Canopy position affects photosynthesis and anatomy in matureEucalyptustrees in elevated CO₂

Abstract

Leaves are exposed to different light conditions according to their canopy position, resulting in structural and anatomical differences with consequences for carbon uptake. While these structure-function relationships have been thoroughly explored in dense forest canopies, such gradients may be diminished in open canopies, and they are often ignored in ecosystem models. We tested within-canopy differences in photosynthetic properties and structural traits in leaves in a matureEucalyptus tereticorniscanopy exposed to long-term elevated CO₂ for up to 3 years. We explored these traits in relation to anatomical variation and diffusive processes for CO₂ (i.e., stomatal conductance,g_s, and mesophyll conductance,g_m) in both upper and lower portions of the canopy receiving ambient and elevated CO₂. While shade resulted in 13% lower leaf mass per area ratio (M_A) in lower versus upper canopy leaves, there was no relationship between leaf nitrogen concentration (N_mass) and canopy gap fraction. Both maximum carboxylation capacity (V_cmax) and maximum electron transport (J_max) were ˜18% lower in shaded leaves and were also reduced by ˜22% with leaf aging. In mature leaves, we found no canopy differences forg_m org_s, despite anatomical differences inM_A, leaf thickness and mean mesophyll thickness between canopy positions. There was a positive relationship between net photosynthesis andg_m org_s in mature leaves. Mesophyll conductance was negatively correlated with mean parenchyma length, suggesting that long palisade cells may contribute to a longer CO₂ diffusional pathway and more resistance to CO₂ transfer to chloroplasts. Few other relationships betweeng_m and anatomical variables were found in mature leaves, which may be due to the open crown ofEucalyptus. Consideration of shade effects and leaf-age-dependent responses to photosynthetic capacity and mesophyll conductance are critical to improve canopy photosynthesis models and will improve the understanding of long-term responses to elevated CO₂ in tree canopies.