Previous studies have demonstrated a clear relationship between diameter at breast height (DBH) and tree transpiration (QT) in multi-specific broadleaved forests. However, these studies were conducted with a limited range of tree sizes and species, and thus many multi-specific broadleaved forests fall outside these conditions. Therefore, this study examined the relationship between DBH and QT in a warm-temperate multi-specific broadleaved forest (n = 12 species) with a wide range of tree sizes (5.0–70.0 cm DBH) using the Granier-type heat dissipation method. The results showed that, although sap flow density varied between individual trees and species, there was a significant relationship between log QT and log DBH (r2 = 0.66, P < 0.001) because of the strong dependence of sapwood area on DBH. This study confirmed the applicability of the relationship for the stand transpiration (EC) estimates even in a multi-specific broadleaved forest with a wide variation in DBH. Our results also revealed that selecting the sample trees in descending order of DBH effectively reduced potential errors in EC estimates for a specific sample size, as larger trees contribute more to EC. This information should be useful for future studies investigating the transpiration of multi-specific broadleaved forests, reducing errors during the scaling-up procedure.
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