Dead and Gone? – The loss of decaying wood communities in urban forests

Reference: Meyer, S., Rusterholz, H-P., & Baur, B. (2021). Saproxylic insects and fungi in deciduous forests along a rural-urban gradient. Ecology and Evolution, 11, 1634-1652.

Dead Wood, New Life

            Forests offer a peaceful escape from the stress in our lives. I, like many others, find comfort in taking a walk through the woods to breathe in the fresh air and admire its beauty. I see the sun peaking through the cracks in the branches of the tree canopy. The trees crackle and creak as a breeze encourages the limbs to sway and bend. The birds flutter and tweet amongst the branches that vines have overtaken. When I gaze at the horizon, I might see a deer munching in the flowering undergrowth. When I look to the ground, I see many dead, decaying logs littering the forest floor. These rotting logs and branches are as beautiful as the surrounding scenery if you discover what is happening inside.

Beyond what meets the eye, decaying wood is teeming with activity. These logs provide homes for the fungi and wood-dependent insects that help to break the wood down. The forest’s health depends on these decomposers, just as the decomposers rely on the forest for food and shelter. Decomposition by fungi and insects adds nutrients back to the soil for a healthier forest with flourishing plants. If the soil lacks nutrients, plants won’t have enough nutrients to pass along the food chain. In essence, decaying logs represent a place where the living and dead work together to make a forest beautiful and productive. All forest habitats depend on this process, including forests in cities.

City Dead Dwellers

An urban forest is any collection of trees growing in cities or towns. These forests or parks often differ in origin, structure, and condition from natural forests. Typically, city forests either originate as fragments of once larger, continuous forests or are designed and planted purposely. Fragmentation in city forests can make it difficult for insects and fungi to disperse properly. Also, urban forests are vital for a city’s health. City parks filter pollution-ridden air, lower city temperatures by providing shade, and supply a refuge for animals that cannot survive in cities. As such, city forests typically experience more noise, human disturbance, and air and light pollution. These factors determine the diversity of organisms a forest can support, including fungi and insects that break down plant material.

As forest well-being depends on decomposition, researchers Sandro Meyer, Hans-Peter Rusterholz, and Bruno Baur wanted to determine if cities disrupt deadwood communities of fungi and insects. In Basel, Switzerland, they set out bundles of beech and oak branches in 25 forests that varied in level of urbanization. After ten months, they collected the wood bundles and put them in a plastic tube with mesh lids to collect the insects that emerged for an additional eight months. To identify the fungi present on the wood, they extracted DNA from wood chip samples.

A Delay in Decay

            The researchers collected and identified 193,534 insects! Beetles and flies were the most abundant, but thousands of wasps and moths were also in the bundles. They also found 97 different types of fungi living in the branches. There were fewer wood-dependent insects and fungi in urban forests. Thus, city conditions harm the decomposing fungi and insects in wood communities. While the types of insects didn’t differ, the types of fungi present changed as forests became more urban. As fungi increased, so did the number of insects. This relationship is crucial because fungi break down plant toxins so that insects can break down the exposed nutrients.

            Wood decomposers decreased in forests closer to cities which could negatively impact forest health. Besides providing benefits to our mental health, city forests are vital for cleaning air pollution, lowering the temperature, and providing homes for animals that cannot survive in cities. Forest management could add deadwood or tree branches to urban forests to increase fungi and insect populations. Doing so would ensure that nutrients are adequately cycled to maintain forest health.

            The next time you take a relaxing hike through the woods, take some time to appreciate the dead, decaying logs and their hosts for helping produce the beauty around you.

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Brandi Pessman

I am a fifth-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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