Light acclimation optimizes leaf functional traits despite height-related constraints in a canopy shading experiment

Area of Science:

• Forest Ecology
• Plant Physiology
• Leaf Trait Analysis

Background:

• Within-canopy gradients in leaf functional traits are associated with light and water potential.
• Experimental manipulations are crucial for understanding drivers of leaf trait variation.
• Sugar maple (Acer saccharum) forests exhibit complex vertical gradients.

Purpose of the Study:

• To experimentally disentangle the effects of height and light on leaf functional traits in sugar maple.
• To investigate the interplay between light availability, height, and leaf morphology/chemistry.

Main Methods:

• Experimental shading of branches along vertical gradients in a sugar maple forest.
• Measurement of leaf mass per area (LMA), leaf density, nitrogen content (N(area), N(mass)), C:N ratio, and carbon isotope composition (δ(13)C).
• Assessment of midday leaf water potential (Ψ(mid)) throughout the growing season.

Main Results:

• Shading reduced LMA, leaf density, N(area), and C:N ratio, while increasing N(mass), confirming light's role in leaf morphology and chemistry.
• Height was the primary driver for Ψ(mid), LMA, and N(area) early in the season; light became dominant later.
• Carbon isotope composition (δ(13)C) correlated with height, not shading, indicating height's greater influence on water use efficiency.

Conclusions:

• Stomatal sensitivity to vapor pressure deficit or Ψ(mid) limits CO2 supply to upper canopy leaves, irrespective of light.
• LMA and N(area) are optimized by morphological acclimation to light with increasing leaf age, despite height constraints.
• Seasonal changes in leaf traits are as significant as microclimatic conditions, highlighting leaf age effects.