-Heat and drought were extreme in summer 2003 in Europe. Climatic data show that most extreme were maximum air temperatures in June and August; maps of these two months show a striking similarity in geographical range. Over wide regions, monthly mean temperatures were more than

The upward movement of water due to transpiration stops when soil water potential ( s ) drops below leaf water potential ( L ). Under these circumstances, water can move in any direction in the plant-soil continuum through the passive conduits of roots and stems towards the lowest  s . This is generally termed as hydraulic redistribution (HR), but the positioning and orientation of the driving water potential gradient may vary. Any experimental method that can measure bi-directional and low flows in the sapwood of roots and stems will be suitable to detect HR. Using one approach for measuring sap flow (the heat field deformation technique, HFD) in several forest species and sites across Europe, we were able to provide evidence on different types of HR: vertical hydraulic redistribution (VHR), horizontal hydraulic redistribution (HHR), foliar uptake (FU) and tissue dehydration (TD). VHR is the vertical water movement through roots in response to water potential differences between deep and topsoil, either hydraulic lift or hydraulic descent. HHR is the lateral water movement through roots in response to horizontal water potential gradients, namely under localised irrigation. FU is the water movement from crown to soil through stems when the crown is wetted by foggy weather. TD is the downward movement of water in stems or roots from above-ground tree tissues to soil under prolonged drought or frost. Results from direct sap flow measurements indicated the vectoral and widespread nature of HR, a phenomenon of paramount importance for overall physiology and ecohydrology.

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In this article, we outline the set-up and the application of an eco-hydrological box model, with the aim to describe the water balance of deciduous (Fagus Sylvatica L.) forest stands. The water balance model (WBM) uses standard meteorological parameters as input variables and runs on a daily time step. It consists of two modules. The aboveground module (1) comprises routines for fog precipitation generation, precipitation interception and snowfall/snowmelt dynamics. Covered belowground processes (2) are bypass flow, percolation, soil evaporation and transpiration, where the latter two processes are considered separately. Preceding to the WBM, a routine is introduced, specifying the intra-annual foliage dynamics of beech. Emphasis is also laid on the inter-annual variation of beech phenology. Leaf sprouting and leaf senescence are calculated as functions of day-length and air temperature. The WBM was applied to four European beech dominated forest stands in the northeastern part of Austria. They are located on a gradient of declining annual precipitation (from west to east). The two easterly sites are located close to the (dry) limit of the natural distribution of beech. Records of soil moisture were used for the adjustment of 26 parameters. On all sites the calibration process (simulated annealing) delivered good predictions of soil moisture (Nash–Sutcliffe efficiency≥ 0.925). Then, the obtained parameterization was used to apply different scenarios of global warming. The temperature was increased step-wisely up to 4 °C. All scenarios were run (1) with present phenological conditions and (2) with phenology responding to higher temperatures. This way, we wanted to assign the effect of higher temperatures and longer growing seasons on the water dynamics of the forest stands. A warming of 1 °C corresponded roughly to an elongation of the growing season of 4.5 days, where the start of the growing season was affected more strongly than the end. Apparently, higher temperatures led to drier soils. The strongest change was observed in early summer, also amplified by an earlier start of the growing season. Rising temperatures led to lower export fluxes of liquid water, simultaneously increasing evapotranspiration (ET). The gain in ET was almost entirely assignable to increased soil evaporation. Drier soils led to a sharp depression of transpiration during summer months. This decline was compensated by the effect of elongated growing seasons. The risk of severe drought was increased by higher temperatures, but here the contribution of growing season length was negligible. Drier soils seem to hamper the stands’ productivity. For all warming scenarios, the estimated increase of the gross primary production, caused by longer periods of assimilation, is nullified by the effect of soil water deficit in mid-summer.

In this article, the setup and the application of an empirical model, based on Newton’s law of cooling, capable to predict daily mean soil temperature (Tsoil) under vegetated surfaces, is described. The only input variable, necessary to run the model, is a time series of daily mean air temperature. The simulator employs 9 empirical parameters, which were estimated by inverse modeling. The model, which primarily addresses forested sites, incorporates the effect of snow cover and soil freezing on soil temperature. The model was applied to several temperate forest sites, managing the split between Central Europe (Austria) and the United States (Harvard Forest, Massachusetts; Hubbard Brook, New Hampshire), aiming to cover a broad range of site characteristics. Investigated stands differ fundamentally in stand composition, elevation, exposition, annual mean temperature, precipitation regime, as well as in the duration of winter snow cover. At last, to explore the limits of the formulation, the simulator was applied to non-forest sites (Illinois), where soil temperature was recorded under short cut grass. The model was parameterized, specifically to site and measurement depth. After calibration of the model, an evaluation was performed, using ~50 % of the available data. In each case, the simulator was capable to deliver a feasible prediction of soil temperature in the validation time interval. To evaluate the practical suitability of the simulator, the minimum amount of soil temperature point measurements, necessary to yield expedient model performance was determined. In the investigated case 13–20 point observations, uniformly distributed within an 11-year timeframe, have been proven sufficient to yield sound model performance (root mean square error <0.9 °C, Nash–Sutcliffe efficiency >0.97). This makes the model suitable for the application on sites, where the information on soil temperature is discontinuous or scarce.