Fire's Effect on Plant-Soil Interactions

(Buzzle.com)

Out of all the previous blog post's relationships, I believe the most exciting (and probably least thought about) has been saved for last! Just as all our previous discussions, plant's relationship with soil is much deeper than just a place for it to grow. The average person sees soil as just 'dirt,' but it is so much more than that. Soil is home to billions upon billions of microscopic bacteria and fungi all that interact with plants in different ways.  Soil is formed through a combination of topography, parent material, climate, biota, and time.  There are thousands of unique soil types that result from varying combinations of these five factors. Soil is really the basis of all other life in an ecosystem.  The type of soil affects the plant communities that are able to survive and thrive at a certain location. There cannot be plants, or animals, or anything living for that matter without 'dirt!' So how does fire affect this incredible relationship between soil and plants?

            In the plant-environment blog post, we scratched the surface on this topic. Often, especially after a high intensity wildlife, many of the nutrients are stripped away from the top of the soil. Also, soil pH levels increase and mineral elements move to the ash layer. The newly bare soil deprived of plants that used to reside there, can lead to lots of runoff and erosion; however, this is not always a negative thing and can actually often redistribute the soil nutrients. Another major change to soil following a fire is water filtration rate. With plants, soil absorbs water pretty rapidly, but after a fire the soil can become "water-repellent." Evapotranspiration is normally a large loss of water from the hydrologic cycle. With fire removing many woody plants from the landscape and replacing them with grasses and herbaceous plants, loss of water from evapotranspiration is greatly decreased.  Both lower evapotranspiration and lower rates of infiltration lead to increase stream flow as well as soil water storage. So while less water can enter the soil, less water is also leaving it. In the short-term fire may negatively effect soil, but nature has some pretty incredible ways of fixing itself.

            Now let's dive in a little deeper. What is fire's effect on soil microbe and fungal communities? Just as every other topic, the intensity of the fire really affects the answer to that question. In one study in the southeast, frequent prescribed fire's effect on the soil fungal communities was examined in comparison to unburned sites. (Remember that prescribed fires are normally very low intensity.) The results of the study indicated no significant changes in both richness and diversity, although often there is a shift in species found (Oliver et al. 2015).  This may indicate that certain fungal communities may actually support plant communities that are composed of desired fire adapted or fire tolerant species that dominate the frequently burned areas (Oliver et al. 2015). A table below displays the lack of much change in richness, diversity, or evenness from the Oliver study. 

This indicates that the argument many people often make that regular burning is ruining the soil is very far from being the truth. Again looking at prescribed fire's effect on fungal communities, but this time across the ocean in the Mediterranean, species richness was observed. Unlike the southeast study, the Rincon study showed a decrease in species richness, but a high ratio of Tuberaceae and Pezizales (Rincon et al. 2013). This still supports the idea of the potential for fire-adapted fungi in fire adapted communities, especially pine communities due to their high reliance on ectomyorrhizal fungal symbiosis.

            High intensity fires have a very different outcome on soil microbial and fungal communities. In a study observing soil carbon and microbial biomass in boreal forests following wildfires, there was a total reduction of 52-56% in soil microbial biomass (Holden et al. 2016).  In addition, mycorrhizal taxa and basidiomycetes showed incredibly low tolerances for high intensity fires (Holden et al. 2016).  This decrease in biomass from unburned to burned sites can be seen in the figure below from Holden's study.

An earlier study from 2013, also compared burned and unburned sites while observing soil fungi communities, but this time in the Mountain Chaco Forest. This study noted the direct relationship between fire and decline in arbuscular mycorrhizal fungi (Longo et al. 2013). The decline was in both richness and diversity, the complete opposite of low intensity fires. However, just as stated before nature has incredible ways of fixing itself.  Another study also looked at arbuscular mycorrhizal fungi post wildfires, but on a long-term scale. They also noted a severe decline in arbuscular mycorrhizal fungi a year after the fire, but 11 years later the community was reestablished (Xiang et al. 2015).  A study in Texas, I believe summarized all the findings in a single study comparing unburned, prescribed burned, and wildfire sites in the Southwest.  They noted that there was a reduced soil microbial biomass and enzyme activity following fire, but the prescribed fire sites did not have long term effects (Fultz et al. 2015).

            Plant communities might also have a strong effect on fire's affect on soil communities.  Varying plant species have different levels of flammability and heat, which affects the intensity of a fire. For example, soil communities were more negatively affected following a fire under pines than kermes oaks (Barcenas-Moreno et al. 2014).  This highlights the importance of vegetation as a fresh organic source in soil ecosystems (Barcenas-Moreno et al. 2014).  This can be important in creating "refuge patches" for soil microbes. Soil fungal communities can reestablish faster, even after a high intensity fire, as long as there are patches of low intensity fires throughout where the fungi can survive (Cowan 2015).

            Fire is an unstoppable part of nature. We as people may try to suppress it, but eventually fire adapted ecosystems will burn. After years of suppression, these fires are high intensity, and as seen by studies in this and previous blog posts, the results can be catastrophic to ecosystems that take many many years to 'fix.' Hopefully now you can at least begin see some of the positives of fire, especially prescribed burns, and the value they can serve to the environment as well as preventing catastrophic wildfires.

Literature Cited

Bárcenas-Moreno, G., García-Orenes, F., Mataix-Beneyto, J., & Bååth, E. (2014). Plant species influence on soil microbial short-term response after fire simulation. Plant and soil, 374(1-2), 701-713. https://www.infona.pl/resource/bwmeta1.element.springer-6521758a-10c8-3d90-affb-4aa7fcbbdf56

Certini, G. (2005). Effects of fire on properties of forest soils: a review. Oecologia, 143(1), 1-10. 

http://www.fsl.orst.edu/ltep/Biscuit/Biscuit_files/Refs/Certini%20Oa%202005%20fire.pdf

Cowan, A. D., Smith, J. E., & Fitzgerald, S. A. (2016). Recovering lost ground: effects of soil burn intensity on nutrients and ectomycorrhiza communities of ponderosa pine seedlings. Forest Ecology and Management, 378, 160-172. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/56543/CowanArielD2015.pdf?sequence=1

Fultz, L. M., Moore-Kucera, J., Dathe, J., Davinic, M., Perry, G., Wester, D., ... & Rideout-Hanzak, S. (2016). Forest wildfire and grassland prescribed fire effects on soil biogeochemical processes and microbial communities: Two case studies in the semi-arid Southwest. Applied Soil Ecology, 99, 118-128. https://www.researchgate.net/profile/Sandra_Rideout-Hanzak/publication/285359708_Forest_wildfire_and_grassland_prescribed_fire_effects_on_soil_biogeochemical_processes_and_microbial_communities_Two_case_studies_in_the_semi-arid_Southwest/links/5695177508ae3ad8e33d4a30.pdf

Graham, R. C., Egerton-Warburton, L. M., Hendrix, P. F., Shouse, P. J., Johnson-Maynard, J. L., Quideau, S. A., ... & Breiner, J. M. (2016). Wildfire Effects on Soils of a 55-Year-Old Chaparral and Pine Biosequence. Soil Science Society of America Journal, 80(2), 376-394. https://www.researchgate.net/profile/Sylvie_Quideau/publication/300081346_Wildfire_Effects_on_Soils_of_a_55-Year-Old_Chaparral_and_Pine_Biosequence/links/5765b33108ae421c4489d48c.pdf

Holden, S. R., Gutierrez, A., & Treseder, K. K. (2013). Changes in soil fungal communities, extracellular enzyme activities, and litter decomposition across a fire chronosequence in Alaskan boreal forests. Ecosystems, 16(1), 34-46. https://www.researchgate.net/profile/Kathleen_Treseder/publication/257469632_Changes_in_Soil_Fungal_Communities_Extracellular_Enzyme_Activities_and_Litter_Decomposition_Across_a_Fire_Chronosequence_in_Alaskan_Boreal_Forests/links/56aed65008aeaa696f2ed73b.pdf

Holden, S. R., Rogers, B. M., Treseder, K. K., & Randerson, J. T. (2016). Fire severity influences the response of soil microbes to a boreal forest fire. Environmental Research Letters, 11(3), 035004. http://iopscience.iop.org/article/10.1088/1748-9326/11/3/035004/meta

Longo, S., Nouhra, E., Goto, B. T., Berbara, R. L., & Urcelay, C. (2014). Effects of fire on arbuscular mycorrhizal fungi in the Mountain Chaco Forest. Forest Ecology and Management, 315, 86-94. https://www.researchgate.net/profile/Carlos_Urcelay/publication/260006674_Effects_of_fire_on_arbuscular_mycorrhizal_fungi_in_the_Mountain_Chaco_Forest/links/0046352f8cfab6163c000000.pdf

Oliver, A. K., Callaham, M. A., & Jumpponen, A. (2015). Soil fungal communities respond compositionally to recurring frequent prescribed burning in a managed southeastern US forest ecosystem. Forest Ecology and Management, 345, 1-9. https://www.srs.fs.usda.gov/pubs/ja/2015/ja_2015_callaham_002.pdf

Reazin, C., Morris, S., Smith, J. E., Cowan, A. D., & Jumpponen, A. (2016). Fires of differing intensities rapidly select distinct soil fungal communities in a Northwest US ponderosa pine forest ecosystem. Forest Ecology and Management, 377, 118-127. https://www.fs.fed.us/pnw/pubs/journals/pnw_2016_reazin001.pdf

Rincón, A., Santamaría, B. P., Ocaña, L., & Verdú, M. (2014). Structure and phylogenetic diversity of post-fire ectomycorrhizal communities of maritime pine. Mycorrhiza, 24(2), 131-141. https://www.researchgate.net/profile/Ana_Rincon/publication/255975104_Structure_and_phylogenetic_diversity_of_post-fire_ectomycorrhizal_communities_of_maritime_pine/links/561e59f208aecade1acbf154.pdf

Xiang, X., Gibbons, S. M., Yang, J., Kong, J., Sun, R., & Chu, H. (2015). Arbuscular mycorrhizal fungal communities show low resistance and high resilience to wildfire disturbance. Plant and soil, 397(1-2), 347-356. https://www.infona.pl/resource/bwmeta1.element.springer-doi-10_1007-S11104-015-2633-Z