Summary: Determining physiological mechanisms and thresholds for climate‐driven tree die‐off could help improve global predictions of future terrestrial carbon sinks. We directly tested for the lethal threshold in hydraulic failure – an inability to move water due to drought‐induced xylem embolism – in a pine sapling experiment. In a glasshouse experiment, we exposed loblolly pine (Pinus taeda) saplings (n = 83) to drought‐induced water stress ranging from mild to lethal. Before rewatering to relieve drought stress, we measured native hydraulic conductivity and foliar color change. We monitored all measured individuals for survival or mortality. We found a lethal threshold at 80% loss of hydraulic conductivity – a point of hydraulic failure beyond which it is more likely trees will die, than survive, and describe mortality risk across all levels of water stress. Foliar color changes lagged behind hydraulic failure – best predicting when trees had been dead for some time, rather than when they were dying. Our direct measurement of native conductivity, while monitoring the same individuals for survival or mortality, quantifies a continuous probability of mortality risk from hydraulic failure. Predicting tree die‐off events and understanding the mechanism involved requires knowledge not only of when trees are dead, but when they begin dying – having passed the point of no return.

The critical amplifying role of increasing atmospheric moisture demand on tree mortality and associated regional die-off

We monitored the radial distribution of sap flux density (v; g H₂O m⁻² s⁻¹) in the sapwood of six plantation-grown Pinus taeda L. trees during wet and dry soil periods. Mean basal diameter of the 32-year-old trees was 33.3 cm. For all trees, the radial distribution of sap flow in the base of the stem (i.e., radial profile) was Gaussian in shape. Sap flow occurred maximally in the outer 4 cm of sapwood, comprising 50–60% of total stem flow (F), and decreased toward the center, with the innermost 4 cm of sapwood (11–15 cm) comprising less than 10% of F. The percent of flow occurring in the outer 4 cm of sapwood was stable with time (average CV < 10%); however, the percentage of flow occurring in the remaining sapwood was more variable over time (average CV ≫ 40%). Diurnally, the radial profile changed predictably with time and with total stem flow. Seasonally, the radial profile became less steep as the soil water content (θ) declined from 0.38 to 0.21. Throughout the season, daytime sap flow also decreased as θ decreased; however, nighttime sap flow (an estimate of stored water use) remained relatively constant. As a result, the percentage of stored water use increased as θ declined. Time series analysis of 15-min values of F, θ, photosynthetically active radiation (PAR) and vapor pressure deficit (D) showed that F lagged behind D by 0–15 min and behind PAR by 15–30 min. Diurnally, the relationship between F and D was much stronger than the relationship between F and PAR, whereas no relationship was found between F and θ. An autoregressive moving average (ARIMA) model estimated that 97% of the variability in F could be predicted by D alone. Although total sap flow in all trees responded similarly to D, we show that the radial distribution of sap flow comprising total flow could change temporally, both on daily and seasonal scales.