Relationship between stem diameter and transpiration for Japanese cypress trees: Implications for estimating canopy transpiration

Previous studies reported relationships between stem diameter at breast height (DBH) and whole-tree transpiration (Q_t) across a variety of species and locations. It might be possible to develop a relationship between DBH and Q_t with smaller variations when we focused on a single species. We attempted to develop such a relationship for Japanese cypress (Chamaecyparis obtusa), which is one of the major plantation species in Japan. We collated Q_t for 51 Japanese cypress trees from nine different-sized and different-aged stands using the sap flux method. We found a strong linear correlation between DBH and the reference value of Q_t at a vapour pressure deficit of 1 kPa (R =.883). This was a consequence of a strong correlation between DBH and sapwood area (A_{S_tree}; R =.973) and the absence of a correlation between DBH and sap flux density (R = −.043). We confirmed that using the relationship between DBH and A_{S_tree}, while assuming typical responses of sap flux density to meteorological factors, provides reasonable Q_t estimates. This study also demonstrated how the DBH–Q_t relationship can be applied to estimate changes in E_C with changing forest management.