Change of whole-tree transpiration of mature Hevea brasiliensis under soil and atmospheric droughts: analyze in intermittent and seasonal droughts under the framework of the hydraulic limitation hypothesis

The changes of whole-tree transpiration under atmospheric and soil droughts were investigated for mature rubber trees (Hevea brasiliensis, clone RRIM 600) in a drought-prone area of North-East Thailand. The changes were analyzed and compared between intermittent drought in the rainy season and seasonal drought where occur periods of lower leaf area (leaf senescence, massive leaf shedding and leaf flushing) and subsoil root growth. The physiological responses were analyzed under the framework of a simple hydraulic limitation model called 'RER_ET0'. This model is mainly based on a critical minimum leaf water potential (ψcrit) and the responses of whole-tree hydraulic conductance (gL) and predawn leaf water potential (ψpredawn) to relative extractable water (REW). Continuous and accurate estimate of whole tree transpiration over a whole year was a key measurement of this study with concurring measurement of environmental conditions and leaf water potential (ψLeaf). We applied the transient thermal dissipation method (TTD) developed by Do and Rocheteau (2002b) which has several advantages but which was never tested in rubber wood The TTD method was calibrated in the laboratory with cut stems of Hevea brasiliensis and two other species (Mangifera indica and Citrus maxima). The results provided a simple linear calibration which was independent of the woody species tested (R² = 0.88, n = 276, P<0.0001). Field comparison with soil water depletion in the dry season validated to a reasonable extent the absolute estimates of transpiration. In rainy season, the first remarkable result was the assessment of saturating transpiration above reference evapotranspiration (ET0) c.a. 2.2 mm day-1 in period of well soil watered conditions. Intermittent soil drought provoked a dramatic decrease of transpiration below a threshold of 50% REW in the top soil which corresponded to a ψpredawn c.a. -0.45 MPa. Transpiration was reduced by 40% at 0.3 REW and 80% at 0.1 REW. The minimum leaf water potential (ψminimum) for sunny day did not decrease according to soil drought and was stable around -1.95 MPa which supported an isohydric behaviour. The decrease of transpiration was mainly due to the change of gL. Results in the seasonal drought showed that the principles of the hydraulic limitation model hold despite changing phenology and leaf area. Moreover, the deciduous pattern showed little influence upon the details of parameters and relationships. Finally, the framework of a simple hydraulic limitation model was successful to describe the main change of whole-tree transpiration under both atmospheric and soil drought. Both our experimental and modeling results demonstrate the isohydric behaviour of the studied mature rubber trees.Les variations de transpiration totale sous contraintes hydriques, à la fois atmosphérique et édaphique, sont étudiées pour des arbres matures d'Hevea brasiliensis (clone RRIM600) dans une zone limitée en eau du Nord-Est de la Thaïlande. Les variations sont analysées et comparées entre une sécheresse transitoire en saison des pluies et la sécheresse saisonnière caractérisée par des fluctuations de la surface foliaire (sénescence foliaire, chute des feuilles and re-feuillaison) et de la croissance racinaire en profondeur. Les réponses physiologiques sont analysées dans le cadre d'un modèle hydraulique simple appelé "RER_ET0". Ce modèle est principalement basé sur une valeur critique du potentiel hydrique foliaire (ψcrit) et sur les réponses, de la conductivité hydraulique totale de l'arbre (gL) et du potentiel hydrique de base (ψpredawn), à la disponibilité en eau du sol (REW). Une estimation précise et en continue de la transpiration totale des arbres durant une année complète était une mesure clé de cette étude avec des mesures concomitantes des conditions environnementales et du potentiel hydrique foliaire (ψLeaf). Nous avons appliqué la méthode à dissipation thermique transitoire (TTD) développée par Do et Rocheteau (2002b) qui a plusieurs avantages mais n'avait pas été testée sur bois d'hévéa. La méthode TTD a été calibrée en laboratoire sur des segments de branches d'Hevea brasiliensis et de deux autres espèces (Mangifera indica and Citrus maxima). Les résultats fournissent une calibration linéaire unique indépendante des espèces étudiées (R² = 0.88, n = 276, P