BACKGROUND
Both abiotic and biotic factors exert selective pressures on plants (Hahn and Maron 2016). Therefore, the variation in plant traits is a reflection of the conditions in which the plants are growing (Kergunteuil et al. 2018). Plant adaptations to these selective pressures have become even more important in the face of climate change as these conditions are changing (Ayres and Lombardero 2000). Due to climate change, the ranges of insect herbivores are expanding to areas that were historically unsuitable and has resulted in naive plant populations being exposed to novel insects and pathogens (Erbilgin 2019). As climate change results in changing abiotic conditions for plants, the ability of plants to defend against novel insects and pathogens is of interest in terms of whether host plant communities will persist or be decimated as a result of climate change.
The lodgepole pine (Pinus contorta) is Alberta's provincial tree and is an ecologically important species (Safranyik and Wilson 2006). Lodgepole pine covers a wide area of western Canada, is a dominant species in some of Alberta's forests and is economically important to Alberta (Smithers 1962; Lotan and Critchfield 1990; Safranyik and Wilson 2006). Loss of lodgepole pine stands will result in a loss of revenue, loss of aesthetic value and a loss of wildlife habitat (Safranyik et al. 1974). A common enemy of the lodgepole pine and by far the most devastating is the mountain pine beetle (Dendroctonus ponderosae, MPB).
Mountain pine beetle is of current interest as a result of its impact on forests in Alberta and British Columbia, especially as the range of MPB has expanded further west and to higher altitudes due to warmer climate (Bentz et al. 2010; Cudmore et al. 2010; Safranyik et al. 2010). With climate change creating unsuitable conditions for pine trees and more suitable conditions for MPB, the ability of pine tree defences to withstand insect attacks is even more critical for the survival of these trees (Kurz et al. 2008, Raffa et al. 2013).
The lodgepole pine (Pinus contorta) is Alberta's provincial tree and is an ecologically important species (Safranyik and Wilson 2006). Lodgepole pine covers a wide area of western Canada, is a dominant species in some of Alberta's forests and is economically important to Alberta (Smithers 1962; Lotan and Critchfield 1990; Safranyik and Wilson 2006). Loss of lodgepole pine stands will result in a loss of revenue, loss of aesthetic value and a loss of wildlife habitat (Safranyik et al. 1974). A common enemy of the lodgepole pine and by far the most devastating is the mountain pine beetle (Dendroctonus ponderosae, MPB).
Mountain pine beetle is of current interest as a result of its impact on forests in Alberta and British Columbia, especially as the range of MPB has expanded further west and to higher altitudes due to warmer climate (Bentz et al. 2010; Cudmore et al. 2010; Safranyik et al. 2010). With climate change creating unsuitable conditions for pine trees and more suitable conditions for MPB, the ability of pine tree defences to withstand insect attacks is even more critical for the survival of these trees (Kurz et al. 2008, Raffa et al. 2013).
Various hypotheses have been proposed as to how plants allocate resources to either defence or to growth. For example, the optimal defence theory predicts that the allocation of defences is driven by the plant's individual overall fitness (McKey 1974). In contrast, the growth-differentiation hypothesis predicts that there is a trade-off between growth and differentiation processes, such as chemical and morphological changes, over a range of environmental conditions (Stamp 2003). In summary, it has been hypothesized that plants face a trade-off between growing in order to compete with neighboring plants and having defences against insects and pathogens.
Conifers integrate both physical and chemical defences against insect herbivores and pathogens (Figure 4) (Franceschi et al. 2005). The primary anatomical defence is through resin ducts which produce, store and translocate oleoresin (Vazquez-Gonzalez et al. 2020). Oleoresin is a complex mixture of chemicals that acts as both a physical and chemical defence (Franceschi et al. 2005; Vazquez-Gonzalez et al. 2020). ). These chemical defences include monoterpenes and diterpene resin acids, that can be stored throughout the bark, acting as a barrier to invaders and making trees less suitable (Franceschi et al. 2005; Erbilgin 2019). Non-structural carbohydrates (NSCs) are comprised of soluble sugars and starches and are essential for the production of defence chemicals in pines (Wiley et al. 2016; Roth et al. 2018). Despite the research that has been conducted on the anatomical and chemical defences in lodgepole pine, the relationship between these defences is inconclusive.
Conifers integrate both physical and chemical defences against insect herbivores and pathogens (Figure 4) (Franceschi et al. 2005). The primary anatomical defence is through resin ducts which produce, store and translocate oleoresin (Vazquez-Gonzalez et al. 2020). Oleoresin is a complex mixture of chemicals that acts as both a physical and chemical defence (Franceschi et al. 2005; Vazquez-Gonzalez et al. 2020). ). These chemical defences include monoterpenes and diterpene resin acids, that can be stored throughout the bark, acting as a barrier to invaders and making trees less suitable (Franceschi et al. 2005; Erbilgin 2019). Non-structural carbohydrates (NSCs) are comprised of soluble sugars and starches and are essential for the production of defence chemicals in pines (Wiley et al. 2016; Roth et al. 2018). Despite the research that has been conducted on the anatomical and chemical defences in lodgepole pine, the relationship between these defences is inconclusive.
Research Objectives
The objective of my research is to fill in pivotal knowledge gaps about the relationship between anatomical defences and resin-based chemical defences in lodgepole pine trees. This information will help to guide future research on lodgepole pine defences against mountain pine beetle. I hypothesize that due to the growth-differentiation hypothesis in which plants face a trade-off between allocating resources to growth or to chemical or morphological changes, plants will invest similarly to chemical and morphological defences while investing fewer resources to growth.
Expected Results
I expect there to be a positive relationship between monoterpenes and resin duct characteristics as well as between diterpenes and resin duct characteristics as trees with enough resources should be able to allocate these to both anatomical and chemical defences. I expect that there is an inverse relationship between the concentration and composition of non-structural carbohydrates and the production of resin ducts as previous research has demonstrated that non-structural carbohydrates support the production of resin ducts (Roth et al. 2018). I also expect that there will be a negative relationship between the concentration of chemical defences and radial growth.