We are limited in our knowledge about coordination of water transport at the whole-tree level, which is influenced by water storage and release along the xylem pathway. Here, we measured water transport and storage at spatially and temporally intensive scales in four tree species of contrasting hydraulic strategies (loblolly pine, southern red oak, Douglas-fir, and trembling aspen) over summer seasons in two environmental systems (mesic/semi-arid). We quantified the coordination of sap flow and volumetric water content at different trunk heights and xylem tissue depths. Further, we compared seasonal patterns of daily water storage and use in the different systems. Coordinated lags in sap flow between upper and lower trunk heights were present in all study species except oak. There was less whole-tree coordination of water transport and storage in the semi-arid compared to the mesic system. The angiosperm species displayed greater water storage use than the gymnosperm species in both systems, with water storage use patterns related to rain events in the mesic system and related to soil-moisture dry down in the semi-arid system. Our findings emphasize the importance of understanding water transport and storage at species- and ecosystem-specific levels inferred from spatially and temporally intensive tree measurements.

Characterizing the transit times or ‘ages’ of water through soil-plant systems is important for modeling ecohydrological processes and improving the accuracy of climate and terrestrial biosphere models (CTBMs). Soil-plant transit times, however, remain poorly characterized. Here, we revisit and leverage a unique isotope labeling dataset from a tropical mesocosm experiment to investigate soil-plant transit times over a 9-month period following a controlled drought. We propose a simple framework for modeling water movement through soil-plant systems, accessible to groups of researchers outside catchment hydrology. We employ two modeling approaches to estimating transit times – parametric (gamma/lognormal) and data-based (phenomenological). Our findings reveal that the parametric approach results in mean transit times (MTTs) that are generally longer than MTTs derived from the data-based approach, particularly in trees. Further, our results demonstrate significant preferential flows in the vadose zone and similar water flow patterns via trees at the scale of this ecosystem. Analogous to preferential flow in soils, we refer to this ecosystem-scale water flow via trees as ‘xylem bias’, whereby some trees have a stronger pull on water and/or larger pool of stored water than others. These results suggest a complex interplay of partitioning, storage, and release mechanisms that remain largely unaccounted for in soil-plant transit time literature. We suggest that our findings have significant implications for CTBMs, underlining the need for improved representations of root water uptake and internal water storage. Our discussion illustrates how interdisciplinary approaches can enhance interpretation of tracer experiments and transit-time analyses.

Characterizing the transit times or ‘ages’ of water through soil-plant systems is important for modeling ecohydrological processes and improving the accuracy of climate and terrestrial biosphere models (CTBMs). Soil-plant transit times, however, remain poorly characterized. Here, we revisit and leverage a unique isotope labeling dataset from a tropical mesocosm experiment to investigate soil-plant transit times over a 9-month period following a controlled drought. We propose a simple framework for modeling water movement through soil-plant systems, accessible to groups of researchers outside catchment hydrology. We employ two modeling approaches to estimating transit times – parametric (gamma/lognormal) and data-based (phenomenological). Our findings reveal that the parametric approach results in mean transit times (MTTs) that are generally longer than MTTs derived from the data-based approach, particularly in trees. Further, our results demonstrate significant preferential flows in the vadose zone and similar water flow patterns via trees at the scale of this ecosystem. Analogous to preferential flow in soils, we refer to this ecosystem-scale water flow via trees as ‘xylem bias’, whereby some trees have a stronger pull on water and/or larger pool of stored water than others. These results suggest a complex interplay of partitioning, storage, and release mechanisms that remain largely unaccounted for in soil-plant transit time literature. We suggest that our findings have significant implications for CTBMs, underlining the need for improved representations of root water uptake and internal water storage. Finally, our results point to the need for interdisciplinary approaches to unravel the complexities of water transport in soil-plant systems.

Water inside plants forms a continuous chain from water in soils to the water evaporating from leaf surfaces. Failures in this chain result in reduced transpiration and photosynthesis and these failures are caused by soil drying and/or cavitation-induced xylem embolism. Xylem embolism and plant hydraulic failure share a number of analogies to “catastrophe theory” in dynamical systems. These catastrophes are often represented in the physiological and ecological literature as tipping points or alternative stable states when control variables exogenous (e.g. soil water potential) or endogenous (e.g. leaf water potential) to the plant are allowed to slowly vary. Here, plant hydraulics viewed from the perspective of catastrophes at multiple spatial scales is considered with attention to bubble expansion (i.e. cavitation), organ-scale vulnerability to embolism, and whole-plant biomass as a proxy for transpiration and hydraulic function. The hydraulic safety-efficiency tradeoff, hydraulic segmentation and maximum plant transpiration are examined using this framework. Underlying mechanisms for hydraulic failure at very fine scales such as pit membranes, intermediate scales such as xylem network properties and at larger scales such as soil-tree hydraulic pathways are discussed. Lacunarity areas in plant hydraulics are also flagged where progress is urgently needed.

The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem regulate ΨL more conservatively than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh., and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves, and xylem anatomical measurements across ten forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future. We end by discussing mechanisms that allow these species to tolerate and/or recover from hydraulic damage.

The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. A legacy of observations in drylands suggests plants with vulnerable xylem more carefully regulate ΨL than plants with resistant xylem. We synthesized over 1600 ΨL observations, 122 xylem embolism curves, and xylem anatomical measurements of Quercus alba L., Liriodendron tulipifera L., and Acer saccharum Marsh. across ten contrasting forests to evaluate if the paradigm linking conservative ΨL regulation to vulnerable xylem applies to temperate deciduous trees. Additionally, we explored generalizable patterns of hydraulic trait acclimation in relation to forest age and climate. Contrary to the dryland paradigm, we found that the tree species with the most vulnerable xylem (e.g., Q. alba) regulated ΨL less strictly (anisohydric behavior) than the species with xylem more resistant to embolism (e.g., A. saccharum and L. tulipifera). This relationship was found across all sites, suggesting coordination among traits was largely unaffected spatio-temporal factors. Our findings indicate drought-response traits of temperate deciduous forest species are coordinated in fundamentally different ways than vegetation in arid climates.