Background & Rationale

Mountain pine beetle (MPB)–associated fungi determine whether beetle attacks on lodgepole pine result in successful colonization and host mortality (Cale et al., 2019; DiGuistini et al., 2011). During attacks, beetles introduce fungi into the phloem, where they contribute to tree death by detoxifying host defences (Rice et al., 2007a; 2007b; Zaman et al., 2023). Unlike mechanically induced damage, these fungi interact directly with host chemical defences, effectively hijacking host biochemistry and immune responses (Wang et al., 2013; 2014). Despite their central role in shaping forest vulnerability and resilience, MPB-associated fungi have rarely been studied independently of the beetle. This underscores the value of assessing fungal performance on its own, as it reveals how fungal activity shapes host susceptibility through biochemical mechanisms, offering insight into tree vulnerability beyond the direct physical effects of beetle colonization. 

Figure X. Spatial representation of the mountain pine beetle (Dendroctonus ponderosae) outbreak from 1999 to 2007 (a, b, c), and the annual area affected since 1928 (d) in western Canada (AB, Alberta; BC, British Columbia) (data from the Canadian Forest Service Forest Insect and Disease Survey and the British Columbia Ministry of Forests and Range) (Safranyik et al., 2010). 

Lodgepole pine exhibits geographic variation in phloem chemistry, particularly in the composition and relative abundance of monoterpenes that form the tree’s primary chemical defence against insects and pathogens Celedon and Bohlmann 2019; Moreira et al., 2014; Sampedro et al., 2011; Smith 1967). This variation reflects specific adaptations to local climatic conditions and to historical exposure to MPB and its fungal associates (Erbilgin 2019; Erbilgin et al., 2007; Keeling and Bohlmann 2006; Raffa et al., 2008; 2013). 

Within regions where lodgepole pine and MPB have coevolved, lodgepole pine populations are expected to possess phloem chemistries that more effectively suppress fungal growth. In contrast, populations at the northern and colder regions of the species’ range, may not have evolved these defences, making them more vulnerable (Erbilgin 2019; Forrest 1980; Liu et al., 2025; Raffa et al., 2017).

Ongoing climate warming allows MPB to migrate and attack lodgepole pine populations further north (Safranyik et al., 2010). As beetles and their fungal symbionts increasingly encounter lodgepole pine populations with limited coevolution, a maladaptation in the host’s chemical defenses may increase vulnerability and mortality (Ullah et al., 2021).

Figure Y. Spatial distribution of the lodgepole pine (Pinus contorta) subspecies. Green for P. contorta subsp. latifolia (Rocky mountain lodgepole pine), red for P. contorta subsp. contorta (Shore pine), and P. contorta subsp. murrayana (Sierra lodgepole pine) (Little 1971).

Understanding how climate-associated variation in lodgepole pine phloem chemistry influences fungal performance is therefore essential for explaining spatial patterns of vulnerability and resilience, and for anticipating the consequences of continued beetle range expansion under future climates (Erbilgin et al., 2017). Forest managers facing ongoing MPB expansion must determine where to intervene and how to source planting material for regeneration. The effectiveness of these strategies depends on whether lodgepole pine populations differ meaningfully in resistance (Ullah et al., 2021).

If I find that northern populations exhibit weaker chemical suppression of fungal growth, foresters can treat these populations as high-risk systems, prioritizing early detection and conservative regeneration strategies (Sharma 2011). Alternatively, if coevolved lodgepole pine populations demonstrate stronger fungal resilience, foresters could introduce genetically resilient populations into areas with limited coevolution through seed transfer or assisted migration (Pedlar et al., 2012; Xu and Prescott 2024). In this scenario, resilient populations could serve as sources of adaptive traits to enhance forest resilience in regions projected to face increased beetle pressure.

To support these management decisions, the following research objectives were explored:

1. Determine whether lodgepole pine populations from Oregon, British Columbia, and Alberta differ in their chemical capacity to suppress the growth of MPB–associated fungi.

2. Assess whether lodgepole pine populations with long-term historical exposure to MPB exhibit greater chemical resilience to fungal symbionts than naïve populations from colder and more northerly regions. 

3. Identify lodgepole pine populations that may represent lower or higher vulnerability to MPB-associated fungi, providing a basis for prioritizing monitoring, seed sourcing, and adaptive management.