Results & Discussion 

Fire effect on fungal communities

Lodgepole pine

Following wildfire in 2023, fungal community responses differed across burn severities. High-severity sites caused change in composition, whereas low and medium severity communities were unaffected (Fig 8). High-severity fire significantly reduced fungal richness, shifting to dominance by fewer fungal taxa (Fig 8). These taxa, Saccharomycetales and Russulales, are primarily saprotrophs (wood-decomposing fungi), but many exhibit flexible lifestyles and can also function as pathogens (disease-inducing fungi) (Paloi et al., 2023; Suh et al., 2006). This shift is commonly observed in severely weakened or late-stage trees, where a variety of substrates become available for fungal colonization (Boddy 1988; Fukasawa 2018).

Figure 8. Top 5 fungal orders in fire-affected lodgepole pine across burn severities in 2023. All burn severities were compared to controls in 2023 as a baseline of pre-disturbance sapwood fungal communities.

White spruce

Like lodgepole pine, white spruce fungal community responses differed across burn severities. High severity sites were again, the only ones affecting community composition, whereas low and medium severity communities were not affected (Fig 9). These communities differed from healthy controls, shifting toward fungi that can both decompose wood and cause disease (Fig 9). In this case, the dominant groups were Russulales and Diaporthales, rather than Saccharomycetales.

Although Diaporthales and Saccharomycetales are both heterotrophic, they differ in feeding styles (Rossman et al., 2007; Suh et al., 2006), suggesting possible differences in the structure of spruce and lodgepole pine wood. Nevertheless, both species are highly vulnerable to high-severity fire and less affected by low or medium fire. This is likely because thicker bark helps protect the cambium and internal tissues (Bär and Mayr 2020).

Figure 9. Top 5 fungal orders in fire-affected white spruce across burn severities in 2023. All burn severities were compared to controls in 2023 as a baseline of pre-disturbance sapwood fungal communities.

Trembling aspen

Trembling aspen fungal communities responded differently from those in lodgepole pine and spruce (Fig 10). There was a shift in community composition across all burn severities, and each severity level was associated with a reduction in fungal richness (fewer taxa) (Fig 10), likely reflecting a loss of resources for fungi.

Most interestingly, within-group dispersion was much higher across all burn severities compared to healthy controls (see Fig 4 of “Data” page), suggesting that individual trees responded very differently to fire exposure.

Across all severities, communities were dominated by Russulales, with high-severity sites showing a slight increase in Xylariales. Both groups are decomposers that typically colonize already decaying wood (Pastorelli et al., 2022). While this pattern was only expected under high-severity fire, its presence at low and medium severities may be due to the thinner bark of aspen, which provides less protection to internal tissues from heat damage (Bär and Mayr 2020).

Figure 10. Top 5 fungal orders in fire-affected trembling aspen across burn severities in 2023. All burn severities were compared to controls in 2023 as a baseline of pre-disturbance sapwood fungal communities.

Changes in fungal communities after one-year post-fire

Lodgepole pine

One-year post-fire, lodgepole pine fungal communities shifted across all burn severities.

Under low-severity fire, communities shifted towards pathogenic Ophiostomatales (Fig 11). These fungi are commonly blue-stain and introduced by secondary bark beetles that are attracted to weakened pine trees (Solheim 1992).

Under medium severity, communities shifted toward saprotrophic Russulales (Fig 11), suggesting that stem continued to deteriorate with little indication of recovery.

Under high severity, saprotrophic communities continue continued to dominate and decompose wood, along with the reappearance of additional fungal taxa (Fig 11). The initial dominance by a few taxa immediately after fire, followed by an increase in the number of taxa after one year, suggests that heat from the fire may have initially reduced fungal diversity, with communities becoming more diverse again after new resources became available (Barbero-López et al., 2019).

Medium and high fire severities showed little development of blue-stain fungi compared to low severity (Fig 11), suggesting that the wood may be too damaged to support bark beetle attack.

Figure 11. Changes in top 5 fungal orders one year after fire in lodgepole pine across burn severities from 2023 to 2024. Each burn severity in 2023 was compared to those in 2024.  

White spruce

One year after fire, spruce fungal communities showed a different pattern than lodgepole pine (Fig 12). Community composition shifted towards fewer taxa and is dominated by saprotrophic fungi (Fig 12), suggesting intensified decomposition by specialist fungi (Semenova-Nelsen et al., 2019).

Low and medium severity sites also showed relatively little development of blue-stain fungi (Fig 12). This is unexpected because there are many spruce bark beetle species that are associated with secondary disturbance. Hence it may indicate that spruce-associated fungi use different infection mechanism compared to those in lodgepole pine.

Under high severity fire, additional fungal taxa reappeared, like lodgepole pine. This reflects an initial loss of fungi due to heat but then recolonized.

Figure 12. Changes in top 5 fungal orders one year after fire in white spruce across burn severities from 2023 to 2024. Each burn severity in 2023 was compared to those in 2024.  

Trembling aspen

Unlike conifers, aspen fungal communities showed little to no change one year after fire (Fig 13). Under all fire severities, community composition continued to be dominated by Russulales, with a slight increase in Hymenochaetales (Fig 13). Emergence of this taxon suggests a transition toward more advanced wood decay (Semenova-Nelsen et al., 2019). 

Figure 13. Changes in top 5 fungal orders one year after fire in trembling aspen across burn severities from 2023 to 2024. Each burn severity in 2023 was compared to those in 2024.  

Implications

These findings show that sapwood fungal communities can be early indicators of tree fate beyond visible damage.

For lodgepole pine, the development of blue-stain fungi after one-year post-low severity fire, suggests that low fires can increase the tree’s risk of secondary attack by bark beetles, causing decline in timber quality and widespread delayed mortality. This indicates that these trees should be prioritized for monitoring and early salvage to prevent fungal spread to other nearby pine forests. In contrast, medium and high fire severity show dominance by saprotrophic fungi, reflecting ongoing wood breakdown. This suggests limited recovery and the need for immediate salvage to reduce the risk of tree fall over on wildlife.

For spruce, the delayed reduction in community richness and dominance by few decomposers suggests that even low and medium fire may lead trees towards decline despite appearing less damaged. This indicates that if retention decisions were made based on visual assessment in the first year, we may greatly underestimate mortality risk. So spruce trees under low and medium severity fire should be further monitored to decide. Trees under high severity however will need to be salvaged immediately, like lodgepole pine.

Aspen fungal communities show a slower and more unpredictable response post-fire regardless the severity. In the context of the forest sector, aspen trees are less prominent as timber but is a good fire break, so retaining wood quality is less important and instead we should focus on conservation. These trees are most suitable for prolonged monitoring and delayed intervention.

Tracking fungal community changes over time can improve the timing of management decisions. Incorporating these indicators to post-fire assessments can better distinguish which trees will survive versus decline, hence refine forest conservation.