almost half of the biological nitrogen fixation.
There are 2 reasons why we should take these communities seriously as part of the carbon sequestration equation. First, comprising 7% of the global NPP may not seem like a lot, but its actually similar in magnitude to the flux we generate by burning fossil fuels according to the authors. The second reason is that primary production in most ecosystems is limited by nitrogen. Most of the Earth's nitrogen is in the atmosphere in a form that's useless to primary producers. A small minority of organisms have the ability to convert this atmospheres nitrogen to essentially a plant fertilizer (nitrogen fixation). This study suggests that nearly half of the biologically fixed nitrogen in terrestrial ecosystems is fixed by these cryptogamic communities. So, in other words cryptogams are largely in charge of the key missing ingredient that would allow for greater plant production and carbon sinking.
In figure 3 from the paper, the authors map where on earth these cryptogam-mediated porcesses are most intense. For carbon (left side), note the importance of the boreal forest. Because we have so little land in the southern hemisphere at similar latitudes, this is primarily a northern hemisphere phenomenon. For nitrogen (right side), look at the drylands coming into play.
Figure 3 (Elbert et al. 2012) Geographic distribution of CO2 uptake and N2 fixation by cryptogamic covers. a–f, The colour coding indicates the flux intensity of carbon net
uptake (a,c,e) and nitrogen fixation (b,d,f) by CGC (a,b), CPC (c,d) and their sum (CGC + CPC, e,f). The flux units are g m−2 yr−1 ; note that the scale bars
for carbon (e) and nitrogen (f) differ by two orders of magnitude. White areas indicate ecosystems for which no data are available; hashed areas were
excluded from global budget calculations (annual mean precipitation <75 mm yr−1 and desert areas designated as dune sand/shifting sands and rock outcrops).
One observation I have is that the authors seem to be trying to estimate current rates of carbon and nitrogen fixation, not the potential. Since I am so used to seeing soil biocrusts compromised by disturbance, I wonder how much higher these rates would be without such disturbances...twice as high, three times? How much C could we sink worldwide if we stopped chronically disturbing soil biocrusts?
Wolfgang Elbert,, Bettina Weber,, Susannah Burrows,, Jörg Steinkamp,, Burkhard Büdel,, Meinrat O. Andreae, & Ulrich Pöschl (2012). Contribution of cryptogamic covers to the global cycles of carbon and nitrogen Nature Geoscience DOI: 10.1038/ngeo1486
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Wolfgang Elbert,, Bettina Weber,, Susannah Burrows,, Jörg Steinkamp,, Burkhard Büdel,, Meinrat O. Andreae, & Ulrich Pöschl. (2012) Contribution of cryptogamic covers to the global cycles of carbon and nitrogen. Nature Geoscience. DOI: 10.1038/ngeo1486
Readers of this blog won't be so surprised, but most people are unaware that mosses grow in deserts and semiarid zones. The reason they can do so is that desert mosses are dessication tolerators, meaning they are capable of drying without dying. While dry, they are in a state of suspended animation, simply waiting for the next hydration period so that biological activity - and hopefully - net photosynthesis can occur. They rehydrate literally in seconds, and are immediately active. You could measure their respiration right away, for example. I always tell people that this is somewhat like spilling a glass of water on Tutankhamun, resulting in him coming back to life. A pretty cool party trick in my opinion. Biocrust organisms in general do this, but it is perhaps most dramatic in the mosses. Here's a video posted by Casey Allen of a simulated rain event.
And another from a class led by Larry Macafee at Badlands National Park. Focus is a little fuzzy, but you can see individual plants hydrating.
Neat trick for sure, but it does come with costs. Biocrust organisms do not regulate water loss with stomates (pores which a plant can open and close to regulate the rate of carbon coming in and water going out), so they lose that water they gained passively due to simply evaporation. They hang onto water very similarly to a piece of a paper towel - soaking it up via contact and losing it to evaporation. Every dry down event is thought to damage membrane integrity a little bit, so that the first order of business when rehydrating is maintenance. Many researchers have shown that upon rehydration there is a period of net respiration, and that after the photosynthetic rate may exceed the respiration rate, resulting in positive carbon uptake. If that happens growth is possible. But if that net photosynthetic threshold is not reached, the hydration event has resulted in carbon loss, and therefore dry mass loss. In other words the mosses would be shrinking a tiny bit rather than growing....bad news. So you can imagine that short hydration events due to small rain events or just high temperature driving fast evaporation, could damage mosses. Further, many such events could even kill them. So nature's tough guys have an Achilles heel, and seem to straddle a knife edge of survival. The climate is changing: what if increased temperatures or altered frequency or magnitude of rain events decrease hydration times? These mosses seem vulnerable to catastrophe.
Recent papers, including an awesome one by Reed et al. in Nature Climate Change, have induced rapid changes in biocrust community composition as a function of experimental climate change manipulations in a 5 year experiment. They induced a 2 degree C warming using infrared lamps in the field in Utah. This is a modest warming effect compared to multi-GCM projections for the area. They crossed this warming effect with a watering treatment which doubled the frequency of rain events but only slightly increased the total amount of rainfall. It's not surprising to me that this had an effect....but what is surprising is that the authors induced a 90% moss mortality in only 1 year!.
Moss dieback in response to increased frequency of small summer rainfall events (Zelikova et al. 2012)
Mosses fail to attain net photosynthesis when experiencing 1.25 mm rain events, contrasting with a 5 mm rain event (Reed et al. 2012).
This effect was essentially exclusively driven by the watering frequency treatment. The group also tracked effects rippling through the entire community. Under the high frequency summer rain treatment, cyanobacterial cover apparently expanded filling the gap left by the mosses, but pigment concentrations suggest that biomass was declining (Zelikova et al. 2012). Meanwhile bacteria and fungi were in decline (Zelikova et al. 2012), and enzyme signatures suggested that decomposition rates were faster under the frequent watering regime. In addition, changes were induced to nitrogen cycling, including increased nitrfication and a shift from an ammonium to a possibly leakier nitrate dominated regime.
Unlike temperature projections, precipitation projections from climate models are notoriously variable. Therefore, we don't know if climate changes will induce this utter tanking of biocrusts and their function. It is a plausible scenario, however, that the summer monsoon in the Colorado Plateau region would bring a higher frequency of storms. This is exactly what we don't want. Not only would we lose soil fertility, but biocrusts would be less able to aggregate soils and prevent dust emissions which could go on to affect Western US water supplies (see previous post).
I should point out that this simulated climate change scenario is not necessarily the most plausible in all drylands, and the apparent indifference of biocrusts to warming may also not be universal. These studies from Utah, contrast nicely with another study which shows a clear negative effect of 2 - 4 degree C warming on lichen-dominated biocrusts in Spain (Escolar et al. in press).
Escolar C, Maestre FT, Martínez, I, Bowke, MA 2012. Warming reduces the growth and diversity of lichen dominated biological soil crusts in a semi-arid environment: implications for ecosystem structure and function. Proceedings of the Royal Society B: in press.
Zelikova TJ, Houseman DC, Grote EE, Neher DA, Belnap J. 2012. Warming and increased precipitation frequency on the Colorado Plateau: implications for biological soil crusts and soil processes. Plant & Soil DOI 10.1007/s11104-011-1097-z
Reed SC, Coe KK, Sparks JP, Houseman DC, Zelikova, TJ, Belnap J (2012). Changes to dryland rainfall result in rapid moss mortality and altered soil fertility. Nature Climate Change DOI: 10.1038/nclimate1596
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Reed SC, Coe KK, Sparks JP, Houseman DC, Zelikova, TJ, Belnap J. (2012) Changes to dryland rainfall result in rapid moss mortality and altered soil fertility. . Nature Climate Change. DOI: 10.1038/nclimate1596
First we had Darwin's finches, Hutchinson's water-bugs, Diamond's islands, and Vitousek's soil age gradients...now possibly we have Gotelli's biocrusts. I'm talking about model systems: objects of study whose characteristics make them especially good for asking certain questions, and from which we wish to gain general knowledge about other systems. Drosophila and E. coli are other models in genetics and microbiology, and rodents have long been a model in toxicology.Even if you do not habitually read Nick Gotelli's work, chances are good that if you are an ecologist you have used his software, have one of his books on your shelf (or at least studied it in Ecology class), or have had him as an editor. I don't know all that much about him personally, but when I met him I learned he is also a bluegrass musician and has a fondness for Hieronymus Bosch. This indicates a creative personality which may explain why he has been such a force in methods development and refinement in ecology, particularly involving null models.Briefly this is how a null model works: 1. You determine a pattern that you expect to arise in your data if a certain process is occurring; classic example is if there is competition among species then you expect spatial segregation among different species, 2. You develop a metric which tells you how strong this patterning is in a given sample, 3. You calculate the metric for a given sample, 4. You compare the calculated metric to those for a large sample of simulated data (usually based on resampling of your real data) which are patterned only by random chance; the algorithm that generates these random samples is your null model, 5. If your real value is more patterned than a large proportion (let's say 95%...but not necessarily) of the simulated samples, you would infer that the process is actively patterning your samples. Not so conceptually challenging, right?The idea came from Diamond's checkerboard concept (Diamond 1975) that certain species combinations do not occur due to competition (based on bird distribution across New Guinean islands). Connor and Simberloff (1979) created the null model to test whether the number of checkerboard pairs differed from chance alone...it didn't in their test. So in this case, the checkerboard (a pair of species never in the same sample) was the pattern being detected, the metric was the number of checkerboards, and it was compared to randomly structured data. This set off a storm of controversy and one of ecology's greatest debates. But regardless of where you stand on that particular debate, there is considerable utility in the null model approach and they can be applied to a wide variety of questions.It's really cool that out of all the possible biota, Drs. Gotelli and Maestre have developed a new method using biocrusts as a model system. They applied it to field data, and Andrea Castillo's constructed experimental crusts. Every chance I get, I like to tell people what a great model biocrusts are in community ecology (and maybe landscape ecology too) for empirical tests of theory, e.g. biodiversity effects on function, the stress gradient hypothesis, intransitive competition as a diversity conserving mechanism to name a few. Fernando has really been a pioneer in this arena. I'm glad to see Dr. Gotelli also sees the utility in this study system. Briefly, this method compares the difference in a given functional response (e.g. photosynthetic rate, enzyme activities, etc.) in samples that contain a particular species and samples which do not. It can be applied to non-experimental field data, taking advantage of natural variation (i.e. a natural experiment). For each species, it calculates a difference (D) between the mean value of some function in samples with the species present and the mean value of the function in samples with the species absent. Then to create the random expectation, the values of function are scrambled randomly among samples, and D is calculated. This process is repeated a large number of times usually until you have thousands of these randomly structured D values. Thus you have an entire distribution of null D-values with which to compare to the real D-value calculated from your data. A standardized effect size can be calculated by subtracting the mean random D from the real D-value, and dividing by the standard deviation of the randomized D-values.Here's a video prepared by the authors about the technique:This paper is especially interesting to me, since I have also worked on some of the data that were used to develop the method in this paper (Bowker et al. 2011). I also have tried to tease out the impact of individual species on ecosystem functions. I had the sense that there was a low level of redundancy among the macroscopic crust components, the mosses and lichens. My reasoning was that if different species exhibit unique functional profiles (a suite of effects on different ecosystem functions) then they were not redundant. My approach to this problem was to seek correlations between certain species and higher functional values in ordination space. The biggest flaw with my approach is that it is difficult to get much information on the rarest or less abundant species. I'm not sure if Gotelli's method gets around this issue. It seems it would be difficult to get a good estimate of D if you have relatively few presences of a particular species. I think it's important to note that in both papers (Bowker et al. 2011, Gotelli et al. 2011), the importance of a species must be understood in a community context. For example if Squamarina lentigera has a strong positive relationship with phosphatase activity, it does not mean that Squamarina is making large amounts of phosphatases. It may be, but it may also be altering the abundance of other organisms which do influence phosphatase. In other words the effects can be indirect. Also, the authors are careful to note that null models can be deceptive. For example, an apparent effect of the species on a function may be observed if: a. the function being considered is actually influencing the abundance of the species, not vice-versa, or b. both arise due to a third mechanism such as spatial variation in soil properties. But if you take steps in your sampling design to minimize these possibilities, you are on firmer ground. For these inferential weaknesses, null models will not supplant experimentation, but they complement it well. An experiment adding or removing all community members to determine their effect on ecosystem function can get overwhelming and impractical fast. A null model analysis can help identify a smaller set of species which can be examined in an experimental context. Furthermore, you may already have a dataset amenable to this null model analysis (I do), whereas designing and running an experiment de novo may take years. In any case, I'm looking forward to applying this new technique and the FORTRAN program supplied with the paper.Bowker MA, Mau RL, Maestre FT, Escolar C, Castillo AP. 2011. Functional profiles reveal unique ecological roles of various biological soil crust organisms. Functional Ecology 25: 787-795.Connor EF, Simberloff D. 1979. The assembly of species communities: chance or competition? Ecology 60: 1132-1140. Diamond JM. 1975. Assembly of species communities. Pages 342-444 in Cody ML, Diamond JM, eds. Ecology and evolution of communities. Harvard University Press, Cambridge, Massachusetts, USA.... Read more »
Gotelli, N., Ulrich, W., & Maestre, F. (2011) Randomization tests for quantifying species importance to ecosystem function. Methods in Ecology and Evolution. DOI: 10.1111/j.2041-210X.2011.00121.x
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