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 R-D, R-L and A(sat) (at 28aEuro...A degrees 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 25-45aEuro...A degrees 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 10aEuro...A degrees C rise in T) and T-max values near 60aEuro...A degrees C (i.e., T where R-D reached maximal values). Thus, while rates of R-D at 28aEuro...A degrees 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.