Drought and defoliation-induced alterations to the structure and function of the vascular tissue in trees

  • Author / Creator
    Hillabrand, Rachel
  • Observations of tree dieback and mortality from climate change-induced drought has been documented across the world, however the mechanisms of tree mortality are not fully understood. Infestations of insect pests may also contribute to the process of drought-induced dieback. Tree physiological processes are linked to the vascular tissue which provides the transport of water and sugars through the xylem and phloem. These processes are critical for tree functioning; recent research has emphasized the importance of xylem functional traits as predictors of mortality. The determination of structural and functional changes to the vascular tissue induced by drought and defoliation can provide evidence for how these stress events increase the vulnerability of trees to dieback and mortality. This thesis advances this objective through different research approaches using the species trembling aspen (Populus tremuloides Michx.) and balsam poplar (Populus balsamifera L.): firstly, by examining drought-stressed trees to provide evidence for previously hypothesized xylem pit membrane damage, secondly, by examining tree rings to discover unknown xylem structural changes due to drought and defoliation, and thirdly, by examining defoliated trees for a comprehensive look at alterations in xylem and phloem structure and function. Results of these analyses indicated a variety of structural and functional changes due to drought and defoliation. Drought-induced, irreversible damage to xylem pit membranes was observed and interpreted as a cause for the phenomenon of cavitation fatigue. The examination of tree rings revealed that the mean xylem vessel diameters correlated more strongly to precipitation when the trees were young, than when they were mature. Additionally, tree rings showed more pronounced alterations to xylem structure in response to insect defoliation than to drought. Xylem structural changes due to defoliation increased the vulnerability of trees to cavitation, and defoliation reduced sieve tube diameters and fiber area in the phloem. Altogether, these data show that vascular tissue alterations due both drought and defoliation increase the vulnerability of trees to drought-induced dieback. Mature trees may be less sensitive to drought than young trees, but when drought is severe, drought promotes mortality through irreversible damage to the xylem pit membranes. The resource limitation resulting from insect defoliation contributes to increased risk for tree physiological dysfunction by reductions in tree hydraulic safety and efficiency through xylem structural changes, and reduced phloem transport efficiency and reduced mechanical strength in stems through phloem structural changes.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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