Long-term variation of the sap flow to tree diameter relation in a temperate poplar forest

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Understanding the relationship between trunk sap flow and tree diameter is crucial for tree-stand transpiration upscaling and sap flow measurement strategy design, because of well-known tree-to-tree variations in sap flow metrics. However, whether and how this relationship will vary intra- and inter-annually, and the underlying mechanisms, are still poorly understood. We measured the sap flow of 126 trees throughout a six-year experiment (2016–2021) in a temperate poplar (Populus tomentosa) forest with five irrigation management regimes. Simultaneously, we also monitored meteorological factors, leaf area indices, and soil water contents. There was a great variability of both sap flux (SF) and sap flux density (SFD) across trees, and this variability tended to be larger in older stands. Patterns of the relationships between SF, SFD, and diameter at breast height (DBH) changed markedly from year to year. A nonlinear positive correlation (P < 0.001) between SF and DBH was found, except in 2016 when canopy closure did not occur. In contrast, no correlations between SFD and DBH were observed in most years, but their correlations became positive and linear in two wet years (2018, 2021) (P < 0.05). The power (Q) of DBH to explain tree-to-tree sap flow variations exhibited enormous change on both intra- and inter-annual scales, underpinned by different mechanisms. The potential transpiration demand of trees mainly determined the intra-annual variation of Q, but its effect depended on stand development, whereas the environmental water supply mostly controlled the inter-annual variation of Q. Based on these results, we provide some recommendations on sap flow measurement and stand transpiration estimation for pure tree plantations in water-limited regions. Our findings should assist the accurate prediction of stand water use in plantation forests of both poplar and other tree species.