Hydraulic acclimation in a Mediterranean oak subjected to permanent throughfall exclusion results in increased stem hydraulic capacitance

Abstract

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. K E Y W O R D Scapacitive water, carbon gain optimization, desorption curve, drought-stress, electrical resistivity tomography, frequency domain reflectometry, rainfall exclusion, sapwood area, stem water content, tree acclimation | INTRODUCTIONIn the early 20th century, Spalding (1905) reported for the first time clear evidence of stem water storage and related diameter changes for the Suaharo desert plant. Later studies described a significant imbalance between leaf transpiration and soil water uptake, and suggested an important role of water storage throughout plant tissues, mainly in the stem, to satisfy the evaporative demand under water limiting conditions (Kramer, 1937, and references therein). This pioneering work demonstrated that tree stems are not inert pipelines along the root-to-leaf continuum. A more recent body of studies has since highlighted the physiological relevance of stem water pools in plant transpiration, hydraulic modelling and survival (Goldstein et al.

Drought acclimation of Quercus ilex leaves improves tolerance to moderate drought but not resistance to severe water stress

“…By delaying the stomatal closure and the turgor loss during drought progression, leaf acclimation to drought would thus benefit tree functioning mostly in periods of moderate water stress. This is consistent with an earlier observation that increased stem hydraulic capacitance in the rainfall exclusion treatment in Puéchabon contributed to carbon gain during well‐hydrated periods, rather than to prevent losses of hydraulic conductance during severe water stress (Salomon et al, 2020). From a gas‐exchange perspective, these responses to rainfall exclusion allow the maintenance of a positive leaf carbon balance for longer periods and the maximisation of the water use efficiency (Holloway‐Phillips & Brodribb, 2011; Limousin et al, 2013).…”

Shifting access to pools of shoot water sustains gas exchange and increases stem hydraulic safety during seasonal atmospheric drought

“…In late dry season, reduced stem capacitance resulted in statistically insignificant amounts of water being discharged from the stem between full hydration and turgor loss (Table 2). Declines in stem water pools and hydraulic capacitance under drought conditions have been observed in several species (Beedlow, Waschmann, Lee, & Tingey, 2017; Hao, Wheeler, Holbrook, & Goldstein, 2013; Matheny et al, 2015; Salomón et al, 2020); however, our results suggest that by late dry season stem water content was highly conserved over the daily operational range, Ψ pd ‐Ψ min . While HSM 50 stem (MPa) in early and late dry season were not statistically different due to large confidence limits associated with stem P 50 estimates, stem water potentials declined between early and late dry season, implying the predicted seasonal decline in HSM 50 stem (MPa).…”

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

“…The inconsistent observations among different studies might be attributed to the scale (branch vs. whole plant) or plasticity of the response. For example, there are reports that the C p of roots contains a comparable amount of extractable water as stems in some species, and C p is also reportedly responsive to environmental moisture ( Salomón et al, 2020 ). In addition, it is also possible that the water provided by C p is not utilized to prevent xylem embolism but to sustain the vitality of other tissues such as the cambium ( Knipfer et al, 2017 , 2019 ).…”