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Researchers Uncover Just How Damaging Fires and Logging Are to Forest Soil

Forest soils recover from disturbances slowly over many years—up to 80 years following a wildfire and as many as 30 years after logging, much longer than previously thought.

According to Elle Bowd, a researcher with Australian National University's Fenner School of Environment and Society, there have been very few studies about the long-term impacts of disturbances like wildfires and logging on forest soils.

Based on what research has been done, we know that post-fire ash can inject large amounts of nutrients that plants need for growth, like phosphorus and nitrogen, into forest soils immediately after a fire. "But [we] know little about what happens eight or 34 years after logging or eight to 167 years after a bush fire to soils, despite their ongoing functional roles," Bowd says.

To fill this gap in our understanding of how long it takes forest soils to recover from disturbance, Bowd led a research team that collected 729 soil cores from 81 sites in the mountain ash forests of southeast Australia. The sampling sites had been subjected to nine different types of disturbances, from wildfires to clearcutting and post-fire salvage logging, at different frequencies in the past. The team used the soil samples to look at 22 different soil measures, including key soil nutrients like nitrate, organic carbon, phosphorus, potassium, and sulfur, and how they'd been impacted by disturbances that occurred eight, 34, 78, and 167 years ago.

By comparing the results to measures taken from sites that hadn't been disturbed in the past 167 years, the researchers were able to gain insights into how disturbance histories influence forest soils. The results of the study are detailed in a paper published in the journal Nature Geoscience last month.

Bowd says the team's findings show that forest soils recover from disturbances slowly over many years—up to 80 years following a wildfire and as many as 30 years after logging, much longer than previously thought. In other words, both natural and human disturbances can have long-lasting effects on forest soils, potentially impacting plant communities and ecosystem health for decades.

During high-intensity forest fires, soil temperatures can top 500 degrees Celsius (932 degrees Fahrenheit), which leads to the loss of soil nutrients, Bowd explains. Meanwhile, logging exposes the forest floor, compacts soils, and alters soil structure in ways that can also reduce vital soil nutrients. And these declines grow more severe as forests experience instances of fire and logging repeatedly without having sufficient time between disturbances to fully recover.

"Long-lasting declines in key nutrients such as nitrate, phosphorus, and organic carbon could have flow-on effects on the functional roles soils have—these include plant growth and survival, nutrient cycling, and sustaining/supporting microbial communities like fungi and bacteria," Bowd says.

Study co-author David Lindenmayer, a professor at Australian National University, noted that the recovery timeframes revealed by the research are far longer than prior estimates. "We thought forests could recover within 10 or 15 years, at most, after these sorts of events," Lindenmayer said in a statement.

The ash forests where the soil samples were collected for the study generate nearly all of the water for the five million people living in Melbourne, the capital of the southeastern Australian state of Victoria and the second-most populous urban area in the country. They also store large amounts of carbon in above-ground biomass and support the timber, pulpwood, and tourism industries.

"Almost 99 percent of Victoria's Mountain Ash forests have either been logged or burnt in the past 80 years, so these forests are facing a huge uphill battle to restore themselves to their former glory," Lindenmayer said.

It's likely that other forests around the world face similar challenges related to soil recovery following bushfires and logging, Bowd adds.

"To maintain the vital roles that soils have in ecosystems, land managers should consider the impacts of current and future disturbances on soils in ecosystem assessments and land-use management and planning," she says. "Specifically, fire (outside the historical fire return interval of 75–150 years) and clearcut and post-fire salvage logging should be limited wherever possible, especially in areas previously subject to these disturbances."

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