First Advisor

Scott Franklin

First Committee Member

Susana Karen Gomez

Second Committee Member

Javier E. Mercado

Degree Name

Master of Science

Document Type

Thesis

Date Created

12-2020

Department

College of Natural and Health Sciences, Biological Sciences, Biological Sciences Student Work

Abstract

Bark beetles are important native insect pests to conifer forests in North America and their irruptive beetle populations are of serious concern both ecologically and economically. As bark beetles bore into the phloem of a host tree to excavate brood galleries, they simultaneously deposit spores of blue-stain fungi. Both the partial girdling of the tree’s phloem by beetles and the blockage of xylem by fungi can overcome the tree’s defenses and cause its death. To combat colonization by insects and pathogens, conifers allocate large amounts of metabolic resources into defense mechanisms, of which, resin ducts play important roles. As resin ducts are severed by invading organisms, high pressure oleoresin is forced out of the wound, either expelling or trapping the invader within the resin, or killing it with its toxic components. Until recently, the physiological and anatomical responses of Douglas-fir to bark beetle infestation and blue-stain infection has been underrepresented in the literature. This thesis aimed to explore the mechanisms associated with Douglas-fir mortality following pheromone mediated Douglas-fir beetle attack as well as to compare the relative strength of defense response of Douglas-fir and lodgepole pine to a simulated bark beetle fungal associate colonization.

The first study aimed to determine whether blue-stain fungi are a major factor in the rate of physiological decline of beetle-infested Douglas-fir trees. Our hypothesis was that beetle-attacked trees would exhibit more rapid declines in physiological measurements than mechanically girdled trees. This was tested by comparing sap flux and leaf water potential values of Control trees, mechanically Girdled trees, and Attacked trees, which were baited with pheromones to attract Douglas-fir beetles. Estimates of transpiration, using Granier sap flow sensors, and mid-canopy pre-dawn water potential were recorded before and after beetle attack, through two growing seasons. Results from the first growing season showed that only a few Attacked trees showed changes in transpiration by the end of the season in late October of 2018, requiring us to split this treatment into two separate groups: Beetle-Attacked and Beetle-Killed. By the second growing season, the Control, Girdled, and Beetle-Attacked treatments showed a larger increase in transpiration and stem water potential in response to the newly available soil moisture than the Beetle-Killed trees, indicating that blockage of xylem by blue-stain fungi had affected water transport in the Beetle-Killed trees. These results showed that Beetle-Killed Douglas-fir do decline more rapidly than Girdled trees, supporting our hypothesis; however, these results brought up several more questions for future investigations: are the differences between the Beetle-Killed and the Beetle-Attacked trees do to beetle attack density, tree physiology, fungal load, or fungal community makeup/ dynamics?

The results of the Douglas-fir mortality study inspired a closer look at differences in the physiological and anatomical defense responses of Douglas-fir and lodgepole pine against the invasion of their respective bark beetles and associated fungi (as simulated by inoculations). The hypotheses were (1) that Douglas-fir would produce a higher density of larger traumatic resin ducts (TRDs) following inoculation than lodgepole pine, and (2) that the highest densities and sizes of resin ducts would be seen in the trees inoculated with blue-stain fungi when compared to other treatments. Seventy trees were selected and assigned to seven treatment groups that included: blue-stain fungi inoculations, non-bluestain fungi inoculations, sterile agar “blank” inoculations, or were left uninoculated. Predawn leaf water potential was measured periodically following inoculation until the end of the season in October of 2019. Tissue samples, including the newest three growth rings were collected two months after inoculation and were analyzed under light microscopy for the formation, density, and position of traumatic resin ducts. Formation of traumatic resin ducts were seen in most of the inoculated Douglas-fir trees. These were visible as a well-defined line of ducts that were estimated to have formed at the same time. The lodgepole pines, however, did not show significant response to wounding or inoculation. No significant differences were observed in the relative location of TRDs within the current annual ring or in leaf water potential between any of the treatments or between species. Inoculated Douglas-fir produced an average of six times more resin ducts than lodgepole pine. The resin ducts produced by Douglas-fir were, on average, almost twice as large on average than those formed by lodgepole pine. Significant treatment effects among inoculation types were limited to Douglas-fir, with the Weak inoculations unexpectedly producing higher resin duct densities than the Control inoculations. All inoculation treatments in Douglas-fir exhibited significantly higher resin duct production than Uninoculated trees. Results from these studies indicate that different conifer species respond differently to bark beetle attack and that different bark beetle fungi associates may elicit distinct defense responses within the same tree species. Understanding the factors involved in conifer mortality and defense responses to bark beetle attack could provide forest managers with vital information to help maintain resiliency within our forests as the climate continues to change.

Abstract Format

html

Language

English

Extent

75 pages

Rights Statement

Copyright is held by the author.

Digital Origin

Born digital

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