Hydraulic architecture of trees: main concepts and results

Abstract

-Since about twenty years, hydraulic architecture (h.a.) is, doubtless, the major trend in the domain of plants (and especially trees) water relations. This review encompasses the main concepts and results concerning the hydraulic of architecture of trees. After a short paragraph about the definition of the h.a., the qualitative and quantitative characteristics of the h.a. are presented. This is an occasion to discuss the pipe model from the h.a. point of view. The second part starts with the central concept of embolism and give a review of important experimental results and questions concerning summer and winter embolism. The last part deals with the coupling between hydraulic and stomatal conductances. It discusses the theoretical and experimental relationships between transpiration and leaf water potential during a progressive soil drought, the increase of soil-root resistance and its consequences in term of xylem vulnerability, the factors controlling the daily maximum transpiration and how stomates can prevent "run away embolism". In conclusion different kinds of unsolved questions of h.a., which can be a matter of future investigations, are presented in addition with a classification of trees behaviour under drought conditions. To end, an appendix recalls the notions of water potential, pressure and tension.hydraulic architecture / cohesion-tension theory / summer embolism / winter embolism / drought resistance Résumé -Architecture hydraulique des arbres : concepts principaux et résultats. Sans aucun doute, depuis une vingtaine d'années, l'architecture hydraulique (a.h.) est devenue une approche majeure dans le domaine des relations plantes-eau (et particulièrement pour les arbres). Cette revue présente les principaux concepts et résultats concernant l'a.h. Après un bref paragraphe sur la définition de l'a.h., les caractéristiques qualitatives et quantitatives définissant l'a.h. sont passées en revue. À cette occasion le « pipe model » est discuté du point de vue de l'a.h. La seconde partie commence avec le concept central d'embolie et continue avec une présentation des principaux résultats et questions touchant l'embolie estivale et l'embolie hivernale. La dernière partie analyse le « couplage » entre les conductances hydraulique et stomatique. Il y est discuté des relations théoriques et expérimentales entre la transpiration et le potentiel hydrique foliaire durant la mise en place d'une sécheresse progressive du sol, de l'augmentation de la résistance sol-racines et de ses conséquences en terme de vulnérabilité du xylème, des facteurs contrôlant la transpiration maximale journalière et de quelle manière les stomates peuvent prévenir l'emballement de l'embolie. La conclusion fait état de différentes questions non résolues, qui pourraient faire l'objet de recherches futures et esquisse une classification du comportement des arbres vis-à-vis de la sécheresse. Pour finir, un appendice rappelle les notions de potentiel hydrique, de pression et de tension.architecture hydraulique / théorie de la cohé...

Hydraulic properties and embolism in small-diameter roots of five temperate broad-leaved tree species with contrasting drought tolerance

“…Mean vessel diameter d was the main determinant of root-specific conductivity and k s increased exponentially with increasing d, but the slope differed between the species. We found no dependence of k s on vessel density in the root xylem which is contrary to the results reported by Tyree and Ewers (1991), Cruiziat et al (2002) and Tyree and Zimmermann (2002) for the hydraulic system in trunk and branches. The lack of a vessel density effect in our study can partly be explained by the negative relation between mean vessel diameter and vessel density in the roots of three of the five species (Fagus, Acer, and Tilia; data not shown).…”

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…Its mechanism is probably as follows: the membrane of pits, allowing inter-vessel connections, may release slight air bubbles as soon as the tension of the liquid column overcomes the threshold allowed by membrane capillarity; as a consequence, the vessel empties in a few microseconds allowing the diffusion of ambient air into the cavitated vessel, and leading to an irreversible embolism when the xylem element is filled up with air [35,147].…”

“…The importance of hydraulic architecture has only recently been re-evaluated and numerous results are now produced, but there are still large gaps in our knowledge of water transport in trees. Resistance to cavitation is possibly one of the most important parameters determining the degree of drought resistance of a tree [35]. The resistance in the soil-root compartment, as well as the anatomical changes with ageing and in branch junctions (hydraulic bottlenecks) need further researches.…”

“…Stomatal control of leaf transpiration and loss of hydraulic conductivity in twigs have been monitored in parallel in a range of species during the course of drought (see review in [35]). A tight coordination was evidenced between stomatal closure and induction of embolism: usually, embolism begins only when stomatal conductance drops below 10% of initial values.…”

“…Drought results in the reduction in soil water availability. Drought stress occurs whenever soil water drops below a threshold inducing restrictions to growth and directly produces and maintains a gradient of water potential throughout the plant [35] and the ratio transpiration/water potential gradient is the whole-plant hydraulic conductance. Water transfer within trees and in the soil-plant water continuum is modelled using an electrical analogy since Van den Honert [150].…”