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Changes in vapour pressure deficit can lead to the depletion and replenishment of stem water pools to buffer water potential variations in the xylem. Yet, the precise velocity at which stem water pools track environmental cues remains poorly explored. Nine eucalyptus seedlings grown in a glasshouse experienced high-frequency environmental oscillations and their stem radial variations (ΔR) were monitored at a 30-s temporal resolution in upper and lower stem locations and on the bark and xylem. The stem ΔR response to vapour pressure deficit changes was nearly instantaneous (<1 min), while temperature lagged behind stem ΔR. No temporal differences in the stem ΔR response were observed between locations. Punctual gravimetric measurements confirmed the synchrony between transpiration and stem ΔR dynamics. These results indicate (i) that stem-stored water can respond to the atmospheric evaporative demand much faster than commonly assumed and (ii) that the origin of the water released to the transpiration stream seems critical in determining time lags in stem water pool dynamics. Near-zero time lags may be explained by the high elasticity of eucalyptus woody tissues and the predominant water use from the xylem, circumventing the hydraulic radial barriers to water flow from/to the outer tissues., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Stem water storage capacity and hydraulic capacitance (C S ) play a crucial role in tree survival under drought-stress. To investigate whether C S adjusts to increasing water deficit, variation in stem water content (StWC) was monitored in vivo for 2 years and related to periodical measurements of tree water potential in Mediterranean Quercus ilex trees subjected either to permanent throughfall exclusion (TE) or to control conditions. Seasonal reductions in StWC were larger in TE trees relative to control ones, resulting in greater seasonal C S (154 and 80 kg m -3 MPa -1 , respectively), but only during the first phase of the desorption curve, when predawn water potential was above -1.1 MPa. Below this point, C S decreased substantially and did not differ between treatments (<20 kg m -3 MPa -1 ). The allometric relationship between tree diameter and sapwood area, measured via electrical resistivity tomography, was not affected by TE. Our results suggest that (a) C S response to water deficit in the drought-tolerant Q. ilex might be more important to optimize carbon gain during well-hydrated periods than to prevent drought-induced embolism formation during severe drought stress, and (b) enhanced C S during early summer does not result from proportional increases in sapwood volume, but mostly from increased elastic water., (© 2020 John Wiley & Sons Ltd.)

To quantify stem respiration (R S ) under elevated CO 2 (eCO 2 ), stem CO 2 efflux (E A ) and CO 2 flux through the xylem (F T ) should be accounted for, because part of respired CO 2 is transported upwards with the sap solution. However, previous studies have used E A as a proxy of R S , which could lead to equivocal conclusions. Here, to test the effect of eCO 2 on R S , both E A and F T were measured in a free-air CO 2 enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E A and R S , which were unaffected by eCO 2 , likely as a consequence of its neutral effect on stem growth in this phosphorus-limited site. However, xylem CO 2 concentration measured near the stem base was higher under eCO 2 , and decreased along the stem resulting in a negative contribution of F T to R S , whereas the contribution of F T to R S under ambient CO 2 was positive. Negative F T indicates net efflux of CO 2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R S on eCO 2 and suggest stimulated root respiration under eCO 2 that may shift vertical gradients in xylem [CO 2 ] confounding the interpretation of E A measurements., (© 2019 John Wiley & Sons Ltd.)

Daytime decreases in temperature-normalised stem CO 2 efflux (E A &#95;D ) are commonly ascribed to internal transport of respired CO 2 (F T ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and F T , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO 2 fluxes was applied to estimate F T . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F T , were reliable predictors of E A &#95;D . After tree manipulation, only turgor pressure was a robust predictor of E A &#95;D . Moreover, F T accounted for < 15% of E A &#95;D . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E A &#95;D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Hydraulic modelling is a primary tool to predict plant performance in future drier scenarios. However, as most tree models are validated under non-stress conditions, they may fail when water becomes limiting. To simulate tree hydraulic functioning under moist and dry conditions, the current version of a water flow and storage mechanistic model was further developed by implementing equations that describe variation in xylem hydraulic resistance (R X ) and stem hydraulic capacitance (C S ) with predawn water potential (Ψ PD ). The model was applied in a Mediterranean forest experiencing intense summer drought, where six Quercus ilex trees were instrumented to monitor stem diameter variations and sap flow, concurrently with measurements of predawn and midday leaf water potential. Best model performance was observed when C S was allowed to decrease with decreasing Ψ PD . Hydraulic capacitance decreased from 62 to 25 kg m -3  MPa -1 across the growing season. In parallel, tree transpiration decreased to a greater extent than the capacitive water release and the contribution of stored water to transpiration increased from 2.0 to 5.1%. Our results demonstrate the importance of stored water and seasonality in C S for tree hydraulic functioning, and they suggest that C S should be considered to predict the drought response of trees with models., (© 2017 John Wiley & Sons Ltd.)

To accurately estimate stem respiration (R S ), measurements of both carbon dioxide (CO 2 ) efflux to the atmosphere (E A ) and internal CO 2 flux through xylem (F T ) are needed because xylem sap transports respired CO 2 upward. However, reports of seasonal dynamics of F T and E A are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO 2 fluxes at three stem heights, together with radial stem growth, temperature, sap flow and shoot water potential, were measured in Quercus pyrenaica Willd. in four measurement campaigns during one growing season. Substantial daytime depressions in temperature-normalized E A were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and E A cannot be uniquely ascribed to diversion of CO 2 in the transpiration stream. Low internal [CO 2 ] (<0.5%) resulted in low contributions of F T to R S throughout the growing season, and R S was mainly explained by E A (>90%). Internal [CO 2 ] was found to vary vertically along the stems. Seasonality in resistance to radial CO 2 diffusion was related to shoot water potential. The low internal [CO 2 ] and F T observed in our study may result from the downregulation of xylem respiration in response to a legacy of coppicing as well as high radial diffusion of CO 2 through cambium, phloem and bark tissues, which was related to low water content of stems. Long-term studies analyzing temporal and spatial variation in internal and external CO 2 fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues., (© The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)