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CO2 is plant food? If only it were so simple

Posted on 27 April 2011 by Dawei

In the climate change debate, it appears to be agreed by everyone that excess CO2 will at least have the direct benefit of increasing photosynthesis, and subsequently growth rate and yield, in virtually any plant species: A common remark is that industrial greenhouse owners will raise CO2 levels far higher than normal in order to increase the yield of their crops, so therefore increasing atmospheric levels should show similar benefits. Unfortunately, a review of the literature shows that this belief is a drastic oversimplification of a topic of study that has rapidly evolved in recent years.

Climate control vs. climate change

The first and most obvious retort to this argument is that plants require more than just CO2 to live. Owners of industrial greenhouses who purchase excess CO2 also invest considerable effort in keeping their plants at optimum growing conditions, particularly with respect to temperature and moisture. As CO2 continues to change the global climate, both of these variables are subject to change in an unfavorable way for a certain species in a certain region (Lobell et al. 2008, Luo 2009, Zhao and Running 2010, Challinor et al. 2010, Lobell et al. 2011). More and more it is becoming clear that in many cases, the negatives of drought and heat stress may cancel out any benefits of increased CO2 predicted by even the most optimistic study. 

But there is a more subtle point to be made here. The majority of scientific studies on enhanced CO2 to date have been performed in just these types of enclosed greenhouses, or even worse, individual growth chambers. Only recently have researchers begun to pull away from these controlled settings and turn their attention to outdoor experiments. Known as Free-Air CO2 Enrichment or “FACE”, these studies observe natural or agricultural plants in a typical outdoor setting while exposing them to a controlled release of CO2, which is continuously monitored in order to maintain whichever ambient concentration is of interest for the study (see Figure 1).

Figure 1 - Example FACE study in Wisconsin, USA with multiple CO2 injection plots; courtesy of David F Karnosky, obtained from Los Alamos National Laboratory.

FACE studies are therefore superior to greenhouse studies in their ability to predict how natural plants should respond to enhanced CO2 in the real world; unfortunately, the results of these studies are not nearly as promising as those of greenhouse studies, with final yield values averaging around 50% less in the free-air studies compared to greenhouse studies (Leaky et al. 2009, Long et al. 2006, Ainsworth 2005, Morgan et al. 2005). Reasons for this are numerous, but it is suspected that in a greenhouse, the isolation of individual plants, constrained root growth, restricted pest access, lack of buffer zones, and unrealistic atmospheric interactions all contribute to artificially boost growth and yield under enhanced CO2.

C3 & C4

Photosynthesis comes in a few different flavors, two of which are C3 and C4. Together C3 and C4 photosynthesis make up almost all of modern agriculture, with wheat and rice being examples of C3 crops while corn and sugarcane are C4. The distinction deals mainly with the specific enzyme that is used to collect CO2 for the process of photosynthesis, with C3 directly relying on the enzyme RuBisCO. C4 plants also use RuBisCO, but unlike C3 plants, they first collect CO2 with the enzyme PEP-carboxylase in the mesophyll cell prior to pumping it to RuBisCO (see Figure 2).

Figure 2 - A simplified diagram contrasting C3 vs. C4 plant photosynthesis. From Nature Magazine.

The relevance of this distinction to excess CO2 is that PEP-carboxylase has no natural affinity for oxygen, whereas RuBisCO does. RuBisCO will just as readily collect oxygen (which is useless) as it will CO2, and so increasing the ratio of CO2/O2 in the atmosphere increases the efficiency of C3 plants; the extra step in the C4 process eliminates this effect, since the mesophyll cell already serves to concentrate pure CO2 near RuBisCO. Therefore excess CO2 shows some benefit to C3 plants, but no significant benefit to C4 plants. Cure and Acock 1986 (a greenhouse study) showed excess CO2 gave a 35% photosynthesis boost to rice and a 32% boost to soybeans (both C3 plants), but only a 4% boost to C4 crops. More recently, Leaky et al. 2006 (a FACE study) did not find any statistically significant boost in photosynthesis or yield for corn (a C4 crop) under excess CO2.

Going a bit deeper, it has recently been found that in some C3 plants—such as cotton and many bean species—a further enzyme known as RuBisCO activase is required to convert RuBisCO into its “active” state, the only state in which it can be used for photosynthesis. The downside of this is that the activase enzyme is much more sensitive to high temperatures compared to RuBisCO itself, and also responds poorly to excess CO2: Heat can destroy the structure of the activase enzyme at temperatures as low as 89.6 F, while excess CO2 reduces the abundance of the cellular energy molecule ATP that is critical for RuBisCO activase to function properly (Crafts-Brandner & Salvucci, 2000, Salvucci et al. 2001). This effect may potentially nullify some of the gains expected from excess CO2 in these plants. 

Chemical Responses & Nutrition

Even within a specific type of photosynthesis—indeed, even within a specific species—the positive responses to enhanced CO2 can vary widely. Nutrient availability in particular can greatly affect a plant’s response to excess CO2, with phosphorous and nitrogen being the most critical (Stöcklin and Körner 2002, Norby et al. 2010, Larson et al. 2010). The ability of plants to maintain sufficient nitrogen under excess CO2 conditions is also reduced for reasons not fully understood (Bloom et al. 2010, Taub and Wang 2008).

It has also been found that excess CO2 can make certain agricultural plants less nutritious for human and animal consumption. Zhu 2005, a three-year FACE study, concluded that a 10% decrease in the protein content of rice is expected at 550 ppm, with decreases in iron and zinc contents also found. Similarly, Högy et al. 2009, also a FACE study at 550 ppm, found a 7% drop in protein content for wheat, along with decreased amino acid and iron content. Somewhat ironically, this reduction in nutrient content is partially caused by the very increase in growth rates that CO2 encourages in C3 plants, since rapid growth leaves less time for nutrient accumulation.

Increased CO2 has been shown to lead to lower production of certain chemical defense mechanisms in soybeans, making them more vulnerable to pest attack and diseases (Zavala et al. 2008 and Eastburn et al. 2010). Other studies (e.g. Peñuelas and Estiarte 1999) have shown production of phenolics and tannins to increase under enhanced CO2 in some species, as well as many alkaloids (Ziska et al. 2005), all of which may have potential consequences on the health of primary consumers. The decreased nutritional value in combination with increased tannin and phenolic production has been linked to decreased growth rate and conversion efficiency of some herbivores, as well as an increase in their relative demand and consumption of plants (Stiling and Cornelissen 2007).

Furthermore, many “cyanogenic” species—plants which naturally produce cyanide, and which include 60% of all known plant species—have been found to increase their cyanide production in an enhanced CO2 world. This may have a benefit to the plants who use cyanide to inhibit overconsumption by pests and animals, but it may in turn reduce their safety as a food supply for both humans and animals (Gleadow et al., 2009a and Gleadow et al. 2009b).

Interactions with other species

Competing plant species have also been shown to drastically alter expected benefits from excess CO2: even in the best FACE studies, most research still involves artificial experimental plots consisting of fewer than five plant species, and often only one species is present. It has long been understood that due to increased growth of competitor species, benefits from isolated experiments cannot be scaled up to explain how a plant might respond in a monoculture plot (Navas et al. 1999). The distinction is even greater when comparing the behavior of isolated species to those of mixed plots (Poorter and Navas 2003). The lack of correlation (r2 = 0.00) between biomass enhancement (BER) of isolated plants and that of plants in mixed plots is presented in Figure 3.

Figure 3 – Isolated vs. mixed biomass enhancement ratios under excess CO2; From Figure 8 of Poorter and Navas 2003

That some plant species may benefit more fully and/or rapidly from excess CO2 also introduces the possibility that the abundance of certain species in an ecosystem will increase more than that of others, potentially forcing the transformation from one type of ecosystem to another (Poorter and Navas 2003). There is also some evidence suggesting that invasive species and many “weeds” may show relatively higher responses to elevated CO2 (Ziska and George 2004), and become more resistant to conventional herbicides (Ziska et al. 2004, Ziska and Teasdale 2000).

There is some evidence that interacting bacterial communities, particularly in the roots, will be affected through elevated CO2, leading to mixed results on overall plant health. Mutualistic fungal  root communities (known as ‘mycorrhizae') are typically shown to increase under excess CO2, which facilitate nutrient transport to the roots (Treseder 2004), although infections of pathogenic species such as Fusarium (the agent of the disease known as ‘crown rot’) have been shown to become more severe under excess CO2 as well (Melloy et al. 2010).

Temperature

It has long been known that stomata (the pores through which plants take in CO2 and exhale oxygen and water) tend to be narrower and stay closed longer under enhanced CO2. This effect is often cited as a benefit in that it increases water efficiency in drought situations.

But there is another key piece to reduced stomatal conductance, considering that 90% of a plant’s water use is actually for cooling of the leaves and nothing more: heat from the sun is absorbed by the water in the leaf, then carried out as vapor in the form of latent heat. So while it is true that the plant may retain water better under enhanced CO2, doing so may cause it to retain more heat. This can potentially carry a plant to less optimal temperature ranges (Ball et al. 1988 and Idso et al. 1993). An image present in Long et al. 2006 (Figure 4) shows this effect quite clearly; while a 1.4 C increase is probably not enough to cause significant damage in most cases, global warming will only serve to exacerbate the effect.  It is also of note that the study above represented a well-watered situation, and so during a drought condition the temperature increase would be even higher. 

Figure 4 - Increase in local temperature under enhanced CO2 due to reduced evapotranspiration. From Long et al. 2006

On the cold end, it has been found that for seedlings of some species of evergreen trees, excess CO2 can increase the ice formation temperature on the leaves, thereby increasing their sensitivity to frost damage (Roden et al. 1998).

Ozone

CO2 is not the only atmospheric gas that is on the rise: concentrations of ground-level ozone (O3) are expected to rise 23% by 2050 due to continuing anthropogenic emissions of precursor gases like methane and nitrous oxides. In addition, Monson et al. 1991 found that natural plant emissions of volatile organic compounds (another group of O3 precursors) increase under excess CO2 in many plant species, thereby introducing the potential that local O3 concentrations around plant communities may rise even higher than the baseline atmospheric level.

O3 has long been known to be toxic to plants: Morgan et al. 2006 found a 20% reduction of soybean yield in a FACE study of 23% excess O3. Similarly, Ainsworth 2008 showed a 14% decrease in rice yield at 62 ppb O3, and Feng et al. 2008 (a meta-analysis of 53 peer-reviewed studies) found on average a 18% decrease in wheat yield at 43 ppb O3. Ozone also appears to reduce the structural integrity of plants as well as make them more vulnerable to certain insect pest varieties such as aphids (Warrington 1988).

Figure 5 - Yield reduction for several crop species under excess ozone. From Wang and Mauzeral 2004

With respect to this effect, excess CO2 may actually prove beneficial in that it causes a narrowing of leaf stomata, thereby reducing the quantity of ozone that can enter the more sensitive internal tissues. Needless to say, the combined effect of excess CO2 and excess O3 is complex, and as it has only recently been given attention it is an area that requires much further research.

Conclusion

A specific plant’s response to excess CO2 is sensitive to a variety of factors, including but not limited to: age, genetic variations, functional types, time of year, atmospheric composition, competing plants, disease and pest opportunities, moisture content, nutrient availability, temperature, and sunlight availability. The continued increase of CO2 will represent a powerful forcing agent for a wide variety of changes critical to the success of many plants, affecting natural ecosystems and with large implications for global food production. The global increase of CO2 is thus a grand biological experiment, with countless complications that make the net effect of this increase very difficult to predict with any appreciable level of detail.

NOTE: This post is also the Advanced rebuttal to "CO2 is plant food".  And a hearty welcome to Dawei to the ranks of Skeptical Science authors

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Comments 1 to 50 out of 106:

  1. And just to make things more complicated it is being reported that CO2 fertilization is killing koalas because CO2 fertilized plants put more of their energy into structure. Eucalyptus leaves become harder for present day koalas to digest because they are more fibrous.

    This is probably not restricted to eucalyptus and might indeed affect some of the plants we eat, or at least affect more animals than just the Koala.
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  2. This post is a balanced and understandable treatment of a very complex topic. Well done!
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  3. I knew nothing about the C3 and C4 pathways until recently when I was trying to understand why the C13/C12 isotope ratios for fossil carbon where different than for atmospheric carbon (thus allowing the identification of fossil carbon on the atmosphere).

    The reason is that while C12 and C13 are chemically identical, but their different masses mean that they diffuse across membranes at different rates. Different diffusion steps mean that the two pathways fix different amounts of C13. The C4 pathway is more efficient, but only evolved recently.
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  4. Yes, excellent summary Dawei.
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  5. Thanks for this. Though I think it is written well enough for intermediate readers to understand as well as advanced.

    I hadn't seen the temperature angle tackled like this before - very interesting.

    Further to the interation with other species section it has been shown that lacwing predators of aphid pests become less efficient consumers under elevated CO2 (Gao et al.; 2010*). Which, if combined with increased fitness of pests & pathogens as hypothesised by Gregory et al. (2010)** signls a possible need for increased pesticide load under elevated CO2.

    Is Sherwood Idso any relation to the CO2"science" Idsos? Speaking of them, I wonder how many of the studies cited above are also cited over there.




    *Elevated CO2 lessens predation of Chrysopa sinica on Aphis gossypii. Entomologia Experimentalis et Applicata. 135

    **Integrating pests and pathogens into the climate change/food security debate. Journal of Experimental Botany. 60
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  6. I still don't get why 'CO2 is plant food' is not a good general point to make. All science (in fact all aspect of human experience) has ever increasing levels of complexity but it doesn't mean that you can't also make generalized statements about subjects.

    Take for example this review which makes many of the same points as made here but which still doesn't shy away from stating "The stimulations of crop seed yields by the
    projected CO2 levels across FACE studies are about 18% on average and up to 30% for the hybrid rice varieties and vary among crops, cultivars, nitrogen levels and soil moisture." in it's abstract.
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  7. For the very simple reason, HR, that your side of the debate use this over-simplification to con people into thinking that it'll be perfectly fine to keep artificially altering the composition of our atmosphere. A 30% increase in crop seed yields might sound great, but are those yields pre-or-post acclimation? What will happen to those yields in warmer conditions, where we are now learning that levels of Rubisco Activase are lower? How will these yields be impacted by changes in insect pests, soil-borne diseases & weeds-all known to be altered by changes in CO2 alone-even before we consider the impacts of eCO2 on mean temperature & hydrology? What happens to Protein, Zinc & Iron levels in those rice plants grown under eCO2 conditions? You can't just avoid these many issues in order to paint a Utopian vision of an eCO2 world-yet that's exactly what you & your fellow contrarians do quite regularly.

    Ultimately, though, its because the statement that "CO2 is a plant food" ignores the fact that water, nitrogen & trace elements have a far more limiting effect on plant growth-in the mid to long term-than CO2 does.
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  8. #6 HR. In a sense, H2S is the "closest" relative to H2O, and it can surely be useful. Why, then, don't we praise its presence as a source of sulfur, which is biochemically essential?

    When we talk generally about different substances, we do of course refer to their effects in general. This post is a general, balanced account of CO2, or you should at least acknowledge that it is a good and honest try at that.
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  9. I am delighted to see this topic receive more nuanced attention as it is generally neglected. We are indeed engaged in a vast experiment that has veered out of control, whereas even the best scientists barely understand all the complex ramifications to plantlife. I would venture to say that we are well behind the curve even compared to the relatively recent revelations about ocean acidification, as far as comprehending the cascading effects in the terrestrial ecosystem by altering the composition of the atmosphere. The consequences of dying trees - losing an essential CO2 sink and the production of oxygen, not to mention habitat for countless species - are at least as dire as losing coral reefs, pteropods and phytoplankton.

    It has so far been impossible to remove existing ambient background levels of tropospheric ozone for controlled experiments on trees and because of this, the long-term impacts are generally ignored. It's kind of like sea level rise - everyone knows it's going to be far higher than IPCC predictions based solely on thermal expansion, but because there is too much uncertainty in modeling the rates of melting ice sheets, it is simply left out of the predictions. Fine - that's science.

    The same is true for trees. Anyone the least familiar with ozone knows it is toxic to vegetation, and knows the background levels are inexorably rising. But because no one has been interested or, perhaps, had the funding to do a gigantic, enclosed, decades-long experiment comparing trees growing with pre-industrial levels (basically zero) to today's (anywhere from 40 ppb to 80 or more) we act as though there is nothing significant to worry about.

    And yet in the real world, in this uncontrolled experiment, it is easy to ascertain that trees everywhere are dying, and at a rapidly accelerating rate. This is true for trees of all species, in all habitats - even young trees being watered in nurseries. Their decline is chalked up to all sorts of other opportunistic causes - insects, disease, fungus, drought, road salt, natural gas line leaks, climate change and on and on. But there is an immense, global pattern here which is attributable to the one thing that trees everywhere share in common - the atmosphere, which is poisonous, and to which they are exposed season after season. They have the tree equivalent of AIDS, a compromised immune system.

    It has been my hope for years that someone with scientific credentials will reveal this existential threat before we lose the ability to preserve seeds and nuts. This article is a great leap forward, with a number of excellent references - there are many more links to published research collected at the top of my blog, http://witsendnj.blogspot.com/
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  10. Try a full accounting of plants vs CO2 at 550 ppm. With tundra and rain forest converting to more CO2 and even temperate plants succumbing to wildfire, and crops in broad continental interiors failing due to heat waves, it looks like CO2 turning existing plants into more CO2 rather than vice versa ;)

    At any temperature, what humans need is not "plant food" but "people food". All us animals need protein, which requires nitrogen. Plants can not turn carbon into nitrogen. Legumes with their nitrogen fixing bacteria may be able to produce more protein given more CO2 and genetic engineering may help other crops to use nitrogen for protein more efficiently but first these plants have to beat the heat. Good luck on that.
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  11. HR @6
    I still don't get why 'CO2 is plant food' is not a good general point to make

    Two reasons:
    1. the implication of your statement is that from this it automatically follows that the agricultural yield of the planet will increase with higher levels of CO2. As Pete Dunkelberg points out @10, there are missing, more questionable steps in that argument

    2. Your statement can have the implied follow-on "so therefore is nothing dangerous about CO2" which, as I'm sure you're aware, has all kind of logical fallacies attached to it.

    Changes to commodity prices over recent history don't suggest any beneficial effect on crop yields yet! (I accept, obviously, that this is somewhat crude measure)
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  12. Pete @10 raises a very interesting point. It could be that elevated CO2 creates more biomass to burn in a warmer world.
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  13. The article states "FACE studies...; unfortunately, the results of these studies are not nearly as promising as those of greenhouse studies, with final yield values averaging around 50% less in the free-air studies compared to greenhouse studies "

    In looking at the reference cited, it's clear that the article should state that the "with the INCREASE in final yield values averaging around 50% less in ..... "

    As the first reference ( Leaky et al. 2009 provided in that section of the article says, there are 6 important lessons learned from FACE studies: "First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale." ..... (read the article to see the rest)

    So CO2 is not only a plant food, but it also decreases water use and improve the efficiency of nitrogen use.
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  14. This is a good article.

    I would come at it from a different angle. Greenhouse studies are not a good indication of a real world scenario; most people agree on this. FACE studies are a bit better, and show less improvement from CO2 enhancements than greenhouse studies. But if you want a more accurate reflection (although still not perfect) of how CO2 increases will affect crop plants in the real world, you have to account for the shifting climate zones. For instance, you try growing crop varieties commonly used in Missouri, but try to grow them in central Texas, adding all the CO2 you want.

    Anyone want to bet on whether the results will continue the downward trend on yield improvements seen in moving from greenhouse to FACE?
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  15. Excellent post! I would nit-pick at you choice of words in the paragraph:
    "Only recently have researchers begun to pull away from these controlled settings and turn their attention to outdoor experiments."
    I would say that the experiments aren't recent; rather, they were started some time ago but it takes several years before the results can be reported, and the results are recent.
    Back to the main idea, I've often replied to "CO2 is plant food" by saying "yes, where CO2 is a limiting nutrient" and these same people usually have irrigated and fertilized lawns and gardens but never thought of trying to add CO2. I also like to point out the tree-topped mountains where I live, and ask why do the trees grow up there and not on desert and chaparral? For the thinner air?
    Reading your post, I'm beginning to think my "where CO2 is limiting" response is too simple. Rising CO2 affects plant growth whether it is or isn't a limiting nutrient.
    thanks,
    jg
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  16. I'm not quite sure whether I have correctly understood your post. Have there been experiments which include changes in CO2, temperature, water, competing plants (weeds) and pests?
    Another question: If the effects of higher CO2 are on balance negative, would a lower CO2 be better? Are there any experiments from which one can conclude what an optimal CO2 level would be for the world's major crops?
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  17. Martin asks a good question. I think this post shows that any rapid change in CO2 is a disturbance that will have complex and numerous effects. Humans can disturb the CO2 level upward faster than natural changes can (barring rare events). I would like to add a question to Martin's: Are there any drawdowns of CO2 that are as fast as the anthropogenic increase? I can't think of a mechanism that would sustain a rapid reduction.

    I have to dig through my notes to find it, but I remember reading about a theory that the drawdown of CO2 leads to a state where forest die, which in turn reduces the rate that global forests sequester CO2, allowing atmospheric levels to rise. In other words, low CO2 invokes a reverse feedback effect.

    I'll hunt for that reference.

    jg
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  18. Great post! Iv'e been waiting to see a sophisticated treatment of this. As even the comments make clear, this is a complex issue as it involves not only the factors limiting plant productivity and biomass but also plant consumers and more broadly food webs. Community ecology is a messy business!

    One issue not really addressed by the post, although touched on in the comments, is how warming will increase plant pathogens and general rates of herbivory. Plenty of work supports the first prediction and the latter is a simple function of physiology: warming increases insect metabolism and thus metabolic demands. Plant photosynthesis also responses positively to increasing temperature, but at a slightly lower rate, eg, see this paper and this summary of it. The end effect, all things being equal and assuming nutrients and water are available, is more primary production and less plant biomass.

    Of course if you consider what limits insect (and other grazer) populations, things get a lot more complicated; it often isn't plant tissue but rather minerals, disease, weather, climate and predators.

    Martin #16, I personally don't believe it is possible to generalize like that (ie, is more or less CO2 "better" for all crops) but you can be sure a warmer world will have more plant disease and likely more plant consumption.
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  19. Martin at 04:22 AM, the FACE trials that concentrate on food crops best replicate real world conditions regarding pests and diseases in that they are conducted alongside control plots generally in zones where the crops that are being trialled are normally grown.
    Rainfall and temperatures are replicated by changing the time of sowing, or by conducting parallel trials of the same varieties in areas that are warmer and drier than the main trial site. Such areas are readily identified from historic records and may only be a couple of hundred km away.

    Trials to ascertain the effect of lower CO2 levels see reduced growth, growth apparently almost ceasing around 180ppm, however these obviously have to be conducted in enclosures.
    Commercial greenhouse growers do have some knowledge of these effects having noted different zones of growth depending on how even CO2 circulation is within the enclosed area, being quickly depleted in those areas least exposed to the extra CO2 introduced, and not able to be replenished as occurs in the open air, but sometimes due to unintended leaks of air from outside do illustrate side by side, how the various concentrations manifest themselves in varying degrees of growth.

    As far as optimal CO2 levels, commercial greenhouse growers where CO2 enrichment has been utilised for several decades, tomatoes being perhaps the main, but just one of the foods you may have been eating all those years, find the optimal level is about 1200ppm. Whether that is optimal for growth or optimal for economic returns I'm not sure.
    In commercial cropping, most fertilisers are applied at rates that find the best balance of offsetting the cost of applying the fertiliser against the anticipated increased returns it will provide, so it is almost always a commercial decision, based on the law of diminishing returns.

    However as far as absolute growth is concerned, application rates for many commonly applied fertilisers more than double the normal rates will still see some growth response over lower rates.
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  20. "Trials to ascertain the effect of lower CO2 levels see reduced growth, growth apparently almost ceasing around 180ppm, however these obviously have to be conducted in enclosures."

    Which is ultimately a straw-man anyway, given that the odds of us ever experiencing CO2 concentrations of less than 200ppm are absolutely zero.

    The fact is that changes in sowing times are a poor simulation of the kinds of conditions being predicted when atmospheric CO2 levels really reach 560ppm-as I've said many times before. As I recall, though, the seed yield increases under those conditions (Rain-fed, slightly warmer) were negligible (when standard errors are accounted for) compared with ambient CO2-& at a cost of reduced nitrogen uptake under those conditions. Of course, other results from FACE trials show that enriched CO2 conditions favor an increase in certain soil-borne pathogens, increase the competitiveness of weeds & make plants more susceptible to attack by insect pathogens. All of which suggest, as I've said before, that the "CO2 is plant food" meme is *dangerously* simplistic.
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  21. A complex subject, and good overview (with citations).
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  22. Grain quality is decreased under high Co2 concentrations:

    Based on the modified physiology and biochemistry of wheat plants under CO2 enrichment, the concentration of total protein in grain was significantly decreased by 7.4% in the FACE treatment. The reduction in grain protein due to elevated CO2 is consistent with previous reports (Kimball et al. 2001; Taub et al. 2008; Wieser et al. 2008; Ho¨gy et al. 2009), resulting in potentially far-reaching consequences for the nutritional value and use by the processing industry...........

    ....Among the grain proteins, the N- and glutamine-rich
    gliadin fraction was significantly decreased under CO2
    enrichment, thereby lowering the gluten concentration
    that is fundamental in determining physical properties of
    dough formation and product quality
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  23. 20 Marcus ".. changes in sowing times are a poor simulation of the kinds of conditions being predicted .."

    Yes, I noticed that. I realise that it's very limiting trying to do these experiments in the open air - and the equipment budget was probably entirely swallowed by the CO2 delivery and monitoring systems - but temp changes, (un)seasonal conditions and precipitation unreliability need attention.

    Who cares if the plants themselves do better if crops are ruined by two weeks of wet/ hot/ windy/ plague conditions just before harvest? I've seen grown men bravely blink away tears over rust ruining a thriving wheat crop - and that was a long time ago.

    Rust one year, locusts the next - following several drought years, makes Hanrahan look hopeful.
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  24. 7 Marcus

    And "your side of the debate" seem to have given up on human ingenuity or forgotten that throughout human history our ancestors have been fighting those problems you list, and generally winning. And all that without even knowing Rubisco Activase existed. Not a utopian but certainly a believer in the human spirit, I'd rather that than an utter pessimist. I'm not here to paint utopian visions just to counter unnecessary pessimism.

    I'm a molecular biologist. Give me a grant of a few hundred thousand dollars and a few research scientists and I'll give you 500 new versions of Rubisco Activase, or more if you want. Give me another ten years funding and I'll give you a field trial on x number of genetically modified crops with a new Rubisco Activase gene. There are always solutions to problem. ( -Questioning of motives snipped- ).
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    Moderator Response: [DB] Don't go there.
  25. HumanityRules - "human ingenuity " and "our ancestors have been fighting those problems" are no excuse for short-sightedness, or hiding our heads in the sand while the tidal wave crests.

    Which, by dismissing the severity of global warming and it's impact on our lives, 'skeptics' appear to endorse.
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  26. 8 SNRatio

    In general I've got no real problem with this article other than it has a bias towards the most pessimistic conclusions.

    For example

    "This effect may potentially nullify some of the gains expected from excess CO2 in these plants."

    Much of the work supporting this is in vitro work performed on isolated enzymes. That is, it's not even in the plant. Earlier Dawei rightly critised greenhouse work for not well representing the real world. How well to you think a couple of isolated enzymes in a tube represent the real world? There is nothing wrong with this scientific approach, it's getting us a basic understanding of these enzymes but it's a long stretch to Dawei's conclusion from this work.

    If Dawei is happy to put a big question mark over the greenhouse work then I'd argue that conclusions drawn from this in vitro work should be in the region of complete speculation. Nothing wrong with that either as long as it's well understood.

    In the meantime all the reports of field trials still seem to show increased CO2 is either beneficial or at worst neutral to crop growth that still doesn't seriously undermine the idea of CO2 is plant food idea.
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  27. 25 KR

    I'm not hiding my head in the sand, I'm standing up against unrealistic pessimism. I suspect under your cloud of doom you can't see that.
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  28. "And "your side of the debate" seem to have given up on human ingenuity or forgotten that throughout human history our ancestors have been fighting those problems you list, and generally winning."

    Tell that to the hundreds of millions of people who are already going hungry-across the world-& they'll probably laugh in your face HR. What do you think the metric will be when ingenious-but costly-solutions to the problems caused by eCO2 & related global warming push the price of basic food-stuffs even *further* beyond their reach? Or what do you think will happen to the price of crops if we have more record breaking droughts, like those in Russia, or massive storms & flood damage to crops, like those here recently in Australia? What's going to happen to the price of crops when some of the most arable land is under half a meter of sea-water? Yet the way you & your ilk would have us believe, with your simplistic reasoning, eCO2 will bring about a New Age of Abundance-whereas the realists amongst us can see that what it will really bring is a whole mess of new problems, in quick succession, that even our most ingenious minds might not be able to solve in time &-even if they do-will come at the cost of much higher prices for staples-like wheat, rice & soy-beans. We realists also recognize that it will be much cheaper-in the Mid to long term-to simply *avoid* this eCO2 scenario altogether, rather than take the chance that the boffins can solve all the problems it will create in time. Still, I guess if your only concern is the profitability of the fossil fuel sector, then you'd be willing to take that chance.
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  29. "If Dawei is happy to put a big question mark over the greenhouse work then I'd argue that conclusions drawn from this in vitro work should be in the region of complete speculation. Nothing wrong with that either as long as it's well understood."

    Well, HR, that just proves how little you know about the relationship between In Vitro & whole organism biology. History has shown us that what occurs at a cellular level tells us a great deal about what will occur at the whole organism level. Greenhouse work, by contrast, only tells us about how an organism will survive in carefully controlled conditions. Once again it seems your "skepticism" only stretches to those things which contradict the propaganda of the Contrarian Movement.
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  30. 11 Phil

    1) All the FACE trials Dawei lists here and mentioned in the review linked in #6 suggest mainly positive, sometimes neutral and never negative effects from increased CO2 from what I can see. The extra information is certainly thought provoking but I don't see that any sufficiently undermine the findings of these trials.

    If you don't want to go as far as saying this points to better agricultural yields in the future that's fine, I don't think I ever put that forward. But it doesn't seem to undermine 'CO2 is plant food'.

    2) It seems a little unfair that I should be held responsible for possible conclusions people might draw from falsely interpreting what I said.
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  31. There are FROZEN FROGS with a label saying "domain unregistered" in place of the figures.

    I hope the glitch is fixed soon
    0 0
    Moderator Response: [DB] Glitch fixed; thanks for pointing that out!
  32. Ian Forrester at 12:29 PM, each time the subject of CO2 enrichment, and particularly FACE trials for wheat is raised, the most common response is, that despite the indicated yield increases, it is the lower protein levels that is the issue.

    Given that, I think something needs to be clarified in the minds of those who have little or no background knowledge on the subject.

    The inverse relationship of crop yield and protein levels is not, repeat, not something new, nor specific to CO2 enrichment.
    Instead it is a well understood, well measured response that happens every year, and has been happening forever, or at least must seem like forever for those growers whose payment is structured not only on weight delivered, but on protein content.
    Routinely, years of high yields show lower protein levels, whilst the lower yields of drought conditions can be offset somewhat by higher protein levels.

    In fact there is even a standard formula that is used to determine the nitrogen requirements of a crop that explains the relationship.
    Using the grain yield (t/ha), the grain protein (%) and the appropriate grain protein factor, (for wheat it is 1.75, and for all other grains it is 1.6), multiplying the yield in t/ha by the % protein by the factor gives the nitrogen requirements in kg/ha.
    For example a 3 t/ha crop of wheat at 12.5% protein removes 66kg N/ha
    (3 t/ha x 12.5% x 1.75 = 66 kg N/ha).

    This formula is used by growers to anticipate the amount of nitrogen that they may need to apply if they want to achieve a certain crop yield.



    http://www.agric.wa.gov.au/PC_92452.html


    However other factors may overtake the planned outcome with the eventual yield higher or lower, but the protein will also have varied inversely if the intended amount of nitrogen was taken up by the crop.

    Another point that is also overlooked is that if calculations are done using the increased yields achieved under CO2 enrichment, and the lower protein levels, it is clear that the amount of protein produced per hectare actually increases.

    Where such limits may be is yet to be determined, but it may eventually be found in the ability of the plants to take up the nutrients, or it could be in the ability of the soil to give them up.

    I don't think anyone should be writing off what may be possible, nor discounting the ability of those involved in such work, it is all really only beginning, and so little is known.
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  33. "1) All the FACE trials Dawei lists here and mentioned in the review linked in #6 suggest mainly positive, sometimes neutral and never negative effects from increased CO2 from what I can see."

    Do some research then HR, there are several FACE trials which have shown that eCO2-alone-will increase the susceptibility of crop plants to insect pests & increase the levels of certain soil-borne pathogens in the soil-both of which I'd define as a *negative*. There are also FACE trials which show that eCO2 can lead to decreased levels of protein, Zinc & Iron in plants. Given that nitrogen is a key component of chlorophyll, the "power-house" of photosynthesis, I'd say that greatly undermines the foundation of the "CO2 is plant food meme". All of that before we even *begin* to consider the impacts of changed hydrology & increased warming on crop yields.

    "2) It seems a little unfair that I should be held responsible for possible conclusions people might draw from falsely interpreting what I said."

    If you make overly simplistic claims, without offering up caveats, then you're actually making it more likely that people will "falsely" interpret what you said-which suggests that this is the outcome you're hoping for, & therefore you *can* be held responsible for making misleadingly simplistic claims.
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  34. "I don't think anyone should be writing off what may be possible, nor discounting the ability of those involved in such work, it is all really only beginning, and so little is known."

    As someone who is 'involved' in such work (amongst others), John D, I can tell you that you are pinning *way* too much hope on the beneficial effects of eCO2 on crop yields-over the longer term-for several reasons. 1) If the protein yield-in g/kg of total biomass-is reduced, then humans & animals will need to consume greater amounts in order to get the same benefits in terms of protein. The same is true of trace elements like iron & zinc, which have also been shown to fall under eCO2 conditions. 2) Under more stressful conditions (lower water, greater warmth), there was almost *no* significant difference in grain yield between those plants grown at eCO2 vs aCO2 conditions-but the drop in N remained about the same. 3) The FACE trials in Horsham have indicated that acclimation sets in after just 3 short years of cropping at eCO2 conditions. 4) We still don't know *exactly* what impact root-pathogens & insect pests will have on total & grain biomass under eCO2 conditions-but the evidence we have to date does *not* bode well. 5) Back on the issue of N, you seem to forget that the quantity of the enzymes that determine the rate of photosynthesis, as well as the amount of chlorophyll in the leaves, is highly dependent on the levels of nitrogen in the plant. Decrease the levels of N, & this would *suggest* that-in the longer term-you'll also get a decrease in total levels of photosynthesis-that might well suggest that any biomass gains will be short-lived.

    What it keeps coming back to is this-is it going to be *more* cost effective to keep adapting our agriculture to suit humanity's "tinkering" with the atmosphere/climate, or is it more cost-effective to simply *stop* with the tinkering? From everything I've read at this site for the past 2 years, I'd argue that the *latter* is true.
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  35. 29 Marcus

    My 25 years as a molecular biologist says the reason you do in vitro work on a gene is because you can further control the conditions in which the gene is operating. You are taking it one step further away from even the slightly messy conditions of a whole organism in a controlled setting. You can certainly get a more refined understanding of how a gene works but only within the artificial setting of a plastic tube. For example there are numerous co-factor and subtle transcriptional and tranlational modifications that are potentially at work in the in vivo setting that have been lost by working in vitro. There are pro's and con's to both in vitro and in vivo work that's why scientists do both but there is no logic to your suggestion that in vitro work trumps controlled in vivo work in helping us understand what will happen to this gene in the real world.

    I just want to emphasise I'm not critising the science. The science is the right approach. I'm questioning how much we consider these results as speculative in relation to the real world. I think Dawei has clearly taken a speculative jump here. He has rightly highlighted the speculative jump made in extrapolating from greenhouse work but has ignored the even greater speculative jump made from extrapolating from in vitro work. I think that has the effect of emphasising a more pessimistc conclusion.
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  36. HumanityRules wrote : "Give me a grant of a few hundred thousand dollars and a few research scientists and I'll give you 500 new versions of Rubisco Activase, or more if you want. Give me another ten years funding and I'll give you a field trial on x number of genetically modified crops with a new Rubisco Activase gene."


    This seems to be a common refrain, and not just with reference to Climate Studies - people have written about 'pots of gold', 'rivers of cash', 'showers of coins', etc. However, when the banks hear similar stories, and various grant organisations hear similar stories (and even when they hear such stories in Dragons' Den), the answer is the same, unfortunately : "If you can prove that your idea has value and potential, the money is yours. If you can't...NEXT".

    That's life, I'm afraid, and (just as with the other examples I've given) you'll get very little sympathy for wishing, especially from those of a certain political persuasion : who will go on about grant-dependency, people expecting money for nothing, etc.
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  37. "but there is no logic to your suggestion that in vitro work trumps controlled in vivo work in helping us understand what will happen to this gene in the real world."

    That is *not* what I suggested-so please stop trying to misrepresent my position. My position is that, from an historical basis, our observations of plant physiology in the real world (outside of a controlled environment) has been backed by what we know at the cellular & molecular level-& vice versa. The same cannot always be said for glasshouse trials-as the FACE trials clearly show. The difference between them becomes even more stark when you factor in the known impacts of soil-borne diseases, competition from weeds & predation by insect pests-yet all 3 of these factors can be understood, & well predicted, by our knowledge of plant physiology gained from In Vitro work. With that in mind, I'd say Dawei is far less guilty of making "speculative leaps" than those who rely on results taken *purely* from Glasshouse trials. I'd also say that he is emphasizing a *realistic* conclusion regarding the future impact of rising CO2, whereas people such as yourself continue to push a blindly optimistic conclusion-only because that's the conclusion which will require the smallest action to be taken on CO2 emissions in the future.
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  38. "Give me a grant of a few hundred thousand dollars and a few research scientists and I'll give you 500 new versions of Rubisco Activase, or more if you want. Give me another ten years funding and I'll give you a field trial on x number of genetically modified crops with a new Rubisco Activase gene."

    Yes, HR, & can you guarantee that *any* of them will perform better than what nature has already provided after millions of years of evolution? As someone with more than 2 decades of experience in Molecular Biology, I've become a little bit more realistic about the potential for molecular biology to solve all our problems. All GMO's have done is to give certain Corporations far too much control over our agriculture, yet with far fewer benefits for farmers & consumers than were originally promised. So forgive me if I still say that it is *more* cost effective to stop stuffing up our climate than it is to use science to adapt our crops to the conditions we're creating.
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  39. Presumably the main point of the "CO2 is a plant food" denier meme is that plants will grow faster and faster and bigger and bigger and take up more and more of the CO2 that selfless giant corporations are making available to them out of the goodness of their hearts. So, problem solved eh? In fact under this approach we should, if possible, speed up the emissions, none of this renewable energy nonsense - get the coal out faster and faster, get into the oil shales without delay, get the methane in the permafrost released as fast as possible. Because it seems in the view of people who keep trotting out this phrase (who was it said "you call it pollution I call it life"? I can't keep the names of these people in my head for some reason) this is a win-win situation. The poor people, and the rich people I suppose, will get more and more food to eat at no cost to themselves, while at the same time all these giant cabbages will just pull the extra CO2 out of the air therefore solving the imaginary greenhouse gas problem.

    But hang on a moment. Hold your horsepower. If those massive Brussels Sprouts do reduce the CO2 then surely all the plants that were benefiting from it stop growing so fast, in which case the CO2 rises again. And since all the cauliflowers get eaten after one year and all that CO2 excreted again, or die and rot after one year, same result, then the following year, with another injection of CO2 (see benevolent corporations above) into the atmosphere we either have to plant even more broccoli, or the ones we do plant have to grow even bigger.

    But I must be misunderstanding something, surely. If this is what is going on then why oh why do the levels of CO2 keep rising and rising and rising as the years and decades of this warming planet go by? And if this is the mechanism that is going to stop us frying eggs on the top of thermometers in parking lots then why didn't it work in the past? temperatures should have stayed pretty constant for millions of years, but didn't Ian Plimer say ... oh, I can't keep track of this.

    My head aches, why can't I get it? Oh, I know that if the good plants are growing and aiming to feed a billion new people every decade, then so are the weeds which are competing with them for light and water and (I suppose) CO2. Probably competing extremely well since the weeds, being weeds, have all evolved to thrive on the smell of an oily rag and a bit of water every few years, and with the new higher CO2 levels it's summertime and the living is easy for thistles and any other weed you want to name.

    And if the weeds thrive then so will the other individual plants in the farmer's field. if every Kale plant is twice the size it used to be because of more CO2, then don't we get half as many Kale plants in the same space? Or do we make paddocks twice (4 times?) as big. Which means 4 times as much water and fertiliser.

    And then there's those pests. Boy, round here one good La Nina has seen Cabbage white butterflies so abundant the roads at times seem to have snow on them from all the white bodies hit by cars. And there are butterflies and moths I haven't seen before, munch munch munching away. Bigger kohlrabi leaves mean more space for caterpillars don't they?

    Still, one good La Nina doesn't make a summer, and in all the years leading up to this annus mirabilis, the low rainfall, dry ground, harsh winds, high temperatures meant that there was bare ground everywhere - even the grass wouldn't grow, and tough native heaths were dying - in spite of all the extra Co2 the big energy companies had been putting into the air for years for these ungrateful plants

    I must be dumb I guess, just don't get it. Not as keen as mustard I suppose.
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  40. Dawei,

    Excellent post. One modification that I would suggest is to add a section (like those on temperature and ozone) on precipitation.

    All of the information provided is important to understand, and I think helps to indirectly address the main flaw behind "CO2 is plant food" as a statement, which is to say that it grossly oversimplifies the problem, trying to reduce something complex and interactive to the level of a parent's explanation to a child of the awkward question of where babies come from ("the stork brings them").

    I also think that precipitation changes are the big bullet in the climate change gun. Certainly, they are the most difficult to predict, but it will not take much in the way of the wrong amounts of rain at the wrong times to obviate any possible benefits from raised CO2 levels and to greatly reduce crop production.

    The state of the Amazon after the 2005 and 2010 droughts will seemingly soon become a prime example of this. My (personal, uneducated) guess is that one more major drought in the next 3-5 years will have huge ramifications; it's like the largest and most disheartening (unintentional) FACE trial ever performed by man.

    BTW, Climate Wizard is a useful site for researching projected changes in temperature and precipitation by region under different scenarios.
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  41. Marcus at 16:46 PM, regarding your point (1), and the requirement to consume more in order to maintain protein input.

    I have no argument with that, it is a well established fact, however what you are continually missing in the bigger picture is that the extra grain needed to be consumed to make up for the lower protein is less than the total extra amount of grain produced, thus leaving a surplus.
    I have continually pointed this out, that despite the lower protein levels, the increased yield means that more total protein is produced per hectare, thus each hectare is able to supply the protein requirements of more people.
    Surely that is what is important in the big picture.

    Regarding your point (4), you seem to intimate that root-pathogens, insect pests and other diseases are going to emerge as new problems, when in fact they will not be new, but are existing problems that are continually having to be overcome.
    What the FACE trials have not been able to replicate so far, as far as I know, are the strategies that are implemented in commercial operations as a matter of course in order to break the cycles of the problems that concern you. The utilisation of break crops, crop rotation etc. Only once the FACE trials have been running long enough utilising such techniques will anyone be able to say whether or not the pest and disease status is going to be any worse than what it is now.
    Of course the safe position is always to be a pessimist, because surely at some point, even if things don't turn out as bad as predicted, it can always be claimed that they are still worse than what they otherwise could have been.
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  42. @Chris S
    Idso aim is to present these papers that deny the devastating impacts of increasing GHG in the atmosphere - so that these papers do not ignore - for the general conclusion. I have the same aim - including on this blog.

    Consider - as before changing the ratio of C3 plants to C4? Is as fast as today?
    Let's look at Figure 3C (C4 %) in Comparison of multiple proxy records of Holocene environments in Midwestern USA, Baker et al. 1998,. As you can see past the changes were often very large and fast, and long-term - where is the "famous" balance?

    @Marcus

    The current "mechanisms" of photosynthesis exist hundreds of millions of years. C4 grasses arose as a reaction to the unusual - in the history of life on Earth - a decrease in the concentration of CO2 - just a 3? million years ago ...

    Being (in my country) at an scientific conference on pests - warming - I heard that you get from us (as a result of global warming), The Western Corn Rootworm, Diabrotica virgifera ...
    I have a question - that the yields and profitability of maize production is higher in my country, or where there is a Western Corn Rootworm ?
    ... otherwise - whether the warming will not compensate the cost increase protection against pests - maize?

    I recommend the conclusion in Leakey 2009: “The effect of elevated [CO2] on C4 crops has received a disproportionate lack of attention compared to the effects of other elements of climate change on C3 and C4 plants. Consequently, adequate data are not available to reliably estimate the extent to which amelioration of drought stress at elevated [CO2] will improve yields over the range of C4 crop growing conditions and genotypes.”

    We should also remember that the increase in CO2 usually:
    - increases the number of leaves and flowers,
    - promotes the regeneration of plants propagated “in vitro”,
    - In some cases can reduce the costs associated with the light made ...
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  43. Humanity Rules, the one thing your endless comments prove about human ingenuity is the astounding ability of climate change deniers to miss the point, ignore the evidence, and generally dash off along a new path of avoidance whenever an inconvenient truth is encountered.
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  44. Johnd, if you had read the paper I cited you would see that you are wrong:

    Unfortunately, results from
    chamber-based experiments suggest that the CO2-induced
    reduction in protein may not easily be overcome by additional
    N supply since this may simply result in additional
    biomass and yield production
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  45. I'm afraid that HR's fantasy about making 500 different rubisco activases is just that, utter fantasy.

    The answers to the following questions will show this to be true:

    Firstly, HR, do you know the primary and secondary structure for, say, wheat rubisco activase? Do you know what part of the 3D structure is causing a low denaturing temperature? Do you know what amino acids need to be replaced and what the replacements will be?

    Secondly, can you give one positive example where this "protein engineering" has ever been shown to actually work? There were lots of people trying this on much simpler proteins 30 or so years ago.

    If you cannot positively answer these questions then you are in the "science fiction arena" and not real science.
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  46. HR @30

    My point @11 was more that world agriculture will suffer other bad consequences from Climate Change, such as increasing desertification, fires (e.g. Russia 2010) and flash floods (e.g. Pakistan 2010) which need to be balanced against any crop yield gain.
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  47. Dawei,

    Very excellent to see all this info on a very hot topic. It cuts through a lot of the oversimplifications. There are a lot of scientists trying to get some sense of what the real net impact will be on plant productivity at the ecosystem scale, given all the higher order impacts on plant interactions, herbivory rates and nutrient cycles. It's a tough nut to crack, which is why no real consensus has yet emerged beyond the obvious physiological responses to CO2.

    I also would like to second Sphaerica's request to put the effects in context with effects of water availability as a controlling factor. Terrestrial ecosystem productivity varies almost 2 orders of magnitude among regions of the earth because of variation in precipitation. We're talking increases of 50% or less in real world production related to CO2.
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  48. @ Stephen Baines and Sphaerica

    Your requests offer a uniquely difficult task to accomplish unfortunately.

    Most drought studies that involve determinations of photosynthetic responses tend to be unfavorable in a peer reviewed context. For plants there is either enough water or it is limiting. In water limiting conditions most plants close their stomata (completely or almost completely) to limit water loss. The unfortunate side effect of this is that gas exchange analysis of photosynthesis cannot be trusted as it depends on stomatal "openess" to accurately measure the exchange of CO2 and water vapor from the leaf mesophyll and the atmosphere.

    Long story short - it is very difficult (if not impossible) to tease apart the decrease in actual photosynthesis from instrument error in relation to reduced stomatal conductance in response to drought.

    More holistic measurements may be taken in drought studies of CO2 enriched plants (biomass, grain yield etc.) but these fail to truly capture the mechanism affecting differences in plant growth and resource allocation that gas exchange captures so well.

    Chlorophyll fluorometry offers promise in quantifying photosynthesis with out relying on gas exchange, but thus far the responses measured by fluorometers deal more with the efficiency and productivity of photosystem II than with the photosynthetic apparatus as a whole.
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  49. 48, david b,

    I was thinking of a more general approach... less in tying precipitation changes to specific biological mechanisms (like photosynthesis), and more along the lines of "it doesn't really matter how good increased CO2 might be for a plant if it dies due to lack of water."
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  50. @ Sphaerica - OK, I see what you're getting at.

    It's a delicate balance between water loss and CO2 aquisition. Nate McDowell et al did an excellent job reviewing drought related mortality in the context of climate change (but not CO2 increases). You'll notice that one of the major hypothesis of (long term) drought mortality is from carbon starvation due to stomatal closure. In this case it is a combination of water and CO2 limiting growth (ultimately resulting in mortality).

    On a shorter time span drought mortality (most often occurring in herbaceous, non-perennial plants) is due to all out interruptions of cellular processes, transport of nutrients and assimilates and overheating in the leaves leading to a denaturation of enzymes. In this case CO2 levels have zero impact on prolonging plant life.

    The opposite would be true as well, in well watered situations with limiting CO2, the plant will die (just not as rapidly).

    It would really require a case-by-case basis to estimate CO2 vs. water status threshold, but the balance and trade-offs between the two would, again, make it difficult to tease apart what the actual cause of mortality would be.

    Sorry for the +/- non-answer.
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