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

Area of Science:

• Forest Ecology
• Tree Physiology
• Insect-Plant Interactions

Background:

• Bark beetle-induced tree mortality is a significant ecological and economic issue.
• The interplay between water stress and tree defenses against bark beetles is not well understood.

Purpose of the Study:

• To investigate how water stress and induced defenses interact to influence ponderosa pine mortality from bark beetle attacks.
• To determine the physiological and chemical responses of ponderosa pines to simulated bark beetle attacks under varying water stress levels.

Main Methods:

• A three-year factorial experiment was conducted on 48 mature ponderosa pines in Arizona.
• Manipulations included water stress (root cutting, snow removal), bark beetle attack simulation (pheromone lures), and phloem biota inoculation.
• Tree responses measured: resin flow, phloem terpene composition, xylem water potential, leaf gas exchange, and survival.

Main Results:

• Bark beetle attacks and associated biota increased phloem mono- and sesqui-terpenes; water stress did not significantly alter these concentrations.
• Resin flow induction by bark beetle attack occurred in unstressed trees but was suppressed in water-stressed trees.
• Water-stressed, beetle-attacked trees exhibited the highest mortality, preceded by reduced resin flow and water stress indicators.

Conclusions:

• Ponderosa pines can induce resin flow and phloem terpenes in response to bark beetles, but resin flow induction is constrained by water stress.
• Water stress experimentally predisposes ponderosa pines to mortality when subjected to bark beetle attacks.
• This research highlights the critical role of water availability in tree defense efficacy against insect herbivores.