A Matter of Mulch: Restoring Post-Fire Pine Forests in the Western United States

Article: Jonas, J. L., E. Berryman, B. Wolk, P. Morgan, and P. R. Robichaud. 2019. Post-fire wood mulch for reducing erosion potential increases tree seedlings with few impacts on understory plants and soil nitrogen. Forest Ecology and Management 453: 117567. DOI: 10.1016/j.foreco.2019.117567

The severe wildfires destroying forests and threatening communities in the western United States have been a devastating consequence of climate change, one that is becoming more frequent. Following severe fires, forest soils can erode, depositing sediment into nearby waterways after it rains and threatening local water quality as a result. To help combat these challenges, the Burned Area Emergency Response (BAER) program, formed by the U.S. Department of the Interior and the U.S. Department of Agriculture in 1974, arrives at the scene after the fire and works to identify and address soil erosion issues early on. One of the main ways that BAER does this is through applying mulch, a plant-based ground covering that is often used in gardens.

Even though mulch is so commonly used to prevent soil erosion following wildfires, by BAER and by land managers in general, there is still much that we do not know about how plants respond to mulch applications in these post-fire forests. Does mulch interfere with the ability of plants to grow back after the fire? Does it matter what the mulch is made of, or how much of it is applied? These questions and more were tackled by the project team using the High Park Fire in Larimer County, Colorado as their test case.   

High Park Fire in Colorado
The High Park Fire in Larimer County, Colorado, which was started by lightning, burned over 86,000 acres in 2012. Source: U.S. Department of Agriculture / Public domain
Monitoring Mulch

The High Park Fire, started by lightning in June 2012, burned over 86,000 acres of lodgepole pine forest in the region, with over half of this acreage experiencing moderate- or high-severity fire. BAER went straight to work afterwards, immediately applying mulch to many affected areas. The U.S. Forest Service provided funding to set up a controlled experiment in a portion of the burned forest not mulched by BAER. They tested three types of mulch (wheat straw, wood strands, and wood shreds) applied at two rates (the rate that BAER uses and 150% of that rate) in replicated sets of plots. From 2012 through 2016, the project team monitored the physical properties of the soil, the thickness of the mulch (which reflects how long it persists), and the amount of nitrogen in soil and in plants. They then recorded the species and abundance of the plants that grew back (that is, how much of the land was covered by each species).

Close-up of wood mulch
Wood mulch minimizes soil erosion, traps in moisture, persists longer than wheat straw, and promotes pine seedling growth in forests devastated by severe wildfire. Source: Public domain

 One aspect of this study that makes it so unique is that, in addition to testing three types of mulch, the team included two types of controls to which they could compared the results. As you might expect, one of these controls consists of a plot with no mulch – how does adding mulch compare to what would happen if you took no action? But the team also implemented a “mulched control”, using a mulch that was made out a synthetic material, not made of plants, so it would not be used by soil microbes as a source of carbon and energy. One concern about using wood mulch is that, because wood has so few nutrients and so much carbon, microbes and fungi might move out of the soil to colonize it, taking advantage of this new carbon-rich food source but reducing the amount of nitrogen in soil; fungi in particular can take nutrients from the soil with them as they colonize wood. By adding a “mulched control”, the team could determine whether the material that the mulch is made of has an effect, and so better understand what might be causing differences they see between mulch treatments – are these differences because of changes in physical properties of soil, like moisture or temperature, or changes in chemical properties, which are affected by what the mulch is made of?

After four years of monitoring, the team found that all mulched areas had higher soil moisture after rain events compared to the non-mulched areas, but mulch did not seem to reduce the amount of nitrogen in soil. Fewer pine seedlings grew in the wheat-straw mulch compared to the wood mulches, and more of these plants were non-native; the wood mulches supported more pine seedlings and persisted longer. This means that the wood mulch helped promote the return of the pine forest to conditions that were present before the fire.

Two plots in a burned pine forest; the plot on the left did not receive mulch and has few pine seedlings, and the plot on the right that did receive mulch has more, taller pine seedlings
Illustrating the effect of mulching on pine seedlings. The plot boundary runs down the center of the image from the blue flag in the foreground toward the road in the back. A mulched plot is on the right and an untreated area is on the left. Source: Dr. J.L. Jonas.
Informing Post-Fire Forest Restoration

Restoring post-fire forests to protect ecosystem services we depend on and promote the return of a valued habitat is a difficult task facing land managers and task forces like BAER. The task is made more challenging by the need to coordinate efforts to reduce soil erosion in a way that does not cause more harm, and that ideally would benefit other parts of the environment as well. It is difficult to make these decisions, but research like this experiment can help inform decisions land managers make to help restore the forests.

Thanks to this study, we now know that wood mulch is better than wheat-straw mulch at promoting the return of pine trees and excluding non-native species from taking over, while also stabilizing the soil, probably because wood mulch persists longer and holds more moisture. In addition, wood mulch does not seem to cause harm by affecting the amount of nitrogen in the soil. With this information, land managers can make informed decisions to help protect and restore these precious ecosystems after devastating fires.

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Jenna Zukswert

I am a PhD student at SUNY College of Environmental Science and Forestry in Syracuse, NY. My research interests are in ecosystem ecology, biogeochemistry, and forest management, and I am interested in bridging the gap between those who conduct research and those who use the results. Twitter: @jzukswert

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