Though noise may change by the moment for humans, its effect on trees and other plants lasts longer, according to a recent Cal Poly study.

The study, the first of its kind to explore the long-term effects of noise on fauna, was published April 14 in the journal Proceedings of the Royal Society B and reveals that human noise pollution affects an ecosystem’s plant life diversity even after the din has stopped.

In a study conducted 12 years ago near natural gas wells in a New Mexico woodland dominated by piñon pine and juniper trees, researchers found that there were 75% fewer piñon pine seedlings in noisy sites as in quiet ones. Researchers recently returned to the sites to find out whether the piñon pine had recovered over time.

The research team found that, in areas where natural gas companies used loud compressors as part of the extraction process, there were fewer seeds and saplings compared to sites where compressors weren't used. The decrease in saplings results from the time when the site was noisy, but the decrease in seedlings revealed that piñon pine seeds still weren’t sprouting despite stopping the noise.

“The effects of human noise pollution are growing into the structure of these woodland communities,” said Clint Francis, a Cal Poly biology professor and senior author of the study. “What we’re seeing is that removal of the noise doesn't necessarily immediately result in a recovery of ecological function.”

While it’s possible that the piñon pine has decreased because of a lack of opportunities to produce, it’s more likely that the Woodhouse’s scrub jay hasn’t returned to the formerly noisy area and so isn’t planting seeds. Piñons depend on scrub jays to carry their pine seeds away from the parent tree, and scrub jays are known to avoid noise.

“Some animals, like scrub jays, have episodic memory,” said Jennifer Phillips, the lead author, who worked on the project while a post-doctoral researcher at Cal Poly and who now is a professor at Texas A&M at San Antonio. “Animals like the scrub jay that are sensitive to noise learn to avoid particular areas. It may take time for animals to rediscover these previously noisy areas, and we don’t know how long that might take.”

Researchers also found differences in juniper seedlings and communities of flowering plants at the site depending on current noise levels and whether the levels had recently changed because compressors were moved. Sites with greater noise had fewer juniper seedlings and different types of plants than quiet locations.

Because of the complexity of ecosystems, the cause of these changes is still unknown.

“Our results reveal that plant communities change in lots of ways with noise exposure,” Francis said. “We have a decent understanding of how and why foundational trees like piñon pine are affected by noise from our previous work with jays, but we are also seeing large changes in plant communities through changes in the abundance of shrubs and annual plants.

“These changes likely reflect impacts of noise on animals that eat plants — deer, elk and various insects — plus the many pollinators that are important for plant reproduction. In essence our research indicates that the consequences of noise are far-reaching and reverberate throughout the ecosystem through lots of species.”

Future studies can offer a more fine-tuned look at how noise causes these ecosystem changes. Researchers want to know more about which herbivores, seed dispersers and pollinators avoid or are attracted to noise, and how changes in insect and animal behavior combine to affect plant communities.

Based on patterns from over a decade of an ecosystem experiencing noise pollution, evidence suggests that plant communities may require a long time to recover from the effects of human noise.

Still, co-author and lead botanist Sarah Termondt, a Cal Poly research affiliate, emphasizes the need to understand the full and lasting costs of noise.

“Continuing to look at long-term changes in floristic inventories over time will elucidate whether communities do eventually recover after long periods of noise pollution, even once it is removed from the landscape,” she said.

When changes to plant communities are viewed alongside the growing evidence for the problems that human noise creates for animals, it is increasingly difficult to ignore the near absence of noise regulations across the U.S., the authors stated.

To see the study, visit the Proceedings of the Royal Society B website: https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2906