Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration ( $E_T$ ), as a component of evapotranspiration (E), behaves differently under wet and dry canopy conditions, yet very few studies have focused on the dynamics of transpiration to evapotranspiration ratio ( $E_T/E$ ) under transient canopy wetness states. This study presents field measurements of $E_T/E$ at the Montmorency Forest, Québec, Canada: a balsam fir boreal forest that receives $\sim 1600$ mm of precipitation annually (continental subarctic climate; Köppen classification subtype Dfc). Half-hourly observations of E and $E_T$ were obtained over two growing seasons using eddy-covariance and sap flow (Granier’s constant thermal dissipation) methods, respectively, under wet and dry canopy conditions. A series of calibration experiments were performed for sap flow, resulting in species-specific calibration coefficients that increased estimates of sap flux density by $34\%\pm 8\%$ , compared to Granier’s original coefficients. The uncertainties associated with the scaling of sap flow measurements to stand $E_T$ , especially circumferential and spatial variations, were also quantified. From 30 wetting–drying events recorded during the measurement period in summer 2018, variations in $E_T/E$ were analyzed under different stages of canopy wetness. A combination of low evaporative demand and the presence of water on the canopy from the rainfall led to small $E_T/E$ . During two growing seasons, the average $E_T/E$ ranged from $35\%\pm 2\%$ to $47\%\pm 3\%$ . The change in total precipitation was not the main driver of seasonal $E_T/E$ variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals.

Drought conditions cause stress to terrestrial ecosystems and make their accurate representation in models challenging. The Canadian LAnd Surface Scheme Including biogeochemical Cycles (CLASSIC) employs an empirical approach to link soil moisture stress with stomatal conductance. Such approaches typically perform poorly during drought. Here, we implemented an explicit plant hydraulics parameterization, that is, Stomatal Optimization based on Xylem hydraulics (SOX), in CLASSIC, thereby connecting the soil‐plant‐atmosphere continuum through plant hydraulic traits. The resulting was evaluated against the carbon and water fluxes measured with eddy covariance at eight North American boreal forest flux tower sites. Compared to CLASSIC, better simulated gross primary productivity (GPP) at all sites (increased 0.51 to 0.59; decreased RMSE 1.85 to 1.54 g C and bias −0.99 to −0.57 g C ). During droughts, identified using the Palmer drought severity index (PDSI), GPP simulated with improved compared to CLASSIC. In contrast, generally overestimated evapotranspiration (ET) compared to CLASSIC ( increased 0.61 to 0.64, but RMSE and bias worsened 0.54–0.78 mm and 0.09–0.32 mm , respectively). The overestimated ET may be due to evaporation parameterization, parameter uncertainties, or SOX limitations. Wood density was used to derive plant hydraulic parameters, enabling a parameter‐parsimonious implementation. This explicit plant hydraulics parameterization in CLASSIC will improve its ability to predict ecosystem responses to increasing frequency and severity of drought in boreal forests. Future work aims to refine the simulated evapotranspiration while retaining the improved GPP drought response.

The boreal forest plays a critical role in global water and carbon cycles, yet the response of its dominant tree species to soil moisture remains poorly studied. Integrated tree-hydrodynamic measurements, including water storage and transport are essential to improve predictions of boreal forest responses to future water limitations. This study analyzed the performance of three distinct instruments: dielectric sensors, dendrometers, and thermal dissipation sap flow sensors, in monitoring tree-water dynamics of three coniferous species characteristic of boreal forests in North America: black spruce (Picea mariana), balsam fir (Abies balsamea), and red pine (Pinus resinosa). We compared these instrument’s abilities to capture diel water dynamics and seasonal responses to soil moisture during one summer, across three Canadian sites with contrasting meteorological and soil conditions. The dielectric sensors effectively detected subtle and rapid changes in stem water storage, particularly in environments with fluctuating or limited soil moisture. The dendrometers reliably tracked tree water deficit in dry conditions and helped indicate when trees are experiencing water stress in wet soil conditions, although their sensitivity to stem growth and bark swelling limited their precision. The thermal dissipation sensors captured variations linked to tree water transport, but showed no response to soil moisture at wetter sites. This suggests that sap-flow dynamics may be more influenced by other drivers of tree-hydraulics, such as vapor pressure deficit and solar radiation, than by soil water availability. Overall, the three instruments captured complementary aspects of tree-water dynamics across varying pedoclimatic conditions. This comparison provides new insights into the utility of each sensor in boreal forests, highlighting the value of integrating multiple sensor types to better characterize forest hydrodynamics under environmental change. These findings also contribute to the integration of tree water dynamics measurements at larger scales and support the refinement of remote-sensing derived datasets, such as vegetation optical depth.