Lawn grasses in the U.S., as it happens, can include both C3 and C4 plants. Wherever C3 and C4 plants grow together, it’s possible to tell from the chemistry of the soil—which is made up, of course, of decayed plants—what their ratio was in past growing seasons. Using this, scientists have been exploring how temperature and CO2 can affect the competition between the two types. In the new study, the researchers, led by Erik Hobbie, an ecologist at University of New Hampshire, looked to see whether the ratio of carbon isotopes in the lawn mushrooms, picked up from dead grasses, could be explained by climate differences.

First, they examined the isotope levels in the samples, which were gathered between 1982 and 2009. “I thought we’d see small shifts in the carbon isotope values,” says Hobbie. “But we saw these quite dramatic shifts that can only be explained by shifts in the competition between C4 and C3 grasses.” Then, they ran regressions to see how much of the shift could be explained by factors like temperature, rainfall, and CO2. They found that despite the rise in temperatures across that period, the relative contribution of carbon from C3 grasses increased. That was in apparent response to the rise in carbon dioxide concentrations, which C3 grasses are better at handling.

It’s intriguing that environmental differences could show up in fungi, because it suggests that researchers interested in the climate change of the very recent past could look to mushroom samples in collections to get a glimpse at changes that have only just begun. “I think it’s an interesting new proxy,” says Hobbie. “To me it points to the potential for archived specimens of fungi as integrators of past environmental conditions.”

Still, Hobbie wonders whether it’s possible to be more precise about exactly how old the carbon is that these mushrooms are eating, and thus a little more certain about their connection to the grasses and environmental change. “Is it this year’s carbon? Or last year’s? Or the last couple years’? We haven’t done those kinds of measurements yet,” he says.

Hobbie notes that people can be carbon-dated, thanks to radiocarbon isotopes in tooth enamel and other human tissues put together after the thermonuclear tests of the mid-20th century. The same should be true of mushroom tissue, with the level of radiocarbon serving as an indicator of the year in which the carbon was assimilated. “That would be the way I would want to look,” he says.

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