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During the 1996 growing season the seasonal dynamics of the Leaf Area Index (LAI) were determined by 3 different methods in two forest types: a mixed oak (Quercus robur L.) â beech (Fagus sylvatica L.) stand and an ash dominated (Fraxinus excelsior L.) stand. The results obtained from the two indirect methods, i.e. hemispherical photography and LAI-2000 Plant Canopy Analyser (Li-COR), were compared with the results of the direct measurement of litter fall collected in litter trap systems. In this study the direct method is considered to be the reference, giving the most accurate LAI-values. Both the hemispherical photography and the LAI-2000 PCA introduced an underestimation of LAI when the actual canopy leaf distribution in the crown layer deviates from a random distribution of leaf area in space as is found in the mixed oak/beech stand. However, when the condition of random leaf distribution is nearly fulfilled (ash stand), the LAI-2000 PCA gave LAI-values which were close to the results obtained from the direct method. Regression curves with R2 > 0.93 could be calculated for both indirect methods.

Assimilation and exchange of carbon for arctic ecosystems depend strongly on leaf area index (LAI) and total foliar nitrogen (TFN). For dry-mesic tundras, the seasonality of these characteristics is unexplored. We addressed this knowledge gap by measuring variations of LAI and TFN at five contrasting subarctic heaths during the growing season 2007, from about 2 weeks after bud burst until about 2 weeks before senescence. The communities generally showed an early season LAI and TFN increase, owing to leaf development of deciduous shrubs, and limited variations later on, owing to concurrent leaf growth and mortality of evergreen shrubs. For most of the communities, the TFN:LAI ratio was constant along the entire sampling period, indicating that leaf growth and mortality, triggers for both LAI and TFN, are more important than leaf N retranslocation, which only affects TFN. A constant TFN:LAI ratio facilitates the determination of TFN from LAI, which is easier to measure or simulate. However, a sheltered community showed an increment in the LAI of the deciduous canopy in mid-late season, paralleled by a decreased TFN:LAI ratio. Key words: leaf traits, leaf area index, foliar nitrogen, seasonal canopy dynamics, northern heath, ecosystem C cycle. Dans les ecosystemes arctiques, l'aasimilation et l'echange du carbone dependent fortement de l'index de l'aire foliaire (LAI) et de l'azote foliaire total (TFN). En toundras mesiques-seches, la saisonnalite de ces caracteristiques demeure inexploree. Les auteurs se sont propose de combler ce vide en mesurant les variations des caracteres foliaires, LAI et TFN dans cinq ericacaies contrastees an cours de la saison 2007, pendant une periode allant de 2 semaines apres l'ouverture des bourgeons allant jusqu'a deux semaines avant la senescence. Les communautes montrent generalement une augmentation precoce des LAI et TFN, due au developpement foliaire des arbustes decidus, et une variation limitee par la suite due a la croissante foliaire et la mortality concomitante des arbustes sempervirents. Dans la majorite des communautes, le rapport TFN:LAI demeure constant tout au long de la periode d'echantillonnage, ce qui indique que la croissance foliaire et la mortality, declencheurs pour les LAI et TFN, ont plus d'importance que la retranslocation de N, qui n'affecte que le TFN. Un ratio TFN:LAI constant facilite la determination du TFN a partir du LAI, ce qui est plus facile a mesurer ou a simuler. Cependant, une communaute abritee a montre une augmentation du LAI de la canopee decidue vers le milieu et la fin de la saison, suivi en parallele d'une diminution du ratio TFN:LAI. Mots-cles : caracteres foliaires, index de l'aire foliaire, azote foliaire, dynamique saisonniere de la canopee, ericacaies nordiques, cyclage ecosystemique du carbone. [Traduit par la Redaction], Introduction Global changes are expected to be particularly pronounced at high latitudes and to profoundly modify ecology and biogeochemistry of arctic terrestrial ecosystems (Callaghan et al. 2004). The large amount [...]

The spatial and diurnal variability of sap flow in a mature beech tree (Fagus sylvatica L.) was investigated on days with different climatic conditions (sunny, cloudy, rainy), during the summer. Sap flux density (v) was measured with six heat field deformation probes placed around the stem circumference. Each probe measured v at six sapwood depths. Daily v exhibited clear radial variation, and the shape of the radial profile of v differed substantially among circumferential positions. At some positions, daily v decreased monotonically towards the stem centre, whereas at others it showed an almost monotonic increase. Hence, the conducting sapwood area of the beech was highly asymmetrical. At all positions, conducting sapwood reached beyond the deepest sampled sapwood depth, precluding correct estimations of total sap flow. The radial profile of v also differed among measuring days. A general trend was that the inner sapwood contributed relatively more to total sap flow under better weather conditions. Besides variations among days, the shape of the radial profile of v also showed within-day variations. The contribution of the inner sapwood to total sap flow increased in the afternoon, with increasing vapour pressure deficit and photosynthetic active radiation. Because of large circumferential and temporal variability, no general function for the radial profile of v could be developed. Key words: circumferential variability, heat field deformation method, multi-point sensor, radial profile, sap flux density, temporal variability. Les auteurs ont mesure la variation spatiale et diurne du flux de la seve chez des hetres matures (Fagus sylvatica L.) au cours de journees differentes (ensoleillees, nuageuses, pluvieuses), en ete. Ils ont mesure la densite du flux de seve (v) avec six sondes de deformation du champ thermique placees autour de la circonference du tronc. Chaque sonde a mesure le v a six profondeurs dans l'aubier. Le v quotidien montre une nette variation radiale et la forme du profil radial de v differe substantiellement selon les positions circonferentielles. A certaines positions, le v quotidien diminue de facon monotonique vers le centre de la tige, alors qu'a d'autres il montre presque une augmentation monotonique. Consequemment, la surface d'aubier conducteur du hetre s'est averee fortement asymetrique. A toutes les positions, l'aubier conducteur a rejoint des profondeurs plus avancees que la profondeur des mesures, empechant ainsi des evaluations du flux total de la seve. Le profil radial du v differe egalement selon les jours de mesure. La tendance generale montre que l'aubier interne contribue relativement plus au flux total de seve sous les meilleures conditions meteorologiques. En plus des variations selon les jours, la forme du profil radial de v montre egalement des variations au cours de la journee. La contribution de l'aubier interne au flux total de seve augmente en apres-midi, avec des augmentations de VPD et de PAR. Du a la forte variabilite circonferentielle et temporelle, les auteurs n'ont pas pu developper de fonction generale pour le profil radial de v. Mots-cles : variabilite circonferentielle, methode par deformation du champ thermique, senseur a points multiples, profil radial, densite du flux de seve, variabilite temporelle., [Traduit par la Redaction] Introduction Xylem sap flow measurements are widely used to investigate tree transpiration, which is an important component of a forest's hydrological balance. Sap flow (F; grams [...]

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 less than 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. [ABSTRACT FROM AUTHOR]