Tree size regulates sugar maple water response to atmospheric evaporative demand and soil water availability

Climate change is intensifying atmospheric evaporative demand for water in forests while altering soil moisture availability. How tree size modulates responses to shifting water dynamics remains poorly understood in North American temperate forest species including sugar maple ( Acer saccharum Marsh.). We examined size-dependent tree water relations by monitoring sap flux density (F d , cm 3 cm -2 h -1 ), stem water content (StWC, %), and tree water deficit (TWD, unitless) in 29 sugar maples across two summers (2023–2024). We analyzed how responses of these indicators to vapor pressure deficit (VPD) and soil relative extractable water (REW) varied with tree size (height, diameter, crown area, canopy status). During summers characterized by high rainfall and moist soils (mean plot-level REW > 0.7), atmospheric evaporative demand strongly governed tree-water relations. Large trees were more sensitive to VPD than small trees. Increasing VPD by ∼45% was associated with a ∼40% increase in F d for large trees and ∼30% for small trees. StWC declined more steeply in large trees, with VPD-driven losses occurring about 30% faster than in small trees. In large trees, greater F d and StWC responses combined to produce nearly double the TWD increase of small trees, despite lower mean TWD. REW had a weaker effect on tree-water relations than VPD but interacted negatively with VPD: under drier soil conditions, increases in VPD produced larger increases in F d and TWD, whereas VPD-driven responses were dampened under wetter conditions. This interaction was consistent across both size classes. Independently of this interaction, large trees maintained moderately stable TWD across the soil moisture gradient, consistent with deeper water access and short-term buffering through stem storage, whereas small trees showed stronger TWD responses to soil drying. Together, these results reveal how tree size mediates the coupling between atmospheric evaporative demand and water status in sugar maple under moist temperate conditions.