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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