Within crown variation in hydraulic architecture in beech (Fagus sylvatica L): evidence for a stomatal control of xylem embolism

Abstract

-The stomatal control of embolism in Fagus sylvatica L. was analysed in response to crown position and experimental changes of trunk hydraulic resistance. On one mature beech tree deep cuts were made in the trunk to increase the resistance to water transfert. We followed the changes in leaf and xylem water potential and stomatal conductance after the cuts at three levels within the canopy. We characterised vulnerability to cavitation for branches taken from two levels of irradiance (sun-exposed branches and shaded ones). Some differences appeared between shade and sun-exposed branches. When the leaf water potential dropped, stomatal conductances decreased earlier and faster in the shade branches. These results are well correlated with vulnerability to cavitation, shade branches being more vulnerable than sun-acclimated branches. Xylem water potential levels producing fifty percent loss of hydraulic conductivity were lower in sun-exposed branches than in shade grown ones (-3.1 MPa vs. -2.5 MPa on average). Xylem water potentials that induced stomatal closure were above the threshold-value inducing cavitation both for shade and sun-exposed branches. We confirmed that vulnerability to cavitation in Fagus sylvatica can acclimate to contrasting ambient light conditions, and we conclued that stomatal response to water stress occured early and sufficiently fast to protect xylem from dysfunction. beech (Fagus sylvatica L.) / xylem embolism / stomatal regulation / irradiance / acclimation Résumé -Variations de l'architecture hydraulique du hêtre (Fagus sylvatica L.) : contrôle de l'embolie du xylème par les stomates. Nous avons analysé le contrôle stomatique du développement de l'embolie chez Fagus sylvatica L. en fonction de l'éclaire-ment des branches et suite à un changement de la résistance hydraulique du tronc. Nous avons fait des entailles dans le tronc d'un hêtre de façon à augmenter la résistance au transfert de l'eau. Nous avons suivi les variations de potentiels hydriques foliaire et de xylème et la conductance stomatique à trois niveaux dans le houppier. Nous avons caractérisé la vulnérabilité à la cavitation de branches de pleine lumière et d'ombre. Lorsque le potentiel hydrique a diminué, la conductance stomatique des branches d'ombre a diminuée le plus tôt et le plus fortement. Ce résultat est bien corrélé avec la vulnérabilité à la cavitation des branches. Les branches d'ombre sont plus vulnéra-bles que les branches de lumière ; ainsi le potentiel hydrique de xylème induisant 50 % d'embolie est plus négatif en plein éclairement qu'à l'ombre (-3,1 MPa contre -2,5 MPa). Le potentiel de xylème induisant la fermeture des stomates est supérieur au potentiel induisant la cavitation à la lumière comme à l'ombre. Nous avons confirmé que la vulnérabilité du hêtre s'acclimate aux conditions d'éclaire-ment et que les stomates protègent le xylème d'un dysfonctionnement.hêtre (Fagus sylvatica L.) / embolie / régulation stomatique / éclairement / acclimatation

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

“…In most plants, petioles are usually less susceptible than branches, and branches, less vulnerable than roots with exceptions like in Alnus glutinosa [63] and Populus euphratica [69]. Apical parts of trees experience the lowest leaf water potential and also the lowest vulnerability to cavitation, as reported for beech [84]: light exposed branches are less vulnerable than shade ones, submitted to lower transpiration and less negative leaf water potential [31]. A very clear hydraulic segmentation was reported for a few species like walnut trees (Juglans regia) [144] with petioles displaying a large vulnerability which aims at disconnecting the leaves through massive cavitation during drought and avoid irreversible damage to perennial parts of the tree.…”

“…At the opposite, in some species cavitation may begin before full stomatal closure like in some poplars [32]. Stomatal control of transpiration appears to be a tool to reduce the risk of deleterious massive embolism in many but not all trees [84,133,135]. Stomatal control and hydraulic architecture (i.e., vulnerability to cavitation and hydraulic conductance) are tightly coordinated in many respects (see review by [46]).…”

Statut hydrique des feuilles et flux xylèmique dans le tronc en relation avec la sécheresse du sol pour cinq espèces d’arbres feuillus tempérés à stratégies de consommation d’eau différentes

“…Below this value beech shoots rapidly lost hydraulic conductivity due to embolism. However, Lemoine et al (2002) reported significant variation in Ψ L within the crown of beech trees and found that stomatal closure and the percentage of loss conductivity in the xylem was more than 50% in sun exposed branches only when Ψ L dropped to almost -2.5 MPa.…”

Beech (Fagus sylvatica L.) branches show acclimation of xylem anatomy and hydraulic properties to increased light after thinning

“…We were interested in how branches acclimate soon after these changes (the first year after thinning). Beech presents strong differences in branch morphology depending on light regime [16]. In the upper parts of the crown, branches are characterised by long internodes in contrast to shade branches where shoots are very short with very short internodes.…”