Modelling water fluxes in plants: from tissues to biosphere

Contents Summary 1207 I. Introduction 1207 II. A brief history of modelling plant water fluxes 1208 III. Main components of plant water transport models 1208 IV. Stand-scale water fluxes and coupling to climate and soil 1213 V. Water fluxes in terrestrial biosphere models and feedbacks to community dynamics 1215 VI. Outstanding challenges in modelling water fluxes in the soil-plant-atmosphere continuum 1217 Acknowledgements 1218 References 1218 SUMMARY: Models of plant water fluxes have evolved from studies focussed on understanding the detailed structure and functioning of specific components of the soil-plant-atmosphere (SPA) continuum to architectures often incorporated inside eco-hydrological and terrestrial biosphere (TB) model schemes. We review here the historical evolution of this field, examine the basic structure of a simplified individual-based model of plant water transport, highlight selected applications for specific ecological problems and conclude by examining outstanding issues requiring further improvements in modelling vegetation water fluxes. We particularly emphasise issues related to the scaling from tissue-level traits to individual-based predictions of water transport, the representation of nonlinear and hysteretic behaviour in soil-xylem hydraulics and the need to incorporate knowledge of hydraulics within broader frameworks of plant ecological strategies and their consequences for predicting community demography and dynamics

A simple field validation of daily transpiration derived from sapflow using a porometer and minimal meteorological data

The original publication can be found at www.springerlink.comHeat-pulse techniques are routinely used to estimate transpiration from canopies of woody plants typically without any local calibration, mainly because of the difficulty of doing so in the field and, frequently, lack of detailed weather data. This is despite concerns that the techniques may produce erroneous values under certain conditions, such as when evaporative demand is high. In this study, we used a micrometeorological approach to validate transpiration from irrigated olives deduced from heat-pulse technique by ascertaining precise values for the parameters that are critical for converting heat-pulse velocity to sapflow. The micrometeorological approach involved limited data on stomatal conductance (gs), obtained hourly with a porometer on four contrasting days, and was used to calibrate a simple model for predicting conductance. Predicted stomatal conductance (gsm) agreed well with that measured, and when both were used to calculate hourly transpiration, they produced values that were within 10% of each other. This was despite brief underestimations of transpiration based on gsm (Tm) in the early hours of the day that arose from poor determination of incident radiation at this time. We then used Tm to iteratively set the values for the various parameters, including the time-out value that accounts for zero-flow conditions, needed to convert heat-pulse velocity to sapflow, for the four days. The best fit between Tm and transpiration from sapflow (Ts) was obtained with time-out value set to 120 s. All heat-pulse velocity data were therefore analysed with this time-out value to obtain sapflow and, hence, transpiration (Ts). Comparison of Tm and Ts for the whole season showed that the former tended to produce higher values on certain days when vapour pressure deficit (D) was high in summer (December–February). While Ts occasionally produced larger values than Tm under the mild conditions of autumn (March–April). Totals of the daily transpiration during the 190-day period were within 10% of each other.Isa A. M. Yunusa, Ian K. Nuberg, Sigfredo Fuentes, Ping Lu and Derek Eamu

Sap flux density measurements based on the heat field deformation method

Accurate measurements of whole tree water use are needed in many scientific disciplines such as hydrology, ecophysiology, ecology, forestry, agronomy and climatology. Several techniques based on heat dissipation have been developed for this purpose. One of the latest developed techniques is the heat field deformation (HFD) method, which relies on continuous heating and the combination of a symmetrical and an asymmetrical temperature measurement. However, thus far the development of this method has not been fully described in the scientific literature. An understanding of its underlying principles is nevertheless essential to fully exploit the potential of this method as well as to better understand the results. This paper therefore structures the existing, but dispersed, data on the HFD method and explains its evolution from an initial ratio of temperature differences proportional to vapor pressure deficit to a fully operational and practically applicable sap flux density measurement system. It stresses the importance of HFD as a method that is capable of measuring low, high and reverse flows without necessitating zero flow conditions and on several sapwood depths to establish a radial profile. The combination of these features has not been included yet in other heat-based sap flow measurement systems, making the HFD method unique of its kind

The effect of short-term flooding on the sap flow, gas exchange and hydraulic conductivity of young apricot trees

The effect of short-term flooding was examined in 2-year-old apricot trees (Prunus armeniaca cv. Búlida). Six apricot trees of similar appearance were submitted to two treatments: three were irrigated daily, while the others were flooded for a period of 50 h by submerging the pots in plastic water tanks. The trees were removed from the water, drained and then placed in the same conditions as the control plants. A decrease in transpiration in the flooded trees with respect to the control plants was evident. The daily pattern of soil O2 concentration and plant hydraulic resistance followed a similar trend during the flooding. However, this relationship was not maintained throughout the experiment, since the O2 values increased rapidly when the waterlogging ceased, while plant hydraulic resistance only recovered at the end of the experiment when the original root system, damaged by flooded conditions, was replaced with new roots. In flooded trees, the midday leaf water potential decreased progressively from the beginning of flooding, but gradually recovered when the waterlogging ceased. Leaf conductance values of treated plants were slow to recover, reaching values of the control plants 8 days after the leaf water potential had recovered. The close relationship observed during most of the experiment between the leaf water parameters, leaf conductance and plant hydraulic conductance indicate that hydraulic messages are likely to play a dominant role in co-ordinating the observed responses of the shoot.The study was supported by two projects: Riego inteligente para un manejo sostenible en frutales (CYCIT-AGL2000-0387-CO5-O4) and Desarrollo de un equipo autónomo para la medición de caudal de savia en plantas leñosas (PETRI-95-0693-01).Peer reviewe