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The Importance of the Post-fire Relationship Between Soil Fungi and Ponderosa Pine Forest Recovery

May 20, 2012

Burned Ponderosa
Stand of Ponderosa pine trees following a burn, note that all understory has been cleared.

In the late 1800s, ponderosa pine forests were characterized by widely spaced trees and large grass openings, with low-intensity fires that burned every 2-5 years (Boyle, 2005) in a cyclic process that resulted in equilibrium (Perrakis, 2006).  Then came the intrusion of man, specifically the European settler, with the addition of roads, his livestock grazing on the grasses, and his fierce desire to put out fire.  During the mid to late 20th century, mounting evidence linked fire exclusion to increased fire hazard, prompting calls for reintroducing fire for purposes of ecological restoration and public safety (Perrakis, 2006).

Previous research has suggested that old growth tree mortality is high when controlled burns are not preceded by the reduction of fuel loads on the forest floor, much of which has accumulated during the past 120 years of fire suppression (Boyle 2005).  In areas with great fuel loads, the ground temperature can exceed 500°C during a fire, where in areas with lesser fuel loads, the ground temperatures typically range in temperatures up to 300°C (Bárcenas-Moreno, 2009).

While wildfires and controlled burns make for widespread and potentially destructive disturbances in forest ecosystems (Cairney, 2007), emerging studies suggest that controlled burns can be effective in reducing subsequent fire severity in ponderosa pine forests (Perrakis, 2006).  These controlled burns typically involve killing small trees, reducing overall forest density, and favoring fire-exclusion vegetation, lessening the fuel loads on the forest floor. (Perrakis, 2006).

Mycorrhizal fungus Laccaria laccata
This is the mycorrhizal fungus Laccaria laccata found on ponderosa pine.

The effects on soil fungal communities appear to be more pronounced with more frequent burnings (Cairney, 2007).  The effects on the forest are dependent upon the severity of the fire, and the temperatures in which the soil is elevated (Cairney, 2007).    Indirect consequences of fire include altered soil moisture and pH, along with increased and physical and chemical effects of coal deposition (Cairney, 2007).  In addition to effects on the physical and chemical characteristics of soil, effects of fire on microorganisms in the soil can be profound (Cairney, 2007).   Since the heat is felt closest to the surface, it is within the top few centimeters of soil where the microorganisms are most significantly affected by the fire (Cairney, 2007).  When looking to identify the microorganisms affected, several different studies suggest that ECM fungi may be more sensitive to burning than bacteria found in the same soil (Cairney, 2007).

Spore longevity in the soil is known for only a few species, but spores of terrestrial fungi in general retain viability for many years; some fungal spores are strongly dormant and require heat treatment to germinate (Claridge, 2009).

The occurrence of “post-fire” ascomycetes in forests is well documented (Cairney, 2007).  Some of the ascomycetes begin fruiting about six weeks after a burn and may continue successionally for up to 2 years (Cairney, 2007).  The underlying causes of this phenomenon have been debated widely over the years, with such factors as tolerance of increased soil pH and other chemical consequences of fire, activation and increased germination of spores, and/or decreased competition from other soil fungi (Cairney, 2007).  Some post-fire ascomycetes may form biotrophic associations with forest trees (Cairney, 2007).  A portion of these fungi may form ect-endomycorrhizal associations with coniferous hosts in the absence of fire, but exist as saprotrophs in the upper soil profile following fire (Cairney, 2007).  Some post-fire fungi are carbonicolous, fruiting on charcoal or partially burned organic debris; others are terrestrial, fruiting on ash or heated soil; still others appear to be mycorrhizal symbionts with, or pathogens on tree roots (Claridge, 2009).  These fungi importantly facilitate the regrowth of trees requiring mycorrhizal formation to survive (Claridge, 2009).  The proliferation of hyphae in ash by post-fire fungi initiates soil aggregation and captures and concentrated nutrients to then be gradually replaced by other fungi as well as root systems of recovering vegetation; the sites with greatest fungal activity were least erosive (Claridge, 2009).

After the Fire
Fresh new life begins to show itself following a burn.

In a research study conducted by Bárcenas-Moreno and Bååth, multiple samples of soil were subjected to various temperatures and then analyzed to see the affect the heat had on the microorganisms within the soil sample (Bárcenas-Moreno, 2009).  Their results indicate that when soil is exposed to temperatures of 400°C or more, there is no apparent recovery of the microorganisms within the soil (Bárcenas-Moreno, 2009).  In temperatures under 300°C, it was the bacteria that first recover, followed by the fungi several days later (Bárcenas-Moreno, 2009).

Research has shown that ECM fungi play a vital role in the recovery of the forest ecosystem following fire providing they themselves survive the disturbance.  Recovery of forest burned by low-intensity fire exudes the preferred outcome of ecologists, the large-sized ponderosa pines survive, the understory is removed, the ECM fungi remains active, and reformation of the prairie grasses begin to be apparent.  Researchers have but scratched the surface in the area of post-fire mycorrhizal importance, but they have determined that there much more to learn.


Bárcenas-Moreno, Gema; Bååth, Erland. Bacterial and fungal growth in soil heated at different temperatures to simulate a range of fire intensities. Soil Biology & Biochemistry. Vol. 41, pp. 2517-2526 (2009).

Boyle, Sarah I.; Hart, S.C.; Kaye, J. P.; Waldrop, M. P. Restoration and canopy type influence soil microflora in a ponderosa pine forest. Soil Science Society of America Journal. Vol. 69, pp. 1627-1638 (2005).

Cairney, John W. G.; Bastias, B. A. Influences of fire on forest soil fungal communities. NCR Research Press. Vol. 37, pp. 207-215 (2007).

Claridge, Andrew W.; Trappe, J.M.; Hansen, K. Do fungi have a role as soil stabilizers and remediators after forest fire? Forest Ecology and Management. Vol. 257, pp. 1063-1069 (2009).

Perrakis, Daniel  D.B.; Agee, J.  Seasonal fire effects on mixed-conifer forest structure and ponderosa pine resin properties. NRC Research Press. Vol. 36, pp. 238-254 (2006).

Smith, J. E., et. al. Short-term effects of seasonal prescribed burning on the ectomycorrhizal fugal community and fine root biomass in ponderosa pine stands in the Blue Mountains of Oregon. NRC Research Press. Vol. 34, pp. 2477-2491 (2004).

Photo Credits:

(After the fire)

(Burned Ponderosa)

(Mycorrhizal fungus Laccaria laccata) {slide by Jim Trappe}

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