If trees could hear, is human noise a threat?

Cover Image Caption: Pinyon-juniper woodlands support diverse plant and animal life in southwestern states, including Arizona, Colorado, Nevada, New Mexico, and Utah (Image Source: “Pinyon-Juniper Ecosystems: Canyon-Ancients” by Desert Landscape Conservation Cooperative is licensed under CC BY 2.0).

Reference: Phillips, J. N., Termondt, S. E., & Francis, C. D. (2021). Long-term noise pollution affects seedling recruitment and community composition, with negative effects persisting after removal. Proceedings of the Royal Society B, 288, 20202906. https://doi.org/10.1098/rspb.2020.2906

It’s Noisy, Can Plants Tell?

In northwestern New Mexico, an expanse of land – called Rattlesnake Canyon Habitat Management Area (RCHMA) – is dominated by woodlands of pinyon and juniper trees. Despite being uninhabited by humans, natural gas wells are dispersed throughout the RCHMA.  Some well pads include compressors that continuously produce a loud noise. While this noise doesn’t deter humans from exploring the beautiful surrounding woodlands, many animals are impacted by the noise produced by compressors.

Compressors at gas wells, like these, can produce loud and continuous noise (Image Source: “Coal seam gas well pad, NSW” by Beyond Coal and Gas is licensed under CC BY 2.0).

Noise is an issue for most animals that communicate using sound, including many pollinating insects and seed-dispersing birds. Some RCHMA birds, such as the scrub-jay, avoid well pads that have noisy compressors. These birds may avoid noise because communication becomes more challenging.

Understandably, animals that produce and hear sound are affected by these noisy compressors. However, could noise affect plants? Whether plants can respond to sound is still debated and undergoing research. Curious researchers at universities in California and Texas questioned and tested if the compressor noise in the RCHMA impact plants. They discovered that noise has negative and long-lasting impacts on tree reproduction, and there may be an explanation that doesn’t involve plants capable of hearing.   

Let’s Talk About Trees

Before we can get into the details of this study, we need to discuss a few things about the trees. Pinyon and juniper trees are two of the most common trees found in the woodlands of northwestern New Mexico. Pinyon trees are called masting trees. This means that the tree only releases seeds after a long time, in this case, every two to seven years. When produced, the seeds get dispersed by many animals, especially corvid birds, like scrub jays. Similarly, juniper seeds are spread by birds and mammals, like the gray fox. Although junipers produce seeds every year, seed survival depends on extreme temperature changes from hot to cold. Covering 15% of the land in the southwest, pinyon-juniper woodlands are important for the ecosystems they support and the resources they provide humans, including food, wood, and recreation.  

Pinyons (left) and junipers (right) are common trees in the woodlands of the southwest (Image Source: left,  “Pinaceae: Pinus edulis” by Dane Larsen is licensed under CC BY-NC 2.0; right, “Utah juniper, Juniperus osteosperma” by Jim Morefield is licensed under CC BY-SA 2.0).

Researchers Jennifer Phillips, Sarah Termondt, and Clinton Francis conducted their research in the pinyon-juniper woodlands of RCHMA. In 2007, they set up plots around well pads that were either quiet or produced noise from compressors.  Between 2010 and 2017, some noisy well pads became quiet, and some quiet well pads began making noise. The remaining well pads were always noisy or always quiet until 2019.  At each plot in 2007 and 2019, the researchers counted the number of pinyon and juniper seedlings (trees less than 20 cm tall). For sites that remained quiet or noisy, noise reduced the number of seedlings found. Noise in once quiet areas did not affect seedling growth. In places that went quiet, juniper seedlings were just beginning to recover, but pinyon seedlings showed no signs of recovery.

The Birds and the Trees

Overall, compressor noise harmed the growth of new juniper and pinyon trees.  However, it is unlikely (yet possible) that noise is directly affecting the plants. The researchers hypothesize that the noise is deterring the animals responsible for dispersing the seeds. Pinyons and junipers rely on scrub jays and mountain bluebirds, respectively, and both birds are known to avoid noise. Also, the pinyon trees likely struggled to recover after the noise was removed due to the long time between seed releases. On the other hand, juniper seeds experienced ideal conditions in 2018 since the winter was frigid after extreme heat in the summer. These drastic changes in temperatures preferred by juniper seeds likely helped them recover more quickly from the noise. However, without these favorable conditions, the junipers may have experienced no recovery, as well.

Scrub jays (top) and mountain bluebirds (bottom) are essential seed dispersers for pinyon and juniper, respectively. Both birds are known to avoid noisy areas (Image Source: top, “Woodhouse’s Scrub Jay (Aphelocoma woodhouseii)” by sussexbirder is licensed under CC BY 2.0 ; bottom, “Mountain bluebird” by YellowstoneNPS is marked with CC PDM 1.0).

This study is the first evidence that noise can harm plants in addition to animals. In the long term, noise can be detrimental to plant-animal interactions, ultimately impacting entire ecosystems. It appears to take time for noise to have adverse effects on pinyon and juniper trees since plots that were only recently exposed to noise showed little change in seedling growth. However, the slow recovery that follows noise removal could suggest a grim future for these woodlands. The researchers have not determined how many years it takes after the noise has stopped for the trees to recover fully. This information will be vital as researchers continue to monitor the health of the woodlands.

Compressors are only one of many sources of noise that humans have introduced to environments. As they continue to grow and expand, cities are another significant source of noise known to affect animal behavior. Many urban seed dispersers and pollinators are negatively impacted by noise. This could mean that urban forests exposed to city noise may be at risk. Traffic noise tends to be a major contributor to city noise. To help reduce noise (and air) pollution, you can choose public transportation or ride a bike.  

Let’s listen to the trees, for the trees are tired of listening to us.

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Brandi Pessman

I am a second-year Ph.D. student at the University of Nebraska-Lincoln in the School of Biological Sciences. Growing up on a farm in a small town in Illinois, I developed an early love for animals and a fascination with their behaviors. When I was younger, however, it never crossed my mind that I would be using spiders to investigate how human presence affects animal behavior, but I am loving every second of it. Studying the behaviors of animals can tell us a lot about the role that we play in their survival (or death), which is becoming increasingly important as human populations continue to grow. When I am not studying spiders, I enjoy playing with my cat or being outdoors!

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