Defoliation constrains xylem and phloem functionality.

Area of Science:

• Plant physiology
• Forest ecology
• Tree vascular biology

Background:

• Insect defoliation is a known stressor contributing to tree mortality, particularly under drought conditions.
• The impact of defoliation on tree vascular structure and its subsequent vulnerability to drought remains understudied.

Purpose of the Study:

• To investigate the physiological and anatomical responses of balsam poplar to manual defoliation.
• To assess how defoliation affects vascular transport efficiency and vulnerability to embolism.
• To elucidate the structural changes in xylem and phloem following defoliation and re-foliation.

Main Methods:

• Manual defoliation of 2-year-old balsam poplar trees.
• Measurement of hydraulic conductivity to assess water transport efficiency and embolism vulnerability.
• Anatomical analysis of xylem and phloem structures in stems, shoot tips, and petioles.

Main Results:

• Defoliated trees exhibited increased vulnerability to embolism and reduced hydraulic conductivity, linked to misshapen xylem vessels.
• Phloem sieve tube diameter was reduced in defoliated trees, indicating impaired phloem transport capacity.
• A reduction or absence of phloem fibers was observed in various plant parts, potentially compromising vascular tissue stability.

Conclusions:

• Defoliation induces significant alterations in tree vascular structure, increasing susceptibility to hydraulic dysfunction and drought-induced mortality.
• These vascular changes, affecting both xylem and phloem, highlight a mechanism by which carbon limitation can exacerbate hydraulic stress.
• Understanding these defoliation-induced effects is crucial for predicting forest responses to climate change and insect outbreaks.