Canopy position affects the relationships between leaf respiration and associated traits in a tropical rainforest in Far North Queensland

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 RD, 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 Q₁₀ 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.