TThe underground networks of tree roots in forests hold more secrets than we think: scientists have now learned that roots determine the time of their growth according to who are their neighborsand the winners in these competitive hierarchies are not always those we expect.
The discovery could have implications for how well the mixed forests that conservationists are planting today will survive in a changing climate.
Most of what we know about the seasonal cycles of tree root growth comes from studying stands of individual tree species. But in nature, forests are usually mixed. Qiwen Guo, a graduate student at the Institute of Forest Ecology at the University of Natural Resources and Life Sciences Vienna, wanted to know how a tree's neighbors influence the dark underground world of its root systems.
Read more: “Never underestimate the intelligence of trees»
To find out, she studied trees in an Austrian experimental forest, where the trees are equipped with minirhizotrons, a non-destructive imaging and root analysis technology that allows one to peer into the underground world through transparent tubes. The experimental forest features four common European tree species in various combinations: oak, hornbeam, linden and maple. Her biggest question was: Do roots adjust their growth calendar based on what trees surround them?
It turns out yes. Guo found that in mixed forests, one species is dominant and sets the rate of root growth for the others, even if that dominant species is not always the most productive tree in the group when grown in a monoculture. “We found that mixing rewrites the underground calendar: Trees change the timing of root growth in mixtures, often coinciding with dominant neighbors rather than adhering to individual schedules,” she writes in a recent behind-the-scenes report. essay about work that was published in the journal Functional ecology. She also found that root growth depended on specific combinations of species, rather than just diversity.
The forest came alive as the scientists worked, and the ambitions of the locals often differed from those of the researchers: “The curious animals stealing our sensors surprised and amused us in equal measure,” she adds. But the reward for their patience was worth it: “Our biggest surprise was the magnitude of the time shift: in monocultures, peak root growth followed full leaf expansion two to three weeks later, but in mixtures this gap widened to about five weeks!”
Roots begin to grow before tree branches sprout new seasonal leaves, but when trees are surrounded by other species, the time lag between peak root growth and full leaf development increases, she notes. This is because some trees wait to suck resources from the soil until there is less competition from other species with conflicting growth calendars. Overall, the results suggest that the dynamics of latent competition may differ significantly from expectations based on monoculture results.
To collect the data, Guo and her colleagues frequently photographed tree roots, creating nearly 7,000 images, and spent up to an hour per image tracking individual roots. It was tedious work: “In mixtures, determining which species each root belonged to was difficult,” she writes. They also measured soil temperatures every two hours and above ground monitored leaf development using fish-eye images.
The findings raise many additional questions for future research: Does mixing affect overall biomass, the ability to collect nutrients from the soil, and the types of fungal colonies that live in the forest? “And most importantly,” she writes, “how will climate-induced shifts in seasonal temperatures disrupt the temperature signals that have regulated these phenological patterns for millennia?”
It's more than just academic issues, she says. Solving these mysteries is essential to predicting how the Earth's forests and roots will fare in a changing world.
Enjoying Nautilus? Subscribe to our free newsletter.
Main image: Chokniti-Studio / Shutterstock
