ALBERT

Description: We explored the impact of canopy position on leaf respiration ( R ) and associated traits in tree and shrub species growing in a lowland tropical rainforest in Far North Queensland, Australia. The range of traits quantified included: leaf R in darkness ( R D ) and in the light ( R L ; estimated using the Kok method); the temperature ( T )-sensitivity of R D ; light-saturated photosynthesis ( A sat ); leaf dry mass per unit area (LMA); and concentrations of leaf nitrogen (N), phosphorus (P), soluble sugars and starch. We found that LMA, and area-based N, P, sugars and starch concentrations were all higher in sun-exposed/upper canopy leaves, compared with their shaded/lower canopy and deep-shade/understory counterparts; similarly, area-based rates of R D , R L and A sat (at 28 °C) were all higher in the upper canopy leaves, indicating higher metabolic capacity in the upper canopy. The extent to which light inhibited R did not differ significantly between upper and lower canopy leaves, with the overall average inhibition being 32% across both canopy levels. Log–log R D – A sat relationships differed between upper and lower canopy leaves, with upper canopy leaves exhibiting higher rates of R D for a given A sat (both on an area and mass basis), as well as higher mass-based rates of R D for a given [N] and [P]. Over the 25–45 °C range, the T- sensitivity of R D was similar in upper and lower canopy leaves, with both canopy positions exhibiting Q 10 values near 2.0 (i.e., doubling for every 10 °C rise in T ) and T max values near 60 °C (i.e., T where R D reached maximal values). Thus, while rates of R D at 28 °C decreased with increasing depth in the canopy, the T -dependence of R D remained constant; these findings have important implications for vegetation-climate models that seek to predict carbon fluxes between tropical lowland rainforests and the atmosphere.