Effects of stem size on stem respiration and its flux components in yellow-poplar (Liriodendron tulipifera L.) trees.

Carbon dioxide (CO2) released from respiring cells in the stems of trees (RS) can diffuse radially to the atmosphere (EA) or dissolve in xylem sap and move internally in the tree (FT). Previous studies have observed that EA decreases as stem or branch diameter increases, but the cause of this relationship has not been determined, nor has the relationship been confirmed between stem diameter and RS, which includes both EA and FT. In this study, for the first time the mass balance technique was used to estimate RS of stems of Liriodendron tulipifera L. trees of different diameters, ranging from 16 to 60 cm, growing on the same site. The magnitude of the component fluxes scaled with tree size. Among the five trees, the contribution of EA to RS decreased linearly with increasing stem diameter and sapwood area while the contribution of FT to RS increased linearly with stem diameter and sapwood area. For the smallest tree EA was 86% of RS but it was only 46% of RS in the largest tree. As tree size increased a greater proportion of respired CO2 dissolved in sap and remained within the tree. Due to increase in FT with tree size, we observed that trees of different sizes had the same RS even though they had different EA. This appears to explain why the EA of stems and branches decreases as their size increases.