We estimated daily use of stored water by Scots pine (Pinus sylvestris L.) trees growing in a temperate climate with the ANAFORE model (ANAlysis of FORest Ecosystems) and compared the simulation results with sap flow measurements. The original model was expanded with a dynamic water flow and storage model that simulates sap flow dynamics in an individual tree. ANAFORE was able to accurately simulate diurnal patterns of measured sap flow under microclimatic conditions that differ from those of the calibration period. Strong relationships were found between stored water use and several tree characteristics (diameter at breast height, sapwood area, leaf area), but not with tree height. Relative to transpiration, stored water use varied over time (between < 1% and 44% of daily transpiration). On days when transpiration was high, trees were more dependent on stored water, indicating that the contribution of internal water to transpiration is not a constant in the water budget of trees.

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Sap flow and potential evapotranspiration rates were analyzed for two coniferous tree species (Douglas-fir and Scots pine) and one broadleaf species (sessile oak) in a mixed Carpineto-Quercetum forest during the growing season 2005. The relationship between sap flow and potential evapotranspiration rates, effective crown area as a measure of the relative transpiration and daily relative proportion of the storage water used for transpiration were used as indicators of the tree water dynamics. These indicators were determined on four consecutive days and all three showed good reliability concerning tree water dynamics.Additional key words: Quercus petraea, Pinus sylvestris, Pseudotsuga menziesii, transpiration rate, water balance, water deficit. ⎯⎯⎯⎯Water relations belong to the most important physiological processes in plants, because water is often growth limiting (Larcher 2003). Continuous sap flow measurements on sample trees provide a valuable background for the analysis of physiological processes and of the water balance (Nadezhdina 1999). Beside transpiration measurements, there is a certain lack of appropriate approaches to interpret the physiological behaviour of trees in relation to the water dynamics in situ (Ortuño et al. 2007). Because of the wide availability of sap flow measurements they could be used as a robust tool to analyze tree water dynamics in field conditions, especially under critical soil water availability.The different indicators of tree water dynamics discussed in this article differ about the means of calculation and how they are characterizing the tree water status. First, water stress conditions can be characterized by a continuous decrease of the sap flow rate (Q wt ) under nearly constant diurnal potential evapotranspiration (PET) conditions (indicator 1). Changes in the slope of the linear regression between hourly fluxes of Q wt and PET corrected for time shift give a specific idea of the tree water dynamics (Table 1). A decrease of the slope can be interpreted as an increase of the tree water deficit. Further, the effective crown area (A eff -indicator 2) can be obtained from the ratio between daily rates of tree transpiration and PET: A eff = Q wt /PET. This indicator works when relating tree level sap flow with stand level PET data (Čermák et al. 1982). The A eff on a reference day (DOY 231 in our study) gives the maximal A eff involved in tree transpiration over the growing season. The ratio of A eff on a particular day to the maximal A eff provides the relative A eff , an estimate of the tree hydraulic conductivity. Finally, the hourly course of PET is scaledup to the sample tree level (PET tree ) by multiplying it with the effective crown area. Since the fluxes of sap flow and transpiration are lagged in time due to the primary use of the water stored in tree tissues, the daily integrated difference between hourly fluxes of Q wt and PET tree is applied to estimate the amount of used storage water. When Q wt is lower than PET tree (mostly in the early morning), the water use...

Platinum electrodes were implanted into the xylem of a lime tree (Tilia cordata) stem and solar-induced electrochemical potential differences of up to 120 mV were measured during the vegetative period and up to 30 mV in winter. The time dependent curves were found to be delayed with respect to solar radiation, sap flow activity, temperature and vapor pressure deficit. A general equation for the potential difference was derived and simplified by analyzing the effect of temperature and tensile strength. The potential determining influence of oxygen concentration on the respective location of the platinum electrode was identified as the principal phenomenon measured. A systematic analysis and investigation of the observed periodic oxygen concentration signals promises new information on sap flow, oxygen diffusion through tree tissues and on oxygen consumption related to the energy turnover in tree tissues.