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.