The thrumming subterranean networks of tree roots in forests have been harboring more secrets than we realized: Scientists have now learned that roots time their growth according to who their neighbors are, and the winners of these competitive hierarchies are not always the ones we might expect.
The finding could have implications for how well mixed forests conservationists are planting today will survive a changing climate.
Most of what we know about seasonal cycles of tree root growth comes from studying stands of single species of trees. But in nature, forests are typically mixed. Qiwen Guo, a doctoral student in 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 underworld 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 fitted with minirhizotrons, a non-destructive root imaging and analysis technology that offers a glimpse into the underworld via transparent tubes. The experimental forest contains four common European tree species in different mixes: oak, hornbeam, linden, and maple. Her biggest question was: Do roots adjust their calendar for growing depending on which trees surrounded them?
It turns out they do. Guo found that in mixed forests, one species is dominant and it sets the pace of root growth for the rest, even if that dominant species is not always the most productive tree of the bunch when grown in monoculture. “We discovered that mixing rewrites the underground calendar: Trees shift their root growth timing in mixtures, often matching dominant neighbours rather than keeping individual schedules,” she writes in a recent behind-the-scenes essay about the work, which was published in the journal Functional Ecology. She also found that root growth depended on specific combinations of species, not merely diversity.
The forest came alive as the scientists worked, and the local inhabitants often had different ambitions than the researchers: “Curious animals stealing our sensors left us equal parts astonished and amused,” she adds. But the payoff for their patience was worth it: “Our biggest surprise was the magnitude of the timing shift: In monocultures, peak root growth followed full leaf expansion by two to three weeks, but in mixtures this gap stretched to about five weeks!”
Roots start growing before new seasonal leaves sprout from a tree’s branches, but when trees are surrounded by other species, the time gap between peak root growth and full leaf development is longer, she notes. That’s because some trees wait to suck up resources from the soil until there is less competition from other species with conflicting growth calendars. In sum, the findings suggest that belowground competitive dynamics may differ substantially from expectations based on monoculture performance.
To collect their data, Guo and her colleagues photographed the tree roots frequently, generating almost 7,000 images, and spent up to an hour per image tracing the individual roots. It was tedious work: “In mixtures, identifying which species each root belonged to was tricky,” she writes. They also measured the temperature of the soil every two hours and, above ground, tracked leaf development with fisheye images.
The findings inspired a slew of additional questions for future research: Does mixing influence total biomass, the ability to gather nutrients from the soil, the kinds of fungal colonies that make their home in the forest? “And critically,” she writes, “how will climate-driven shifts in seasonal temperatures disrupt the thermal cues that have regulated these phenological patterns for millennia?”
All more than academic questions, she says. Solving these mysteries is essential for predicting how Earth’s forests and roots will fare in a changing world. 
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