CO2 and tree growth

Anyone who has ever engaged a hardcore denier in conversation has come up against the “CO2 is plant food” canard. The fervent denier will then tell you about this or that glasshouse study that demonstrated that CO2 increased plant growth. Of course, that’s all fine and good…in the glasshouse, but these kinds of studies are useless for conveying what happens in the real world. In the glasshouse, variables can be tightly controlled so that the only thing being manipulated is CO2. These plants are not subject to other variables like

  • increased temperature
  • altered rainfall
  • changes in water table
  • increased nitrification
  • increased or changes in pest and disease incidence, especially fungal pathogens
  • variable ecosystem responses (e.g. changes in species composition)
  • nutritional limiting factors

The clever denier when confronted with this information will resort to highlighting a few studies in cereal crops that purport to show increased biomass and yield, however, these are few and far between. More and more as these studies increase in duration, negative effects begin to appear due to some of the factors I mentioned above and cast doubt on the “CO2 is plant food” meme. For example see here, here, here. Of course, the denier can tell you that many of these things can be overcome by increasing irrigation, fertiliser and pesticides which of course that adds to the environmental and monetary costs of production. In many cases, this is going to be inevitable anyway.

What the denier can’t explain away though is the effect on natural ecosystems. There’s no farmer to go around spraying chemicals, applying water or fertilisers. There’s certainly no way to prevent range shifts. So, are natural ecosystems starting to feel the effect of human induced climate change? The authors of a new study published in Global Ecology and Biogeography certainly think so.

tropical forest trees – most at risk from increased CO2 and climate change

Analysing dendrochronological and isotopic records of trees from all over the globe, Silva and Madhur evaluate the impacts of atmospheric changes on tree growth and intrinsic water use efficiency. They have summarised their key findings as follows.

” In 37 recently published case studies changes in iWUE were consistently positive, increasing by between 10 and 60%, but shifts in growth varied widely within and among forest biomes. Positive RC values were observed in high latitudes (> 40°N), while progressively lower (always negative) responses were observed toward lower latitudes. Growth rates declined between 15 and 55% in tropical forests. In subtropical sites growth declined by between 7 and 10%, while mixed responses occurred in other regions.”

They conclude, “Over the past 50 years, tree growth decline has prevailed despite increasing atmospheric CO2. The impact of atmospheric changes on forest productivity is latitude dependent (R2 = 0.9, P < 0.05), but our results suggest that, globally, CO2 stimulation of mature trees will not counteract emissions. In most surveyed case studies warming-induced stress was evoked to explain growth decline, but other factors, such as nutrient limitation, could have overridden the potential benefits of rising CO2 levels.”

This study only looked at two aspects of climate change effects being growth and water use efficiency. The effects of changing phenology of pests could be potentially worse as demonstrated by the effect of mountain pine beetles in North America and altered rainfall and water table issues as seen in Western Australia. Given that these issues are manifesting after an average 0.8 degrees temperature increase since the 1950’s, what the hell is it going to look like at 2.0 degrees?


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17 responses to “CO2 and tree growth

  1. Many talk of increased biomass and show side-by-side pictures of wheat grown at different CO2 levels (higher co2 = taller, of course)

    However, you do not eat biomass you eat grain and you require protein.
    There is less evidence that yeild increases or total protein increases. There is one study I found (Indian I think) showing a reduction in protein with co2. (will try to find this when at home!)

    Increase in biomass makes it better as a fuel crop!!! but not nutritionally.

  2. I must confess I remain baffled as to how and why – no matter how many times the truth is explained to them – some people repeat lies like this.

    The planet Mars is further from the Sun and smaller than the Earth. It’s molten core cooled faster and its volcanic activity ceased and then it lost its atmosphere. There is (it seems) no intelligent life on Mars.

    The planet Venus is closer to the Sun and smaller than the Earth. It’s volcanic activity did not stop and the degassing of its core triggered a runaway greenhouse effect that has left it with surface temperature and pressure 90 times that here. There is (it seems) no intelligent life on Venus.

    The planet Earth is thankfully where it is, its volcanic activity is moderate and it is big enough to retain its atmosphere; containing enough greenhouse gases to keep the temperature above freezing most of the time. Unfortunately, despite realising over 100 years ago that artificially doubling the CO2 content of the atmosphere would raise average temperatures at least 2 Celsius, humans are not doing anything to stop this happening. There is (it seems) no intelligent life on Earth.

    • Bob

      Where did you get the 2 degrees Martin. I couldn’t find the reference.

      • The amount of warming expected for a doubling of CO2 in the atmosphere is climate sensitivity. I did not provide a reference because this is such a well-documented subject. However, I think it is fair to say that most relevant scientists think climate sensitivity is in excess of 2.5 Celsius. About the only barely-credible researchers that dispute this are Richard Lindzen, Pat Michaels and Roy Spencer – all of whom are partly-funded by right-wing think tanks because they say things that such ideologically prejudiced people want to hear.

      • Bob

        You are an intellectual midget. I don’t know why I bothered to ask. Btw, look up the word naive, you might see your picture.

      • Tell me what you really think, Bob; why don’t you?

        However, is there any chance of having a logical argument;, or some explanation for that outburst? Have you got any facts or references of your own to falsify my assertion that most relevantly qualified scientists think 2 Celsius is a minimum value? If you know better, why did you ask the question?

        How did you get hold of my IQ result from the UK government; they won’t even tell me what it is? Also just so that I can evaluate the importance of your opinions (of me and climate science) can you tell me what psychological and/or scientific qualifications you have?

  3. I love that song. Now I won’t be able to get it out of my head.

  4. Bob

    “I chose “silver bullet” because some idiot I was talking to a few weeks ago referred to “the giant bloodsucking vampire that is the global warming movement” or some such thing. He was nuts, but I hadn’t made the coffin
    nail/cancer stick connection. Duh to me. I’ll use that from now on.” You’re a scientist, eh? No, you’re an warmicon ecologist. Don’t confuse the 2.

  5. Bob

    “John, you need to adopt the attitude that if WUWT or any denier blog for that matter says they have a peer reviewed paper supporting their lunacy, it means they have something that either isn’t peer reviewed, is peer
    reviewed but being misinterpreted, or is peer reviewed and being misrepresented.” Atatboy “scientist!!

  6. Hi,
    Still cannot find the Indian article, but links to a few papers are here – by all means copy any you like!!!