(This exercise is based on Ehleringer, J. R., Cerling, T. E., and Helliker, B. R. 1997. C4 photosynthesis, atmospheric CO2, and climate. Oecologia 112: 285–299.)
As you learned in Chapter 5, high rates of photosynthesis that occur at high temperatures can result in increased O2 concentrations relative to CO2. Under these conditions, typical plants with C3 metabolism experience high rates of photorespiration. Photorespiration is costly and results in the loss of energy that could otherwise be used for plant maintenance or growth. Apparently this has been a strong selective force on plant metabolism, because an alternative pathway, C4 metabolism, that avoids photorespiration, has evolved multiple times. Most C4 plants are grasses (monocots) and they include some of the most important food crops such as corn (Figure 1) and sugarcane.
Although many factors interact to determine which species of plants coexist in an ecological community, photosynthetic efficiency at various temperatures can be an important large-scale predictor of plant distributions. In the paper by Ehleringer et al., (1997), they suggest that on a large scale, temperature can predict the interaction and mix of C3 and C4 plants. In addition, atmospheric concentration of CO2 can influence the likelihood of photorespiration and thus the relative efficiency of C3 and C4 plants. If concentrations of CO2 are high, photorespiration by C3 plants will be negligible even at high temperatures. Ehleringer et al. (1997) provided the following panel of plots (Figure 2) to describe the relationship of C3 and C4 plants to temperature and CO2 concentrations.
Notice that the measure of photosynthesis rate is quantum yield (y-axis above), which can be considered as a surrogate for plant growth. C3 plants show a strong decline in quantum yield with increasing temperature (left and middle panel above); whereas C4 plants show no change in quantum yield with temperature (right panel above). This difference sets up a trade-off between the two types of metabolism at various temperatures. We will use these relationships, along with a simulation of temperature at various latitudes in the plains region (grassland biome) of the central United States and Canada, to determine what plant communities should look like at high or low latitudes. You will simulate the mix of C3 and C4 plants that should exist at various latitudes for different CO2 levels. You will consider how the distribution of C3 and C4 plants should change from preindustrial CO2 levels (180 ppmV) to potential future CO2 levels (500 ppmV). We are simulating a mixed grass-forb prairie (i.e., includes both monocots and dicots).
|Latitude = ° N||
Preindustrial CO2 (180 ppmV)
Current CO2 (350 ppmV)
Future CO2 (500 ppmV)
Notice in the simulation window you can choose the CO2 level corresponding to three time periods (preindustrial to future). You can also choose the latitude at which to make the comparison (values can range from 25° to 60° North latitude). C3 plants are shown in green and C4 plants are in brown. Bars show quantum yield (our measure of growth) of each type of plant. We assume that plants that have lower relative growth will not coexist over long periods of time with plants that have higher relative growth. If one type of plant shows substantially higher growth rates over the year, then we would predict a grassland dominated by that plant type. If growth rates are relatively even over the course of the year, then we predict coexistence and a resulting mixed grassland. For our purposes, a difference of less than 10% (of the smaller value) is predicted to result in a mixed grassland.
Start at preindustrial CO2 levels and run the first simulation at 30°N. Note how photosynthesis changes over the course of the year, and the annual quantum yield. Now rerun the simulation for 40°, 50°, and 60° North. Plot annual yield by latitude for both photosynthetic types. Which photosynthetic type will dominate? At what range of latitudes do you predict C4 dominance, mixed prairie (both C3 and C4), C3 dominance?
Now change the CO2 level to current (350 ppmV) and run the same range of latitudes noting the relative growth rate of both photosynthetic types at each latitude. Plot annual yield by latitude for both photosynthetic types. Which photosynthetic type will dominate? At what range of latitudes do you predict C4 dominance, mixed prairie (both C3 and C4), and C3 dominance?
Run the same series of simulations for a future CO2 level (500 ppmV). Run the same range of latitudes noting the relative growth rate of both photosynthetic types at each latitude. Plot annual yield by latitude for both photosynthetic types. Which photosynthetic type will dominate? At what range of latitudes do you predict C4 dominance, mixed prairie (both C3 and C4), and C3 dominance?
Considering all three of your graphs, how is the mix of C3 and C4 plants likely to change with future changes in CO2 levels? Explain why. Note that we are only considering changes in CO2 and not corresponding temperature change. How might higher temperatures affect the relative growth of these two photosynthetic types?