Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root–soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root–soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root–soil interface.

Using data from 20 even-aged and homogeneous mature beech and oak study plots in Flanders (Northern Belgium), an analysis of the empirical relationships between the rates of leaf area index (LAI) change throughout the leaf development of 2008 and stand, site and meteorological variables was performed. Species-specific multiple linear regressions were fitted between the rates of LAI change and the predictors for two distinct periods from April until August. After a sharp increase in LAI following budburst, the seasonal LAI development for both species showed a marked period of stationary LAI development over all study plots. The cause for the cessation of LAI growth was assumed to be the decline of air temperature and radiation during this period. Later on, the rate of LAI development restarted similarly in every plot. The influence of weather on LAI development was high and its effects were different between species, with beech mostly affected by radiation and oak negatively related to minimal and maximal values of air temperature. Furthermore, our analysis suggested that stand structural (tree density and stand basal area for both species) and tree growth characteristics (average tree-ring width ratio for oak) variables were major drivers of the LAI development during early spring. Later during the growth period, stand variables became less predominant in affecting LAI development. Site quality variables affected LAI development to a lesser extent. The seasonal LAI development was found very similar among stands. This study adds a more accurate and comprehensive approach to the modelling of LAI development during leaf growth of two important European temperate deciduous forest species

Summary• Increasing evidence about hydraulic redistribution and its ecological consequences is emerging. Hydraulic redistribution results from an interplay between competing plant and soil water potential gradients. In this work, stem-mediated hydraulic redistribution was studied in a 53-year-old Douglas-fir tree during a period of drought.• Sap flux density measurements using the heat field deformation method were performed at four locations: in two large opposing roots and on two sides of the tree stem. Hydraulic redistribution was induced by localized irrigation on one of the measured roots, creating heterogeneous soil water conditions.• Stem-mediated hydraulic redistribution was detected during night-time conditions when water was redistributed from the wet side of the tree to the nonirrigated dry side. In addition to stem-mediated hydraulic redistribution, bidirectional flow in the dry root was observed, indicating radial sectoring in the xylem.• It was observed that, through stem-mediated hydraulic redistribution, Douglasfir was unable to increase its transpiration despite the fact that sufficient water was available to one part of the root system. This resulted from the strong water potential gradient created by the dry soil in contact with the nonirrigated part of the root system. A mechanism of stem-mediated hydraulic redistribution is proposed and its possible implications are discussed.

Different multiple linear regression models of maximum leaf area index (LAI max ) based on stand characteristics, site quality, meteorological variables and their combinations were constructed and cross-validated for three economically important tree species in Flanders, Belgium: European beech (Fagus sylvatica L.), Pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.). The models were successfully tested on similar datasets of experimental sites across Europe. For each species, ten homogeneous and mature stands were selected, covering the species' entire stand productivity range based on an a priori site index classification. LAI max was derived from measurements of leaf area index (LAI) made by means of hemispherical digital photography over the whole growing season (mid-April till end October 2008). Speciesspecific models of LAI max for beech and oak were mostly driven by management practice affecting stand characteristics and tree growth. Tree density and dominant height were main predictors for beech, while stand age and treering growth were important in the oak models. Scots pine models were more affected by site quality and meteorological variables. The beech meteorological model showed very good agreement with LAI at several European sites. Scots pine's stand model predicted well LAI across Europe. Since the species-specific models did not share common predictors, generic models of LAI max were developed for the 30 studied sites. Dominant height was found to be the best predictor in those generic models. As expected, they showed a lower predictive performance than speciesspecific ones.

8Forest canopy density can be highly variable within one stand. The accuracy of indirect 9 methods to quantify stand leaf area index (LAI) is often unknown, and intensive sampling strategies are 10 required. Our objectives were to study the drivers of the spatial LAI variability, and to improve the 17inventory data were used to study the drivers of the deviation of patch LAI (LAI dev ) relative to the 18 average plot LAI. Average tree distance from a patch centre was negatively related with the LAI dev . Tree 19 structural characteristics (diameter at breast height (DBH), tree height, crown length and crown cover) 20were all positively related to the LAI dev . Based on our findings, we suggest that for the analysed forest 24 Introduction 26The leaf area index (LAI; m 2 leaf m -2 ground) is a generic ecosystem characteristic 27 determined by and, in turn, influencing, several key processes of the forest ecosystem. LAI is a key 28 determinant of the total photosynthetic carbon uptake, and therefore, of forest productivity (Waring 29 1983; Maguire et al. 1998). LAI also determines other crucial aspects of forest canopy functioning, 30such as rainfall interception and evapotranspiration. Furthermore, LAI directly influences radiation 31 intensity reaching the ground (Smith 1981). It is therefore important to accurately quantify LAI 32 (Eckhardt et al. 2003). 33Direct methods of assessing LAI are the most accurate (Bréda 2003; Jonckheere et al. 2004). 34However, they are extremely labour intensive and time consuming which makes them less applicable 2004; Fuentes et al. 2008). Yet, the sampling strategy (design and intensity) for indirect LAI 44 measurements remains a source of inaccuracy (Strachan and McCaughey 1996). 45Within every forest -even inside homogeneous even-aged stands -LAI is spatially highly 46 variable. Furthermore, site parameters and management practices might over time induce canopies to 47 become heterogeneous regarding foliage distribution (Seidling 2007 61The current study focused on the relationships between the spatial variability of LAI and tree 62 structural characteristics in small-scale plots (around 1000 m 2 ) during the period of peak LAI for two 84A study plot of about 1000 m 2 was established in each of the 30 forest stands. We assumed 85homogeneity of site quality parameters within these plots. Every study plot consisted of a circular 97Tokyo, Japan) mounted on a Canon 5D digital professional camera (Canon Inc., Tokyo, Japan). 98Hemispherical photographs were taken at the highest camera resolution (13.3 mega pixels) and with 99ISO set at 200. The focus ring was set to infinity as depth of field was practically infinite. The camera 100 was placed on a tripod with the top of the lens 1.3 m above the ground and the camera oriented in such a 101 way that the magnetic north was always located at the top of the photograph. The lens position was 102 manually fitted in the vertical and horizontal axes with the help of a double water-level. Calibration of 103 the camera's...