Effects of High Severity Wildfire on Mycorrhizal Fungi in Boreal Forests

Introduction

Boreal forests are warming faster than the rest of the globe, leading to increased wildfire activity in the past 23 years [11]. Specifically, boreal wildfires have increased in frequency and severity, resulting in a loss of ecological resilience [9]. Decreased resilience in boreal forests means the ecosystem’s ability to tolerate and fully recover from wildfire disturbance is reduced [9]. Additionally, post-fire recovery in boreal forests is slower and more difficult than in other forest biomes, because of the cold climate, shorter growing seasons, and acidic soil [1,2,4]. A main component of boreal forest recovery and function in poor growing conditions is mycorrhizal fungi, a non-pathogenic fungi that colonizes the roots of vascular plants, including forest trees [2, 4, 12,13]. Mycorrhizal fungi establish mutualistic symbioses with host plants, benefitting both the mycorrhizal fungi and the host plant through the exchange of growth-limiting resources [13]. These symbioses also facilitate important boreal forest processes such as nutrient cycling, soil formation, and reforestation [17]. However, these processes are hindered by high severity fires, which burn the canopy of trees and the soil [14]. The effect of high severity fire on soil threatens boreal forest mycorrhizal fungi [1,2,3,4,5,6].

Presently, research studying the effect of increased fire activity from climate warming on mycorrhizal fungi is relatively new [1,2,4]. Despite limited research, current literature has identified trends among forests of similar ecologies. The following review will evaluate the current literature on high severity wildfire and mycorrhizal fungi in boreal forests. Understanding the effects of high severity wildfires on mycorrhizal fungi is important to predict post-fire forest recovery and influence decisions concerning forest management practices as boreal forest wildfire activity shifts.

Background

Mycorrhizal fungi are associated with over 90% of plant species in boreal forests [13]. The mutualistic symbiosis between mycorrhizal fungi and vascular plants benefits both the host plant and the fungi in nutrient limited boreal forests [13]. The association provides the fungi with a source of fixed carbon from plant roots, while the plant receives a supply of water and essential nutrients from the mycorrhizal fungal network [12]. The fungal network consists of branching filaments of fungi, known as hyphae, that extend out in the soil  [12,13]. Hyphae have a greater surface area than roots, allowing the fungi to absorb more essential nutrients like phosphorus and nitrogen [13]. Consequently, mycorrhizal symbiosis supports early tree establishment and growth, protects hosts against pathogens, aids soil nutrient cycling, and improves host adaptations to stressors [1,8]. However, this symbiosis is threatened by high severity fire, as high soil temperatures can cause heat-induced fungal mortality. Additionally, high tree mortality can induce shifts in mycorrhizal fungal abundance because these fungi are sensitive to changes in tree species as they are strongly linked to their host [1,2,3,5,6]. The coupled effects of heat-induced fungal mortality and host mortality influence the ability of mycorrhizal fungi to recover after the event of a fire [1,2,3,4,6].

Heat-Induced Fungal Mortality

Wildfire severity is directly linked to heat-induced mortality of mycorrhizal fungi. High severity wildfires can cause an increase in soil temperatures, up to 100-700° C, that most fungi are unable to tolerate [6]. To study the correlation between fire severity and fungal mortality in boreal forests, researchers used chronosequence, a method comparing ecologically similar sites that differ in age since a disturbance, and DNA extraction [2,3,5]. The DNA was analyzed to identify the fungal taxa present in soil samples from burned and unburned sites [2,3,5]. The results suggest that wildfire severity determines the significance of the reduction in fungal biomass [3,5]. Pérez et al. (2020) found that mycorrhizal fungal richness in topsoil decreased with increasing fire severity. In burned stands of varying severity (ground fire, surface fire, and crown fire) the quantity of two types of mycorrhizal fungi, ectomycorrhizal and ericoid, decreased by 84% and 52% respectively compared to control stands in topsoil [5]. Ectomycorrhizal fungi associate with several dominant boreal plant species, including Pinaceae, Salicaceae, and Nothofagaceae, by surrounding and penetrating root cells [15]. Ericoid mycorrhizal species are restricted to associations with ericaceous host plants common in the shrub-dense understory of boreal forests [16]. Further, Franco-Manchón et al. (2019) observed a similar but more significant effect on mycorrhizal fungi exposed to high severity fire. In burned stands, there was a total absence of mycorrhizal fruiting bodies [2]. The slight variation in the results between these two studies could be attributed to differences in the location-specific ecologies of the sites, as the studies were conducted in different forests and with differences in the degree of the high severity fires. Overall, though, increased severity of fire significantly reduces mycorrhizal fungi. The loss of mycorrhizal fungi in boreal forest soils indicates a lack of adaptation to high severity wildfires.

There are some boreal mycorrhizal fungi taxa that do not experience heat-induced mortality from high severity fires. Although Pérez et al. (2020) found an overall decrease in ectomycorrhizal fungi in the topsoil of burned stands, the majority of ectomycorrhizal fungi in mineral soil (deep soil) were found to be resilient to high severity fire [5]. DeVan et al. (2023) also found one ectomycorrhizal taxa, Suillus, that was resilient to high severity fire and induced post-fire pine tree expansion in Alaska [1]. In both studies, the fungi were considered resilient if there was no significant difference in fungal richness between the burned and unburned sites [1,5]. Additionally, Pérez et al. (2020) found two ectomycorrhizal species, Sphaerosporella sp. and Laccaria oblongospora, that were abundant only in burned stands. The absence of these species in unburned stands suggests that they proliferate after fire from the soil spore bank. Therefore, high severity wildfire doesn’t result in complete loss of mycorrhizal fungi from heat-induced mortality. Some species proliferate after fire or are not significantly affected. This provides evidence of a tolerant supply of fungal diversity that may be vital for post-fire tree recovery. However, studies identifying resilient fungi are limited. Future studies should explore if the same resilient fungi are present after crown fire across all boreal forests.

Host Identity & Composition

High severity wildfires also impact mycorrhizal fungal communities by causing high tree mortality, as the fungi are strongly linked to their hosts. When host species composition changes after fire, fungal biodiversity is altered [1,5]. Research by DeVan et al. (2023) found that high severity fire had the greatest effect on black spruce and lodgepole pine hosts. Black spruce pine ectomycorrhizal fungi was significantly reduced in burned sites, allowing opportunistic fungi like ericoid mycorrhizae to colonize other host species. Consequently, ericoid mycorrhizal fungi increased with high severity fire. Researchers found higher ericaceous than ectomycorrhizal hosts in burned sites, with no ectomycorrhizal host seedlings found in high severity burn sites [1]. The replacement of ectomycorrhizal fungi with ericoid mycorrhizal fungi can cause fungal homogenization, lowering the competitiveness of black spruce pine because ericoid mycorrhizal fungi is a less beneficial symbiont than ectomycorrhizal fungi [1]. This shift from ectomycorrhizae to ericoid fungi dominance can change the natural vegetative and fungal composition, altering nutrient cycling and the resilience of boreal forests. Furthermore, the increased abundance of less beneficial symbiont species means  post-fire recovery may be more difficult.

Coniferous trees are the most common mycorrhizal hosts in boreal forests. However, less common deciduous hosts are becoming more dominant with increasing fire severity [1,10]. Research by DeVan et al. (2023) found that Aspen, a tree species native to cold regions, hosts low-diversity fungi that are resilient to high severity fire [1]. As Aspen becomes more dominant due to hosting resilient fungi, overall mycorrhizal fungal diversity decreases. The reduction in fungal diversity can threaten forest resilience. However, DeVan et al. (2023) recognized that this is one of the only studies assessing the relationship between host and fungal community dynamics in response to fire severity [1]. Therefore, more studies must be conducted to further explore this relationship.

Post-fire Colonization & Recovery

Mycorrhizal fungal colonization after wildfires directly impacts the recovery of mycorrhizal fungal communities. To study the effects of high severity wildfire on mycorrhizal community recovery, all researchers used fire chronosequence and DNA extraction. Research by Lou et al. (2023) suggests that the colonization rate of mycorrhizal fungi is significantly reduced by high severity wildfire [3]. Additional research by Franco-Manchón et al. (2019) found no mycorrhizal taxa colonization one year after fire, and only 25% colonization of mycorrhizal taxa 5 years after fire [2]. Overall, reduced colonization rates following high severity fires cause slower mycorrhizal fungal community recovery [1,2,4,6]. A different study in Canadian boreal forests found no significant difference in fungal communities between a 46 year old site and the unburned control site. The results suggest that the recovery of fungi to pre-fire levels may take a minimum of 46 years [6].

After the fungal community has recovered to pre-fire levels, mycorrhizal fungal communities can continue to increase in biodiversity. Research by Orumaa et al. (2022) found that ectomycorrhizal fungal diversity was highest in the oldest burn site 181 years after fire [4]. High severity fires cause increased mycorrhizal community recovery times. As fires become more severe and frequent, mycorrhizal communities will need more time to recover. However, they may not have enough time between disturbances to return to pre-fire levels, reducing mycorrhizal biodiversity and resilience in boreal forests.

Conclusion

The purpose of this review was to evaluate the effects of high severity fire on mycorrhizal fungi in boreal forests. Specifically, this review explored the impact of high severity wildfire on heat-induced fungal mortality, mycorrhizal hosts, and fungal post-fire recovery. The current research reveals that high severity wildfire poses a threat to the biodiversity and biomass of mycorrhizal fungi in boreal forests. High severity wildfires increase heat-induced fungal mortality and shift host identity and composition. They also reduce colonization rates and increase mycorrhizal recovery time. The effects of high severity wildfires on mycorrhizal fungi directly threaten the resilience of boreal forests; decreased resilience is problematic as climate change is increasing wildfire severity and frequency. Therefore, further research should be conducted to gain a better understanding of the mycorrhizal response to high severity wildfires in boreal forests. Additional research can provide more insight into boreal forest post-fire recovery and influence forest management practices to promote ecological resilience in boreal forests.

Author's Note

I have been fascinated by forests since I was young as I spent many summers visiting Yosemite National Park. Growing up in southern California, I grew accustomed to seeing forest fires on the news. My interest in forests and fire ecology developed further after taking ecology and forest focused courses at UC Davis. It wasn’t until reading Entangled Life: How Fungi Make Our Worlds, Change Our Minds & Shape Our Futures by Merlin Sheldrake did I begin to wonder how forest fires affect life beneath the surface. When I was tasked with writing a scientific literature review in my UWP 102B course, I was excited to learn about the effects of forest fires on the complex fungal communities in the soil. With this review, I hope to shed light on the importance of fungi in forest ecosystems and how they are impacted by wildfires as many times they go unrecognized.

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