André L.C. Franco, Laureano A. Gherardi, Cecilia Milano de Tomasel, Walter S. Andriuzzi, Katharine E. Ankrom, Elizabeth M. Bach, Pingting Guan, Osvaldo E. Sala, and Diana H. Wall
Climate change poses a threat to grasslands due to, among other factors, the increased occurrence of extreme rainfall events. More frequent extreme droughts under climate change can increase the abundance of root-feeding soil nematodes (roundworms) in grasslands by suppressing their nematode predators. These root feeders are important drivers of plant biomass production and above-belowground allocation patterns in grasslands, and it is important to understand how their responses may impact the way grasses are affected by drought. Some evidence shows that, under drought conditions, grasses can allocate less resources to leaves, which lose water, and more to produce roots, which capture water. But does that hold true when drought causes an increase in the population of root feeders?
In order to find out, we set up an experiment in which we exposed a dominant shortgrass steppe species, blue grama (Bouteloua gracilis), to extreme low and high amounts of water under a variable amount of root feeders based on the total nematode community, which besides root feeders also includes bacterivores, fungivores, predators, and omnivores. We tested whether high levels of root herbivory by nematodes would prevent the high grass biomass allocation to roots normally expected in response to drought.
The results showed that the effects of drought on grass biomass allocation depend on the abundance of root-feeding nematodes. High abundance of those nematodes impeded grass responses that otherwise allocated relatively more biomass to roots than leaves under drought conditions to benefit water uptake. This suggests that when the root-feeding nematodes thrive, they undermine an important plant mechanism that buffers grasses and ecosystems against the effects of drought.