In an electron micrograph, mycorrhizal fungi (bright gray) penetrate root cells, where the fungi supply water and nutrients in exchange for carbon.
Eye of Science / Science Source
By Elizabeth Pennisi, Warren Cornwall
The future of the world’s flora may depend as much, if not more, on what is under the ground as what is above. Under 90% of all plants lie an invisible support system – underground fungal partners that form a network of filaments that connect plants and bring nutrients and water to their roots. In return, the plants supply a steady supply of carbon to the fungi. Now researchers are learning that these hidden partners can shape how ecosystems respond to climate change.
The right mold partners can help plants survive warmer and drier conditions, according to a study reported earlier this month at the Ecological Society of America’s online annual meeting. But other studies at the meeting show that climate change could also disrupt these so-called mycorrhizal fungi, possibly accelerating the decline of their host plants. “The picture becomes clearer that we can really not ignore the responses of mycorrhizal fungi to climate change,” says Matthias Rillig, an ecologist at the Free University of Berlin.
These fungal associates come in two forms. Arbuscular mycorrhizae (AM), common in tropical and some temperate forests, such as fields and meadows, invade root cells and extend thin hairs called hyphae in the soil. Ectomycorrhizal (EM) fungi, in contrast, are associated with conifers as well as hickory, alder and beech. They attach themselves to the outside of the roots, and their networks of hyphae give rise to the fungi that emerge on moist forest floors.
Both species absorb phosphorus and other nutrients, capture nitrogen from decomposing organic matter, and help store carbon in the soil. “Mycorrhizal compounds are probably the most important symbiosis in terrestrial ecosystems because of their importance to plant productivity,” said Christopher Fernandez, a soil economist at the University of Minnesota, Twin Cities.
Climate change could change these associations, he says. Fernandez is part of the B4WARMED (Boreal Forest Warming at an Ecotone in Danger) project, a large-scale effort to control the effects of warming and drought on boreal forests that extend across northern latitudes. The study is also artificially heating and drying plots of forest, and Fernandez studied the impact of simulated climate change on the hidden fungi by sequencing soil and root samples of plots.
As conditions got warmer and drier, the variety of EM fungi declined, and “weedy” EM fungi took over, Fernandez reported at the meeting. These “weeds” do not spend much energy on building extensive underground networks, breaking their connection. If the same disturbance occurs as climate change develops, fewer seedlings can establish their critical cooperative relationships with fungi, which can deprive the trees of nutrients.
Monitoring carried out by B4WARMED has already shown that the warmer, drier climate of recent years is taking a toll on the boreal forest. What role each change in EM fungi plays is not yet clear, but “the change in mycorrhizal fungal communities in response to climate change is deeply concerning,” Fernandez says.
The results of B4WARMED “show that in the future there may be some major shifts in both above and below ground communities,” says Sarah Sapsford, an ecologist at the University of Canterbury. “What we see now, we may not see again.”
Another ecosystem, the Pinyon pine forest of the American Southwest corner, shows that existing variations in mycorrhizae also affect the resilience of trees. Decades ago, a counter-stunt stunted some of the pinyons (Pinus edulis) grows at Arizona’s Sunset Crater Volcano National Monument. The hard-hit trees have different EM fungi from their longer neighbors, suggesting that the two types of trees may be genetically different.
When a mega-drought hit the region in 2002 and 2003, twice as many of the taller trees died. To see if the mycorrhizae made a difference, Catherine Gehring, an ecologist at the University of Northern Arizona, Flagstaff, and her colleagues grew seedlings of the two species of plants with and without their fungi in a greenhouse, under different water regimes.
“We found that ectomycorrhizal fungi play a critical role in drought tolerance,” Gehring reported at the meeting. Sanna Sevanto, a biophysicist from the Los Alamos National Laboratory, saw the fungi in action by dipping the roots of seedlings in heavy water, which served as a tracer. Water moved much faster into the drought-tolerant roots infected with their mold than when they were sterile, Gehring reported.
She and her colleagues are working with local researchers to replant pines in the Navajo Nation, not far from Sunset Crater, and they are adapting what they learned. Because genetic differences between the trees seem to determine which of the two groups of EM fungi colonizes them, the team will be careful to plant seedlings with the right genotype to attract the drought-resistant fungi. That “could mean the difference between life and death in drought,” says Gehring.
A third study suggests mycorrhizae could shape how not only trees but entire ecosystems respond to environmental change. Because AM and EM fungi are associated with different tree species, Colin Averill, an ecologist at ETH Zurich, asked herself if the fungi themselves help determine which forest grows in a particular region.
Earlier investigators suspected they were doing so, but Averill and his colleagues were looking for evidence in a large set of U.S. Forest Service data that tracked the species, growth and death of any tree larger than a sapling in 6965. forest plots across the eastern United States. They found that many plots are dominated by AM-related trees rather than by EM-related trees; a mix of the two is rarer.
A statistical analysis found that fungi were the key to this strong cleavage, and the researchers found a probable reason: The dominant mycorrhizae can help lock a forest in a stable state. Tree measurements taken at 5-year intervals on each plot showed that an AM tree was at least 10 times more likely to take root in an AM forest than in an EM forest and twice as likely to survive. Meanwhile, EM trees were more likely to bloom in EM forests.
The fungi can enforce this monopoly by altering the soil in ways that prevent specific species – for example by controlling nitrogen levels. A more established fungal network can also help young trees tolerate heavy shade that interferes with photosynthesis, as older trees are resistant to drought or disease. “If you are an EM tree, you can plug in an EM network, and that can help you survive,” says Averill.
The grip of these fungi can slow down the response of forests to external pressures such as climate change. Trees attached to AM, for example, do better at hot temperatures, but they may be slower than expected to colonize an EM-dominated forest as the climate warms, Averill says. “These kinds of dynamics can become really important as we try to predict how the global forestry system will change in the future.”
