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Internal water storage within trees can be a critical reservoir that helps trees overcome both short- and long-duration environmental stresses. We monitored changes in internal tree water storage in a ponderosa pine on daily and seasonal scales using moisture probes, a dendrometer, and time-lapse electrical resistivity imaging (ERI). These data were used to investigate how patterns of in-tree water storage are affected by changes in sapflow rates, soil moisture, and meteorologic factors such as vapor pressure deficit. Measurements of xylem fluid electrical conductivity were constant in the early growing season while inverted sapwood electrical conductivity steadily increased, suggesting that increases in sapwood electrical conductivity did not result from an increase in xylem fluid electrical conductivity. Seasonal increases in stem electrical conductivity corresponded with seasonal increases in trunk diameter, suggesting that increased electrical conductivity may result from new growth. On the daily scale, changes in inverted sapwood electrical conductivity correspond to changes in sapwood moisture. Wavelet analyses indicated that lag times between inverted electrical conductivity and sapflow increased after storm events, suggesting that as soils wetted, reliance on internal water storage decreased, as did the time required to refill daily deficits in internal water storage. We found short time lags between sapflow and inverted electrical conductivity with dry conditions, when ponderosa pine are known to reduce stomatal conductance to avoid xylem cavitation. A decrease in diel amplitudes of inverted sapwood electrical conductivity during dry periods suggest that the ponderosa pine relied on internal water storage to supplement transpiration demands, but as drought conditions progressed, tree water storage contributions to transpiration decreased. Time-lapse ERI- and wavelet-analysis results highlight the important role internal tree water storage plays in supporting transpiration throughout a day and during periods of declining subsurface moisture. 

Abstract The subsurface processes that mediate the connection between evapotranspiration and groundwater within forested hillslopes are poorly defined. Here, we investigate the origin of diel signals in unsaturated soil water, groundwater, and stream stage on three forested hillslopes in the H.J. Andrews Experimental Forest in western Oregon, USA, during the summer of 2017, and assess how the diurnal signal in evapotranspiration (ET) is transferred through the hillslope and into these stores. There was no evidence of diel fluctuations in upslope groundwater wells, suggesting that tree water uptake in upslope areas does not directly contribute to the diel signal observed in near‐stream groundwater and streamflow. The water table in upslope areas resided within largely consolidated bedrock, which was overlain by highly fractured unsaturated bedrock. These subsurface characteristics inhibited formation of diel signals in groundwater and impeded the transfer of diel signals in soil moisture to groundwater because (1) the bedrock where the water table resides limited root penetration and (2) the low unsaturated hydraulic conductivity of the highly fractured rock weakened the hydraulic connection between groundwater and soil/rock moisture. Transpiration‐driven diel fluctuations in groundwater were limited to near‐stream areas but were not ubiquitous in space and time. The depth to the groundwater table and the geologic structure at that depth likely dictated rooting depth and thus controlled where and when the transpiration‐driven diel fluctuations were apparent in riparian groundwater. This study outlines the role of hillslope hydrogeology and its influence on the translation of evapotranspiration and soil moisture fluctuations to groundwater and stream fluctuations.