Effects of Brush Pile Burning in the Chicago region

by Antonio Del Vallé, Emma Leavens, Meghan Midgley

Research project summary

Over the past three years, we have conducted research on brush pile burning and its effects on plants, fungi, and soil across the Chicago region. We’ve had the pleasure of collaborating with many different organizations, forest preserves, stewards, and volunteers across the region to determine where brush has been burned and collect data related to plant and soil communities. Our project has three main themes/questions: 1) how do plants and soil change through time following brush pile burning?; 2) how do differences in habitat, burning techniques, and type of wood being cleared/burned impact the succession of plant/soil communities following burning?; 3) are there post-burn restoration techniques that encourage desirable plant and soil communities? As we have begun to produce preliminary results from the different sections of this research, we have delivered information to managers and stewards through reports and presentations. We thought a blog post would help deliver these preliminary findings to audiences that have not yet seen our results first-hand. 

Throughout this blog post we pose a number of common questions and topics that we’ve received from the restoration community, and we deliver our answers to these questions based on our results to-date. We will mainly look into questions related to the 1st research theme listed above. We address where we are at with the other two research themes at the end of this post, and we will provide more results once we have more thoroughly analyzed our data.

Note on Terminology

Many folks who conduct brush pile burns refer to the post-burn areas as “burn scars”. This term makes sense as the area immediately following a brush pile burn often lacks plants. We believe this term may have a lasting negative connotation that might not reflect some of the potential benefits of brush pile burning in certain contexts. You’ll find that we use a variety of terms to refer to areas where brush pile burns have occurred, including: burn scars, burn pile footprints, burn pile locations, post-burn areas, etc. Our use of these terms is not intended to imply that brush pile burning is causing net-negative or positive effects.

Should we conduct brush pile burns? Are there alternatives to brush pile burning?

Removal of invasive woody species (e.g. buckthorn, honeysuckle, etc.) and overstory thinning of native weedy species (e.g. maple, basswood, etc.) are important/essential restoration techniques to restore oak woodland, savanna, and grassland habitats in the Chicago region. Many regional practitioners and stewards eliminate resulting woody debris by conducting brush pile burns. This approach reduces effort, cost, and potential soil compaction of removing wood from natural areas. Additionally, it fosters social engagement and camaraderie amongst stewards through group “cut and drag” work days and gathering around the fire afterwards. 

As researchers, we want to understand what happens when brush piles are burned in natural areas. Brush pile burning may change soil chemistry and plant and fungal biodiversity. In our research, we’re working with stewards and managers of natural areas throughout the Chicago region to assess the above- and belowground effects of brush pile burning, identify the burning strategies that maximize or minimize these impacts, and characterize post-burn succession. As many stewards and managers also remove ash, add mulch, or reseed brush pile burn scars, we’re also evaluating the efficacy of these strategies. 

Alternative techniques to brush pile burning include 1) cutting/piling brush and leaving woody debris, 2) girdling and/or basal bark treatment and leaving dead snags standing, and 3) forestry mowing and leaving mulched woody debris. How brush is cut/treated and removed (or not removed) are decisions that are site-specific and require the expertise of managers/stewards to decide what is best for an individual site. 

This research project is not designed to answer questions such as “is brush pile burning good or bad?”or “is removing wood or allowing it to decay a better alternative to brush pile burning?” These are interesting questions, but are outside the scope of what we can say based on our data. Similarly, we have not studied whether or how the impacts of brush pile burns differ from those of downed logs that may burn for days following a landscape burn. Instead, we are focused on helping stewards and managers learn more about what happens to the soil, fungi, and plants in different brush pile burn scenarios and how different post-burn restoration techniques impact these post-burn areas.

How hot do brush pile burns get? 

Soil temperatures can reach values of 80-350 ℃ at 2 centimeters depth within the centers of burn piles and stay above 60 ℃ for 3-5 days after active flames are put out. Below is an example of soil temperatures observed underneath a brush pile at Harms Woods in Cook County.

How does brush pile burning affect bacteria and fungi?

For the next three sections, we will draw from our results from studying brush pile burns conducted at The Morton Arboretum and Forest Glen Woods (Cook County) from 2015-2023. We present data for each preserve separately, since each preserve has slightly different habitats/plant communities, and brush pile burns may consist of different levels of burn intensity.

Short term: Microbial biomass (amount of bacteria and fungi) decreases dramatically after burning. Some unusual pyrophilic fungi species (Pyronema omphalodes) are often observed 0-1 years after burning.

Long term: Mycorrhizal fungi appear to return to pre-burn abundance within 4 years post-burn. Microbial biomass remains significantly different 8 years post-burn. 

How does brush pile burning affect soil chemistry?

Nutrient levels: There is a significant increase in soil nutrients like nitrate and phosphate immediately after burning. Nitrate returns to pre-burn levels within 2 years. Phosphate returns to pre-burn levels within 5 years. 

pH levels: Pre-burn soil pH was ~6.25 and ~7.25 for Morton Arboretum and Forest Glen respectively. Despite these pre-burn differences, both sites saw significant increases resulting in post-burn soil pH of ~8.25 at both sites. Soil pH lowers to ~7.5 after 8 years post-burning. These values are still significantly higher (more alkaline) than pre-burn pH levels at Morton Arboretum, but have nearly returned to normal levels at Forest Glen.

How does brush pile burning affect plant communities?

Burn pile footprint plant communities go through a successional change in species composition. In the short term you might observe weedy plant species. Burning may provide the right habitat/disturbance/conditions for rare plants within or around burn scars. Over time the burn scar plant community will reflect the surrounding plant community.

Short term: Plant cover and quality is greatly reduced in the first growing season after burning. Weedy/hardy plants are observed 0-3 years after burning. Some examples of species that we frequently observe during this time period include Barbarea vulgaris, Solidago altissima, and Phytolacca americana. We also frequently observe burn scar moss (Funaria hygrometrica) in recently burned piles.

Long term: Plant cover and abundance return to pre-burn values within 7 years post-burn. Plant quality (FQI) also returns to pre-burn values 2-6 years after burning. Variation in plant quality within burn scars is dependent on the surrounding plant quality within the natural area. Plant quality variation may also be related to burn intensity and other factors. We have more work to do to uncover the relationship of burn intensity and other management decisions to plant communities in burn scars.

Are there rare plants observed in burn pile footprints?

We’ve observed a number of rare plant species growing at the center or edge of burn scars including but not limited to: Allium tricoccum, Allium burdickii, Carex bromoides, Pycnanthemum verticullatum pilosum, Senna hebecarpa, Spiranthes ovalis erostellata. Most of these species are present in the surrounding plant communities (e.g. Allium spp, Carex bromoides, etc) and likely seeded in from those direct sources. Others such as Spiranthes ovalis erostellata were not readily observed in the surrounding areas. This particular plant was found in an 8 year old burn scar. 

The state endangered Geranium bicknellii is known to exclusively grow on the edge of burn pile footprints in a few natural areas in the region where it grows and where it has been purposefully seeded by stewards. We have not conducted surveys at the preserves where it has been found/seeded, but we are aware of the importance that brush pile burning may have on this particular species.

Review: How does burning brush affect the ecology of the burn pile footprint?

• Direct impacts of brush pile burning:

• The temperatures achieved during brush pile burning and the durations of those elevated temperatures are high/long enough to negatively impact seed viability and mycorrhizal fungi. 

• Brush pile burning increases soil pH, nitrate, and phosphate through ash deposits.

• Impacts of brush pile burning through changes in the microbial community:

• Microbes (fungi and bacteria) are needed to balance soil nutrients, so their fluctuations through time may impact soil nutrient changes through time.

• Decreases in mycorrhizal fungi abundance or changes in the fungi community may inhibit the growth of plant species that require specific mycorrhizal associations. It takes ~4 years for mycorrhizal fungi abundance to return to normal following brush pile burning, and we are still analyzing changes in the fungi diversity.

• Impacts of brush pile burning through changes in soil nutrients and pH:

• Brush pile burning creates more alkaline soils and increases soil nitrate and phosphate. This may provide unique habitat conditions for rare/pyrophilic plants, create conditions that support invasive species, and/or make the area difficult for more conservative plants to grow in. As soil pH and nutrients balance through time, the plant community successionally returns to a more normal composition.

We are still working on identifying the relationships between changes in soil dynamics, microbial communities, and plant communities within brush pile burn scars and what that means for the ecology of the burn pile footprint. We will continue to keep you updated on our findings.

Research Theme #2: How do different burning techniques, habitat context, and type of wood being burned impact the succession of burn pile plant communities?

We’ve received many questions related to this topic such as: 

• Should burn scars be reused?

• Should you burn on top of a car hood?

• Should you build burn piles in “weedy” areas?

• Does the type of wood you burn dictate the “recovery” of your burn scar?

We don’t yet have the answers to these questions, but we have collected data on 100 scars across over 30 preserves to attempt to answer some of these questions. We are still analyzing data and critically thinking about our results. We will follow up with any interesting results on these questions.

Research Theme #3: Are there effective post-burn restoration techniques?

We are testing the efficacy of restoration techniques that are commonly used or are easily available to the steward community: reseeding with native species or a non-native cover (e.g. Lolium spp.), adding leaf litter or mulch, and removing ash off the top of the burn pile footprint and scattering it in the surrounding area. We have not yet finished analyzing data on the efficacy of these techniques, but we will be glad to share results when we do.