Drought-Mediated Changes in Tree Physiological Processes Weaken Tree Defenses to Bark Beetle Attack

  • Thomas KolbEmail author
  • Ken Keefover-Ring
  • Stephen J. Burr
  • Richard Hofstetter
  • Monica Gaylord
  • Kenneth F. Raffa


Interactions between water stress and induced defenses and their role in tree mortality due to bark beetles are poorly understood. We performed a factorial experiment on 48 mature ponderosa pines (Pinus ponderosa) in northern Arizona over three years that manipulated a) tree water stress by cutting roots and removing snow; b) bark beetle attacks by using pheromone lures; and c) phloem exposure to biota vectored by bark beetles by inoculating with dead beetles. Tree responses included resin flow from stem wounds, phloem composition of mono- and sesqui-terpenes, xylem water potential, leaf gas exchange, and survival. Phloem contained 21 mono- and sesqui-terpenes, which were dominated by (+)-α-pinene, (−)-limonene, and δ-3-carene. Bark beetle attacks (mostly Dendroctonus brevicomis) and biota carried by beetles induced a general increase in concentration of phloem mono- and sesqui-terpenes, whereas water stress did not. Bark beetle attacks induced an increase in resin flow for unstressed trees but not water-stressed trees. Mortality was highest for beetle-attacked water-stressed trees. Death of beetle-attacked trees was preceded by low resin flow, symptoms of water stress (low xylem water potential, leaf gas exchange), and an ephemeral increase in concentrations of mono- and sesqui-terpenes compared to surviving trees. These results show a) that ponderosa pine can undergo induction of both resin flow and phloem terpenes in response to bark beetle attack, and that the former is more constrained by water stress; b) experimental evidence that water stress predisposes ponderosa pines to mortality from bark beetles.


Dendroctonus Drought Pinus ponderosa Terpenes Tree chemical defense Induced defenses Tree mortality 



This work was supported by McIntire-Stennis Program project accession no. 230732 from the USDA National Institute of Food and Agriculture. John Kaplan, Ansley Roberts, Teresa Reyes, and Patrick Dunn provided valuable help in the field and laboratory; Roberts and Reyes were supported by the Hooper Undergraduate Research Program at Northern Arizona University and the National Science Foundation Research Experience for Undergraduates Program, respectively. The Northern Arizona University Centennial Forest provided the study site.

Supplementary material

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Authors and Affiliations

  1. 1.School of ForestryNorthern Arizona UniversityFlagstaffUSA
  2. 2.Departments of Botany and GeographyUniversity of Wisconsin-MadisonMadisonUSA
  3. 3.Forest Health ProtectionUSDA Forest ServiceFairbanksUSA
  4. 4.Forest Health ProtectionUSDA Forest ServiceFlagstaffUSA
  5. 5.Department of EntomologyUniversity of Wisconsin-MadisonMadisonUSA

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