Year of Award

2016

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Anna Sala

Commitee Members

Lila Fishman , Cara Nelson

Subject Categories

Ecology and Evolutionary Biology

Abstract

Despite formidable tree defenses against insect pests, including resin produced in resin ducts, bark beetles have recently caused extensive outbreaks in western North American forests. To date, our understanding of the extent to which resin duct defenses remain effective at the peak of an outbreak, the relative genetic vs. environmental controls on resin duct production, the degree to which drought reduces resin defenses in naturally occurring outbreaks, and whether and when defense production competes for resources with growth remains limited. Resolving these questions is relevant in the face of climate change, and increased risk of outbreaks. I took advantage of a 40-year-old ponderosa pine (Pinus ponderosa) genetic trial where a recent mountain pine beetle outbreak (Dendrochtonus ponderosae) killed 36% of trees. I examined phenotypic and genetic variation in resin ducts, their consequences on survival during the outbreak, the influence of drought on resin duct defenses, and potential growth-defense tradeoffs. Using dendrochronology, I retrospectively measured growth and resin duct traits in thirty families from low to high mortality from before to during the outbreak. Resin duct defenses significantly decreased mortality during the peak of the outbreak both at the phenotypic and genetic levels. However, differences in resin duct defenses between trees that survived and those that died and between high and low mortality families were small prior to the outbreak but were amplified during the outbreak, which was preceded by a drought. Resin duct production, but not size, was under strong genetic control. Analyses at both the phenotypic and genetic levels indicated that drought significantly predisposed some trees and families to mortality via reductions of resin duct defenses, particularly duct size. I found no evidence of a resin duct defense-growth tradeoff. Rather, faster growing trees and families produced more resin ducts than slower ones. Despite this, however, faster growing families did not suffer lower mortality. These contradictory results suggest breeding for fast growth increases resin duct defenses and survival, but at high beetle densities during an outbreak, other factors possibly involving induced defenses, appear to ultimately offset this effect.

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© Copyright 2016 Sean Pinnell