Since the Industrial Revolution, concentrations of carbon dioxide in the atmosphere have rapidly increased. How does this influx of atmospheric carbon affect ecosystems, such as forests, croplands, and the 40 million acres of American lawns? Clues to answer this may lie in an unexpected source: mushrooms.
Trees and grasses pull carbon out of the atmosphere during photosynthesis and thus play a key role in the global carbon cycle. Theoretically, researchers can study how vegetation changes over time to assess the effects of increasing concentrations of carbon dioxide. Unfortunately, studying historical changes in grass communities is difficult. Unlike trees, which build tree rings from year to year, grasses leave little behind when they die and decompose, so scientists must use creative methods to look at grassland ecosystems from years past.
One method involves using the two stable isotopes of carbon, 13C and 12C, as natural tracers. But where can record of these isotopes be found?
Perhaps in mushrooms, Hobbie et al. hypothesized. The authors tracked the 13C to 12C ratios in mushrooms from lawns in America’s Midwest to study the historical shift in grass varieties in the region. The fungi feed on dead plant matter, so changes in carbon isotopes within mushrooms from samples collected over time can allow researchers to look at what kinds of grasses the fungi had consumed from season to season and year to year.
To see how changes in temperature, precipitation, and carbon dioxide in the atmosphere can affect vegetation, the researchers looked at competition between two kinds of plants: C3 and C4 grasses, which use different metabolic pathways for photosynthesis. These different pathways produce different 13C:12C ratios in plant tissues.
C3 grasses—such as wheat, oats, and ryegrass—are called cool-season plants and thrive in a temperature range of 65°F–75°F. These grasses are highly productive in the spring and fall, but high summer temperatures reduce growth. C4 plants, on the other hand, flourish in warmer and drier environments. These warm-season plants include corn, crabgrass, and bluestem grasses and are more efficient than C3 plants at photosynthesis under low concentrations of carbon dioxide.
The researchers used isotopic data from samples of the fungus Amanita thiersii collected between 1982 and 2009 from 26 locations in the southeastern and south central United States. The scientists combined these data with information on temperature, precipitation, and carbon dioxide concentrations over the same period to study changes in the balance between C3 and C4 plants.
They found that high temperatures were good predictors of a higher percentage of C4 grasses, whereas higher precipitation favored C3 productivity. Over the 1982–2009 period, C3 grass productivity increased 18.5%, which the scientists attributed to a 13% increase in atmospheric carbon dioxide during that time.
The researchers point out that shifting lawn management also could have played a role in the changing grass landscape. Despite this, the novel method of using mushrooms to study the vegetation landscape and plant competition over time could be used in the future to assess how grasslands are adapting to climate change and to increasing carbon dioxide concentrations. (Journal of Geophysical Research: Biogeosciences, http://doi.org/10.1002/2016JG003579, 2017)
—Alexandra Branscombe, Freelance Writer