Water relations of European silver fir (Abies alba Mill) in 2 natural stands in the French Alps subject to contrasting climatic conditions

This paper reports on the diurnal and seasonal variations in water potential, stomatal conductance, and transpiration of twigs from silver fir in a mesohygrophilic stand of the external French Alps, and in a mesoxerophilic stand in the inner French Alps where this fir grows near its ecological limits. In both stands, predawn needle water potential was always 0.2-0.4 MPa below the potential of the driest soil layer. In the first one, it was maintained at about -0.4 MPa. Maximum stomatal conductance and maximum transpiration, which could reach 200 mmol/m2 /s and 1 mmol/m2/s, respectively, occurred at the same time which corresponded to minimum leaf water potential. In the dry stand, predawn needle water potential never dropped below -1.14 MPa, yet a general browning of older needles was already observed. The decrease of predawn needle water potential was accompanied by the decrease of maximum stomatal conductance and transpiration to 15% of their highest value, which reached 150 mmol/m2/s and 1 mmol/m2/s, respectively, at this stand. Maximum stomatal conductance occurred in general before UT 07.00, and maximum transpiration 5-6 h later, irrespective of predawn needle water potential. Furthermore, in both stands, stomata closed at vapor pressure deficit value as low as 0.3 kPa. This extremely early reaction to water stress exhibited by European silver fir is consistent with its well-known sensitivity to atmospheric humidity and soil water availability. It indicates a strong avoidance strategy, which we have hitherto attributed only to species better adapted to drought.Comportement hydrique du sapin pectiné (Abies alba Mill) dans 2 stations des Alpes françaises climatiquement contrastées. L'article décrit les variations diurnes et saisonnières du potentiel hydrique foliaire, de la conductance stomatique et de la transpiration de rameaux de sapin dans une station mésohygrophile des Alpes externes, et dans une station mésoxérophile des Alpes internes en limite écologique de l'essence. Dans les 2 stations, le potentiel hydrique de base est toujours inférieur de 0,2 à 0,4 MPa au potentiel hydrique des couches de sol les plus sèches. Dans la première, il s'est maintenu aux environs de -0,4 MPa. La conductance stomatique et la transpiration maximales, pouvant atteindre respectivement 200 mmol/m2/s et 1 mmol/m2/s, ont toujours eu lieu au même moment, qui correspondait au potentiel hydrique foliaire minimum. Dans la station sèche, le potentiel hydrique de base n'est jamais descendu en dessous de -1,14 MPa, mais on pouvait déjà observer un brunissement généralisé des plus vieilles aiguilles. Cette diminution du potentiel de base s'est accompagnée d'une diminution de la conductance et de la transpiration maximales pour atteindre 15% de leur plus forte valeur, qui pour cette station sont respectivement de 150 mmol/m2/s et 1 mmol/m2 /s. La conductance stomatique maximale a le plus souvent eu lieu avant 7 h TU, et la transpiration maximale 5 ou 6 h après, indépendamment du potentiel de base. De plus, dans les 2 stations, les stomates se ferment quand le déficit de pression de vapeur atteint seulement 0,3 kPa. Cette réaction extrêmement précoce au stress hydrique est cohérente avec la légendaire sensibilité du sapin à l'humidité atmosphérique ainsi qu'à l'eau dans le sol. Elle dénote chez cette essence une nette stratégie d'évitement que l'on croyait jusqu'alors être l'apanage d'espèces mieux adaptées à la sécheresse

Supervised-distributed control with joint performance and communication optimization

This paper is concerned with the control of systems composed of multiple coupled subsystems. In such architectures, communication between different local controllers is desired in order to achieve a better overall control performance. Any resultant improvement in control performance needs, however, to be significant enough to warrant the additional design complexity and higher energy consumption and costs associated with introducing communication channels between controllers. A practical distributed control design aims, therefore, to achieve an acceptable balance between minimizing the use of communication between controllers and maximizing the system-wide performance. In this article, a new approach to the problem of synthesizing stabilizing distributed control laws for discrete-time linear systems that balances performance and communication is presented. The approach employs a supervisory agent that, periodically albeit not necessarily at every sampling instant, solves an optimization problem in order to synthesize a stabilizing state feedback control law for the system. The online optimization problem, which maximizes sparsity of the control law while minimizing an infinite-horizon performance cost, is formulated as a bilinear matrix inequality (BMI) problem; subsequently, it is then relaxed to a linear matrix inequality (LMI) problem, and (i) convergence to a solution as well as (ii) that early termination guarantees a feasible (but suboptimal) control law are proved. Stability of the closed-loop system under what is a switched control law is guaranteed by the inclusion of dwell-time constraints in the LMI problem. Finally, the efficacy of the approach is demonstrated through numerical simulation examples

Simultaneous and sequential control design for discrete-time switched linear systems using semi-definite programming

The control of switched linear discrete-time systems occurs in multiple engineering fields, where it has been used to deal with complex and non-linear systems. This paper presents two strategies to design control laws for discrete-time switched linear systems, whilst guaranteeing asymptotic stability of the closed loop. Firstly, an arbitrary switching signal is considered. In this scenario a common quadratic Lyapunov function is used for stability, but subsystem Lyapunov functions are employed to improve local subsystem performance. Secondly, a constrained switching signal, associated with subsystem lower dwell time bounds is studied. In this case, a decrease in Lyapunov cost is achieved by design, based on dwell time constraints only, thus removing the need for both a common quadratic Lyapunov function or direct stable switches. It is shown in both cases that the control design problems can be formulated as one or a sequence of semi-definite programming problems, and therefore can be solved efficiently. Finally, two examples are provided in order to illustrate the different techniques presented

Essai de typologie des symptomes de deperissement dans les massifs dauphinois externes (Vercors-Chartreuse-Belledonne)

CNRS RP 185 (2331) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Le point des connaissances relatives à la croissance et au développement du Frêne commun (Fraxinus excelsior

International audienc