- Blue spruce (Picea pungens) and Engelmann spruce (Picea engelmannii) experience varying levels of spruce beetle (Dendroctonus rufipennis (Kirby)) colonization, yet the underlying differences and mechanisms resulting in lower colonization for blue spruce are not known. Both spruce species have important roles in subalpine ecosystems where examining changes in mortality, distribution, and growth are key to understanding climate change effects at high elevation. I use a variety of disciplines and methodologies to better understand the differing levels of spruce beetle colonization between blue spruce and Engelmann spruce.
In Chapter 2, I evaluate mixed stands of blue spruce and Engelmann spruce trees at three sites to ascertain levels of water stress and water use efficiency at the time of the study and for the previous five years. This involved measuring water potential (Ψp) and carbon stable isotope ratios (δ¹³C) obtained from tree-ring cellulose. While water potential did not differ significantly between blue spruce and Engelmann spruce, water potential did follow predictable seasonal precipitation patterns. Carbon stable isotope ratios were corrected for atmospheric ratios (Δ¹³C) giving an estimate of discrimination, which is directly related to intrinsic water use-efficiency (iWUE). Higher iWUE implies greater relative stomatal limitation of photosynthesis. Discrimination was higher in blue spruce than Engelmann spruce. This demonstrates higher iWUE in Engelmann spruce, which may affect growth and tree carbon-based defenses related to spruce beetle colonization.
Tree physical attributes associated with spruce beetle colonization were also compared. Bark and phloem thickness may conversely provide both a barrier and a habitat for spruce beetle colonization. Bark was thicker in blue spruce compared to Engelmann spruce, while phloem was thinner for blue spruce compared to Engelmann spruce. Resin flow has been shown to encapsulate pioneering beetles, slowing or halting colonization. Resin flow was higher in blue spruce compared to Engelmann spruce. These physical attributes may indicate that blue spruce has higher constitutive defenses against spruce beetle.
Other tree attributes that influence spruce beetle colonization are related to tree chemistry, in particular terpene compounds. Volatile terpenes affect spruce beetle host tree selection, and phloem-based terpenes may affect spruce beetle aggregation as precursors to pheromones. In addition, specific phloem-based terpenes negatively affect spruce beetle survival and reproduction due to toxic effects. While species-level significance was rare in volatile terpene comparisons, trends indicate overall higher rates of volatile terpenes emanating from Engelmann spruce than blue spruce. Phloem terpenes exhibited species-level differences, with blue spruce having higher concentrations of the most abundant terpenes, potentially toxic terpenes like 3-carene, and total terpenes. These results may indicate less tree attraction and higher levels of defense for blue spruce than Engelmann spruce.
To evaluate the effect of spruce beetle vision on tree colonization, I measured bark reflectance of the two species and analyzed possible spruce beetle visual cues. Bark reflectance was higher in Engelmann spruce than in blue spruce and highest for both species in the visible spectrum. Electro-Retino Gram (ERG) results indicate spruce beetle responds to multiple wavelengths of the visual spectrum including violet/purple, blue, green, and red. Spruce beetles also responded to wavelengths outside the visual spectrum in the infrared. Due to spruce beetle responding to multiple wavelengths of light and no difference between bark reflectance trends, it is unclear if the spruce beetle makes a visual selection of Engelmann spruce rather than blue spruce.
The final part of Chapter 2 compares the effects of synthetic pheromone lures on colonization at the two sites with spruce beetle present. A subset of trees from both species were randomly selected to receive spruce beetle lures. As expected, lures significantly increased spruce beetle landing and colonization on Engelmann spruce. However, even with the presence of spruce beetle lures, fewer blue spruce trees were landed upon and colonized than Engelmann spruce.
My dissertation demonstrates a unique approach to compare spruce beetle colonization between two species, utilizing physiological, physical, chemical, and spectral components of both tree species to better understand differences in spruce beetle colonization. Change in climate may affect physiological mechanisms including carbon allocation and resin flow resulting in changes to both spruce species’ resistance to spruce beetle colonization. Factors that inhibit colonization and are more species specific, such as thin phloem, thick bark, and bark reflectance, are likely unchanged under future climate predictions. This study will provide researchers and land managers vital information and establish a knowledgebase for future scientific studies relating to spruce beetle colonization in blue and Engelmann spruce.