Defensive Traits during White Spruce (Picea glauca) Leaf Ontogeny

Area of Science:

• Plant-insect interactions
• Chemical ecology
• Forest entomology

Background:

• Herbivore feeding preferences are influenced by plant ontogeny and defense strategies.
• Optimal defense theory predicts higher investment in chemical defenses in vulnerable, developing foliage.
• Eastern spruce budworm (Choristoneura fumiferana) is a specialist feeder on conifer foliage.

Purpose of the Study:

• To investigate the ontogeny of white spruce (Picea glauca) needle toughness.
• To review the literature on defensive compounds in developing white spruce needles.
• To test predictions of optimal defense theory regarding plant defenses during leaf development.

Main Methods:

• Measurement of needle toughness and water content throughout white spruce needle ontogeny.
• Literature review of chemical defense compounds in developing and mature white spruce needles.
• Comparison of toughness between sun- and shade-grown needles.

Main Results:

• Needle toughness decreased in buds and increased in expanding leaves, with reciprocal changes in water content.
• Mature foliage toughness slightly decreased through the growing season, independent of water content.
• Literature review did not support higher chemical defenses in expanding versus mature white spruce needles.

Conclusions:

• White spruce (Picea glauca) exhibits dynamic changes in needle physical properties during ontogeny.
• The lack of increased chemical defenses in young needles suggests a potential fast-growth, low-defense strategy.
• This challenges the universal applicability of optimal defense theory in explaining plant defense allocation.