Results & Discussion 

Lodgepole pine chemical defences differentially suppress fungal growth

Lodgepole pine monoterpene blends differed significantly in their ability to suppress fungal growth (Kruskal–Wallis test: χ² = 52.35, df = 2, p < 0.001) (Fig 6). Pairwise comparisons revealed that coevolved populations from Oregon strongly inhibited fungal growth, whereas evolutionarily naïve northern populations from Alberta exhibited weaker inhibitory effects (Z = −2.66, p = 0.016). Although less inhibitory than Oregon populations, Alberta populations still showed significant suppression relative to the ethanol control (Z = −2.35, p = 0.032). Unexpectedly, British Columbia populations exhibited weaker inhibition than the naïve Alberta populations, despite their long-term historical exposure to MPB (Z = 2.09, p = 0.046). Consistent with this pattern, British Columbia monoterpene chemistry did not differ significantly from the ethanol control in its effect on fungal growth (Z = 0.38, p = 0.76) (Fig 6).

Figure 6. Box plots displaying overall fungal biomass of G. clavigera and O. montium under different lodgepole pine defences. Lower case letters above box plots indicates significant groupings. 

Fungal volatiles can inform about fungal growth and lodgepole pine vulnerability

Different lodgepole pine populations also differentially influence volatile emissions by MPB-associated fungi. Fungi emitted higher concentrations of verbenone and cis-grandisol under Alberta and Oregon chemistries (conditions that were more inhibitory to fungal growth), suggesting that these volatiles may negatively influence fungal performance. Verbenone concentrations were significantly higher under Alberta chemistry than under all other amendments: ethanol (Z = 9.51, p < 0.001), British Columbia (Z = 6.50, p < 0.001), and Oregon (Z = 3.24, p < 0.001) (Fig. 7a). Similarly, cis-grandisol concentrations were elevated under Alberta chemistry relative to all other amendments: ethanol (Z = 7.62, p < 0.001), British Columbia (Z = 7.62, p < 0.001), and Oregon (Z = 4.25, p < 0.001) (Fig. 7b).

In contrast, isobutanol was positively associated with fungal growth, with higher concentrations observed under less inhibitory chemistries (ethanol and British Columbia). Isobutanol levels were significantly higher under ethanol and British Columbia chemistries than under Alberta and Oregon chemistries (ethanol–Alberta: Z = 4.42, p < 0.001; ethanol–Oregon: Z = 1.57, p < 0.001; British Columbia–Alberta: Z = 4.11, p < 0.001; British Columbia–Oregon: Z = 1.25, p < 0.001) (Fig 7c).

Figure 7. Multi-panel bar charts of mean fungal volatile concentrations under different lodgepole pine defences. Standard errors are of 95% confidence interval.

Conclusion & Implications

This study demonstrated that lodgepole pine populations differ substantially in their chemical capacity to suppress MPB-associated fungi based on their historical exposure to the MPB. Monoterpenes from Oregon populations, most strongly inhibited fungal growth, which is consistent with the expectation that long-term MPB exposure has favoured more effective chemical defences. Alberta populations exhibited weaker fungal suppression, supporting concerns that colder, historically beetle-limited regions may be more chemically naïve. Although Alberta populations reduced fungal growth relative to controls, they may be insufficient to fully constrain fungal establishment under increased attack pressure.

In contrast to expectations, British Columbia populations exhibited the weakest fungal inhibition despite long-term beetle exposure, indicating that historical beetle presence alone does not guarantee strong chemical resilience. This suggests that regional climate or selection pressures can drive divergent defence strategies, and that forest vulnerability cannot be inferred solely from geography or beetle history.

Fungal volatile responses mirrored these growth patterns. More inhibitory host chemistries were associated with elevated verbenone and cis-grandisol, whereas less inhibitory chemistries favoured high production of isobutanol, a compound well-known to support cell growth in microbes (Hazelwood et al. 2008). These results indicate that host chemistry influences not only fungal growth but also fungal signaling environments. Foresters can utilize the presence or absence of these volatiles to inform forest resilience.

Overall, this study provides evidence that climate-associated variation in lodgepole pine chemistry shapes fungal performance in ways that directly affect forest vulnerability and resilience. These findings support two key management strategies: 1) lodgepole pines in northern regions (Alberta) exhibit weaker chemical defences and should be prioritized for early monitoring and conservative regeneration (Sharma 2011); 2) Oregon populations with stronger defences could contribute adaptive traits to Alberta forests through the introduction of seeds, pollen, or nursery-grown seedlings, helping to enhance resilience as beetle pressure continue to increase (Pedlar et al., 2012; Safranyik et al., 2010; Xu and Prescott 2024).