Biochemistry, physiology, and ecology of paper birch defenses to bronze birch borer and their responses to anthropogenic greenhouse gases

Periodic outbreaks by bronze birch borer (Agrilus anxius) have caused widespread birch (Betula) mortality in the boreal forests of North America. Mechanisms by which deciduous trees resist wood-borers are not understood but have been hypothesized to result from integrated physical and biochemical phloem defenses. Historically, wood-borer outbreaks have been associated with environmental stress, which is thought to weaken tree defenses. Therefore, the overarching goals of this dissertation research were to characterize birch defenses to bronze birch borer, their relationship to whole plant physiology, and how their expression is affected by environmental stress.

Objectives of this research were to use a metabolomics approach to compare the constitutive (Chapter 2) and induced (Chapter 3) biochemical profiles of paper birch (B. papyrifera) phloem, which is resistant to bronze birch borer by nature of its coevolutionary history, with that of European white birch (B. pendula), which is much more susceptible. Then, to investigate the relationship of these putative defenses to whole plant physiology, paper birch stems were girdled to disrupt transport of current photosynthate and alter within-tree resistance to bronze birch borer (Chapter 4). Finally, to determine how environmental factors modify expression of resistance, the effects of elevated CO₂ and ozone (O₃) on paper birch resistance and bronze birch borer colonization were analyzed (Chapter 5).

Rate of wound periderm (callus) formation did not differ between paper birch and European white birch, but may contribute to resistance if it interacts with defensive chemicals or nutritional factors that slow feeding, allowing wound periderm to encapsulate larvae. Constitutive and induced phenolic profiles of paper birch and European white birch differed. Paper birch had two phenolics that were not detected in European white birch, and concentrations of six were higher in paper birch than European white birch, which may contribute to higher resistance of paper birch to bronze birch borer.

Trunk girdling decreased phloem resistance to bronze birch borer below the girdle, confirming that stress compromises resistance and supporting the hypothesis that the flux of current photosynthate from leaves to trunk provides the energy to drive defensive responses. Total phenolic concentrations and rate of wound periderm formation were higher above the girdle, corresponding with higher resistance in that portion of the tree.

Elevated atmospheric CO₂ and O₃ decreased paper birch resistance to bronze birch borer in a four-year study at the Aspen FACE facility in Rhinelander, WI. In combination, however, elevated CO₂ and O₃ had no effect on paper birch resistance, which is consistent with previous studies showing elevated CO₂ to ameliorate effects of elevated O₃ on tree physiology. Elevated CO₂ and O₃ altered phloem chemistry but not in ways that could explain patterns of bronze birch borer colonization. The results suggest that on-going changes in atmospheric composition could facilitate bronze birch borer outbreaks, possibly altering the distribution of paper birch in North America.