Hidden tipping point revealed

Adding a new dimension to the climate crisis, U of M researchers have found that it matters what types of fungi decay dead trees.

It matters because if one common type spreads northward, it means significantly greater releases of carbon dioxide into the atmosphere. Unfortunately, according to scientists’ projections, climate change raises the odds that will happen.

This scenario springs from work led by College of Biological Sciences professor Jonathan Schilling and graduate student Hunter Simpson. Using data from some 12.5 million fungal samples and 30 million tree records from around the world, they examined the ecology of two groups of fungi—white rot and brown rot—named for the color of wood infected by them.

The researchers found that brown rot fungi tended to occur along with conifers such as pine trees, while their white rot cousins tended to be found with deciduous trees.

Conifers dominate in northern areas, but as the climate warms, deciduous trees—and white rot fungi—will likely move northward and displace them.

This is worrisome because the two types of fungi differ in how they digest the molecules that give wood its stiffness. One such molecule is lignin, which accounts for about 40 percent of the above-ground carbon stored in wood.

Both types of fungi release carbon dioxide as a by-product of digesting tree wood. However, brown rot fungi leave most of the lignin, and the carbon contained in it, behind. White rot fungi, however, aren’t so picky. They also digest the lignin and release its carbon as CO2. Thus, as white rot fungi move north and start digesting lignin, northern forests will see a rise in CO2 emissions.

Currently, the ratio of white rot to brown rot fungal occurrence is about 87% to 13%, respectively. If that ratio shifts even a little toward white rot, the consequences would be devastating, Schilling says.

“If my calculations are even close to correct, a loss of two percent of brown rot relative to white rot would release the same amount of carbon dioxide into the atmosphere that we release from burning fossil fuels,” he says.

“If this is really true, then someone needs to stop this madness.”