By 2050, What Will Be The Greatest Human Impact on Plants?
I first encountered the term “anthropocene” back in 2007 when I was writing a manuscript regarding some of our research in Yellowstone National Park (see Ref. 1 below).
A more detailed view regarding the nature and implications of the anthropocene is presented on YouTube here. Yet another YouTube video that’s a bit more technical can be viewed here. (And if you’re really pressed for time, then check out this 2-min YouTube video.)
If you’ve watched any of these videos, then you probably have a pretty good basic understanding of the anthropocene. If you didn’t, then the anthropocene can be briefly defined as the current geologic age in which “…human impacts such as land use and industrial pollution have grown to become significant geological forces, frequently overwhelming natural processes.” (from Ref. 1 below)
There are several major impacts on plants as result of the anthropocene.
OK, but let’s get back to the question I’m posing here: Which of the above will probably have the greatest overall impact on plants within the next 35 years?
I’m Placing My Bet On CO2
First off, let’s acknowledge that atmospheric CO2 levels will likely continue to increase within the next 35 years, and probably well beyond that, primarily from human activities (please see here, for example). At the current rate of increase (please see here for the data), atmospheric CO2 will likely reach 500 to 600 ppmv by 2050. (It was about 310 ppmv in 1950 and is currently at about 400 ppmv.)
Although this may not seem like a big increase, it will likely have profound effects on plants, for several reasons.
As mentioned in the previous post, most green plants will likely benefit photosynthetically from this increase in CO2.
Mainly because the enzyme in plants that captures CO2 from the atmosphere for photosynthesis is currently working at a sub-optimal rate. This is because the current level of atmospheric CO2 is so low (relative to when green plants colonized the land, for example) that it may actually limit the activity of this key enzyme, known as RuBisCo.
Let me put it this way: Imagine that RuBisCo is like a high-performance car that can go 100 mph when you give it high-octane gasoline (petrol). But now, only low-octane gas is available, so the car can only go 50 mph max.
By steadily increasing the CO2 available to green plants, we are enabling RuBisCo to “fix” CO2 into sugars at a faster and faster rate, resulting in improved photosynthetic productivity of the plants.
This is great, right? More productive plants means higher crop yields, bigger vegetables, more fruit….without having to add any more fertilizer or water.
But wait. This isn’t the whole story…
Other Effects of Elevated CO2 on Plants
Increasing the rate of photosynthesis by accelerating RuBisCo is not the only effect that elevated levels of CO2 have on most green plants.
And some of these other effects may actually negatively impact crop plants.
Elevated CO2 levels have at least two effects on leaf stomata:
1. High CO2 tends to close stomata.
2. High CO2 may lead to fewer stomata per leaf during plant development.
Perhaps above all else, land plants try to minimize water loss via transpiration, which occurs mainly through the stomata. Thus, it makes sense that, if there is a relative abundance of CO2 for doing photosynthesis, plants would tend to close stomata, make fewer of them, or both.
But isn’t this another good thing? On the one hand, yes. But on the other hand, keep in mind that leaves cool themselves via leaf transpiration through stomata. So less transpiration may mean that the plants become more susceptible to heat stress, which many believe is on the increase do to global warming. And plant heat stress typically inhibits photosynthesis. (For example, please see Ref. 4 below.)
A recently published report (see Ref. 5 below) provides evidence that high-CO2 crop plants may have less protein, zinc and iron (see news report about this paper here).
Scientists are uncertain why elevated levels of CO2 cause a decrease in zinc and iron in plants, though variation among different species indicates that it’s likely a complex mechanism. “Of all the elements, changes in nitrogen content at elevated [CO2] have been the most studied, and inhibition of photorespiration and malate production, carbohydrate dilution, slower uptake of nitrogen in roots and decreased transpiration-driven mass flow of nitrogen may all be significant.” (from Ref. 5 below)
In a previous post we explored the origin of C4 plants in a past, relatively low-CO2 world and their fate in a future high-CO2 world. Though C4 plants likely arose as a result of decreased levels of atmospheric CO2, their fate is very uncertain in the face of the increasing levels of CO2 that will likely occur in the centuries to come. (If some C4 plant species are displaced by C3 species, primarily as a result of elevated levels of atmospheric CO2, then this may contribute to a loss in plant biodiversity.)
Though it seems reasonable that many C4 plant species might lose their CO2-concentrating edge over C3 plants in relatively higher CO2 environments and be displaced, there is little evidence for this to date. Admittedly, this hypothesis is very difficult test, and, indeed, there is evidence to the contrary (e.g., see Ref. 6 below).
Bottom Line: In 2011, I posted on how increased CO2 will affect plants. It was clear then that, because plants’ responses to elevated levels of CO2 were so significant and complex, it was difficult to make reliable predictions. Three years later, though we know more about how some crop plants will likely respond to a high-CO2 world, it’s still hard to make general predictions with high confidence. What I think is clear, however, is that, considering all the major impacts of the anthropocene, increased atmospheric CO2 will probably have the greatest overall effect on plants.
“Prediction is very difficult, especially if it’s about the future.” – Nils Bohr, Nobel laureate in Physics
“I never think of the future, it comes soon enough.” – Albert Einstein
1. Tercek, M. T., T. S. Al-Niemi and R. G. Stout (2008) “Plants exposed to high levels of carbon dioxide in Yellowstone National Park: A glimpse into the future?” Yellowstone Science, Vol. 16, pp. 12-19. (PDF)
2. Terashima,I., S. Yanagisawa and H. Sakakibara (2014) “Plant responses to CO2: Background and Perspectives.” Plant & Cell Physiology, Vol. 55, pp. 237-240. (Full Text)
3. Leakey, A. D. B., et al. (2009) “Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE.” Journal of Experimental Botany, Vol. 60, pp. 2859-2876. (Full Text)
4. Ruiz-Vera, U. M., et al. (2013) “Global warming can negate the expected CO2 stimulation in photosynthesis and productivity for soybean grown in the midwestern United States.” Plant Physiology, Vol. 162, pp. 410-423 (Full Text)
5. Myers, S. S., et al. (2014) “Increasing CO2 threatens human nutrition.” Nature, Vol. 510, pp. 139-142. (Abstract)
6. Morgan, J. A., et al. (2011) “C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland.” Nature, Vol. 476, pp. 202–205. (Abstract)
Thanks to Prof. Lisa Ainsworth (USDA/ARS & University of Illinois) for generous input regarding the potential displacement of C4 plants by C3 plants in a high-CO2 world.
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