22 research outputs found

    Limitation of the Cavitron technique by conifer pit aspiration

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    The Cavitron technique facilitates time and material saving for vulnerability analysis. The use of rotors with small diameters leads to high water pressure gradients (delta P) across samples, which may cause pit aspiration in conifers. In this study, the effect of pit aspiration on Cavitron measurements was analysed and a modified 'conifer method' was tested which avoids critical (i.e. pit aspiration inducing) delta P. Four conifer species were used (Juniperus communis, Picea abies, Pinus sylvestris, and Larix decidua) for vulnerability analysis based on the standard Cavitron technique and the conifer method. In addition, delta P thresholds for pit aspiration were determined and water extraction curves were constructed. Vulnerability curves obtained with the standard method showed generally a less negative P for the induction of embolism than curves of the conifer method. Differences were species-specific with the smallest effects in Juniperus. Larix showed the most pronounced shifts in P(50) (pressure at 50% loss of conductivity) between the standard (-1.5 MPa) and the conifer (-3.5 MPa) methods. Pit aspiration occurred at the lowest delta P in Larix and at the highest in Juniperus. Accordingly, at a spinning velocity inducing P(50), delta P caused only a 4% loss of conductivity induced by pit aspiration in Juniperus, but about 60% in Larix. Water extraction curves were similar to vulnerability curves indicating that spinning itself did not affect pits. Conifer pit aspiration can have major influences on Cavitron measurements and lead to an overestimation of vulnerability thresholds when a small rotor is used. Thus, the conifer method presented here enables correct vulnerability analysis by avoiding artificial conductivity losses

    Die hard: timberline conifers survive annual winter embolism

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    During winter, timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X-ray micro-computer tomography (micro-CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze-thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro-CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long-term data and micro-CT observations demonstrate that timberline trees can survive annual cycles of pronounced winter-embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the timberline and its relevance to general plant hydraulics

    Herb hydraulics : inter- and intraspecific variation in three Ranunculus species

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    The requirements to the water transport system of small herbaceous species differ considerably from those of woody species. Despite their ecological importance for many biomes, knowledge regarding herb hydraulics remains very limited. We compared key hydraulic features (vulnerability to drought-induced hydraulic decline, pressure-volume relations, onset of cellular damage, in situ variation of water potential and stomatal conductance) of three Ranunculus species differing in their soil humidity preferences and ecological amplitude. All species were very vulnerable to water stress (50 % reduction in whole-leaf hydraulic conductance (kleaf) at -0.2 to -0.8 MPa). In species with narrow ecological amplitude, the drought-exposed R. bulbosus was less vulnerable to desiccation (analysed via loss of kleaf and turgor loss point (TLP)) than the humid-habitat R. lanuginosus. Accordingly, water stress exposed plants from the broad-amplitude R. acris revealed tendencies of lower vulnerability to water stress (e.g. osmotic potential at full turgor, cell damage, stomatal closure) than conspecific plants from the humid site. We show that small herbs can adjust to their habitat conditions on inter- and intraspecific levels in various hydraulic parameters. Coordination of hydraulic thresholds (50 and 88% loss of kleaf, TLP, minimum in situ water potential) enabled the study species to avoid hydraulic failure and damage to living cells. Reversible recovery of hydraulic conductance, desiccation-tolerant seeds, or rhizomes may allow them to prioritise towards a more efficient but vulnerable water transport system while avoiding the severe effects that water stress poses on woody species

    Xylem cavitation resistance can be estimated based on time-dependent rate of acoustic emissions

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    In this study, we focused on the use of acoustic activity as a function of time, rather than absolute cumulative emission counts. We hypothesized that the highest acoustic activity should occur near the steepest part of a typical vulnerability curve, that is, near its inflection point (P50), when most embolism is forming within a narrow range of water potential (Ψ). Therefore, the Ψ at maximum AE activity should be correlated to a species’ hydraulically measured P50

    Height-related changes in xylem hydraulics and anatomy of leader shoots of Picea abies trees

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    In trees, water is conducted from roots to leaves under tension. Increasing tree height constrains water transport in terms of both safety and efficiency. From one hand, the gravitational pressure drop imposes an extra tensile strength to the xylem water equal to -0.01 MPa every m of height, with potential negative consequences due to higher risks of embolism formation. On the other hand, the total root-to-leaves path length increases with increasing tree height, with potential negative effects on water flow due to the increased frictional forces. However, it is still unknown how these requirements of hydraulic safety and efficiency are managed in taller trees in order to avoid critical water potentials and/or the risk of embolism formation. We assessed the vulnerability curves (VCs) and measured xylem anatomical traits of the leader shoots of Picea abies varying from 2 to 37 m of height, in two different sites in the Dolomites (Italian Eastern Alps). Both xylem hydraulics and anatomy changed with tree height. The xylem water potential triggering 50% of loss of conductivity (P50) significantly increased from small (-5.86 MPa) in to tall trees (-3.40 MPa) (P=0.007); the total tracheid number (N) and the tracheid hydraulic diameter (Dh) significantly increased with tree height (P=0.005 and P=0.007 respectively), with Dh varying from 10.92 in small to 14.88 \u3bcm in tall trees. We found a strong trade off between efficiency vs. safety, with Kh (total xylem conductivity) significantly varying with P50 (R2=0.60, P<0.001). Taller trees prioritize xylem efficiency vs. safety, with the xylem becoming more vulnerable to cavitation with increasing tree height. These results could represent a mechanistic explanation of why taller trees are commonly more prone to top dieback and vigour decline compared to smaller ones. Therefore, we suppose that these phenomena will more frequently occur with the forecasted climate change scenarios of increasing occurrence of drought events
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