SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Area of Science:

• Plant Physiology
• Forest Ecology
• Photosynthesis Research

Background:

• Deciduous hardwood trees exhibit seasonal changes in leaf function.
• Understanding these changes is crucial for forest productivity and carbon cycling.

Purpose of the Study:

• To determine seasonal patterns of leaf photosynthetic capacity and conductance in deciduous hardwood tree species.
• To compare these patterns between canopy and understory leaves.

Main Methods:

• Measurements of CO2 Exchange Rate (CER) under light-saturation and low light conditions.
• Assessment of dark respiration rates and leaf conductance.
• Monitoring of leaf intercellular CO2 mole fractions (ci) throughout the season.

Main Results:

• Canopy leaves (bigtooth aspen, red oak) showed higher maximum CER (10-15 μmol m⁻² s⁻¹) than understory leaves (4-5 μmol m⁻² s⁻¹).
• CER peaked in early June, remained constant until mid-September, then declined. Respiration was highest in young leaves.
• Leaf conductance was stable until autumn, with canopy leaves having higher water vapor conductance (2-5 mm/s) than understory leaves (1-2 mm/s).

Conclusions:

• Leaf photosynthetic capacity and conductance vary seasonally and with canopy position in deciduous trees.
• These physiological adjustments influence overall forest carbon assimilation and water relations.