Photosynthesis and respiration decline with light intensity in dominant and suppressed Eucalyptus globulus canopies

Within canopy gradients in light-saturated photosynthesis (A_max), foliar nitrogen ([N]_area) and leaf dark respiration (R₁₅) were studied in the canopies of dominant and suppressed trees within an even-aged (4-year-old) Eucalyptus globulus (Labill) stand in southern Tasmania. Despite being an even-aged stand growing in a relatively uniform environment with respect to nutrient and water availability, the stand exhibited considerable structural complexity. Diameter at 1.3 m ranged between 3 cm and 21 cm, trees average 12 m height and stand leaf area index was ~6 m² m^-2 leading to strong gradients in light availability. We were interested in understanding the processes governing canopy production in trees of contrasting dominance classes. Vertical gradients in photosynthesis and foliar respiration were studied within the canopies of dominant and suppressed trees during 2006 and 2007. A_max varied from ~18 μmol m^-2 s^-1 in the upper canopy to 3 μmol m^-2 s^-1 at lower canopy positions. On average, A_max were higher in the dominant trees than in the suppressed trees. However, at any given height, A_max were similar in suppressed and dominant trees and were strongly related to leaf nitrogen content. Dark respiration varied from ~1.4 μmol m^-2 s^-1 in the upper canopy to 0.2 μmol m^-2 s^-1 in the lower canopy positions. In contrast to the patterns for A_max, dark respiration rates in the suppressed trees were higher than dominant trees at similar canopy positions. Respiration rates were also strongly related to [N]_area and to A_max.