Ground-Based Optical Measurements at European Flux Sites: A Review of Methods, Instruments and Current Controversies

This paper reviews the currently available optical sensors, their limitations and opportunities for deployment at Eddy Covariance (EC) sites in Europe. This review is based on the results obtained from an online survey designed and disseminated by the Co-cooperation in Science and Technology (COST) Action ESO903—“Spectral Sampling Tools for Vegetation Biophysical Parameters and Flux Measurements in Europe” that provided a complete view on spectral sampling activities carried out within the different research teams in European countries. The results have highlighted that a wide variety of optical sensors are in use at flux sites across Europe, and responses further demonstrated that users were not always fully aware of the key issues underpinning repeatability and the reproducibility of their spectral measurements. The key findings of this survey point towards the need for greater awareness of the need for standardisation and development of a common protocol of optical sampling at the European EC sites

Validity of leaf areas and angles estimated in a beech forest from analysis of gap frequencies, using hemispherical photographs and a plant canopy analyzer

Using both a Li-Cor Plant Canopy Analyzer (PCA) and the hemispherical photographs technique, we measured the gap fraction in two young beech pole stands of known leaf tip angle distribution. The average contact number at various zenith angles (K(θ) function) was determined and leaf area index was calculated using the method proposed previously. The following cases were examined: 1) data from PCA using five, four or three rings, 2) data from hemispherical photographs, arranged in rings, and divided into azimuth sectors (90, 45 and 22.5°) or averaged over azimuth (360°). These results were compared with a semi-direct estimation of the leaf area index derived from allometric relationships established at tree level. We also compared the G(θ) functions calculated using direct measurements of the leaf tip angle distribution with those deduced from transmittance data. The two indirect techniques gave the same estimation of the gap fraction at all zenith angles. When data were processed using the random model (averaged over azimuth), the PCA and photographs provided the same values of leaf area index, these values being considerably lower than those from allometric relationships (-25 %). When data from hemispherical photographs were divided into narrow azimuth sectors (22.5°), assuming a quasi-random model, the estimate of leaf area index was improved, but remained about 10 % below the allometric estimates. Leaf area index estimated using the random model was found to be 75 % of that estimated using allometric relationships. It is shown that the underestimation of the leaf area index observed considering all five rings on the PCA is due to an inappropriate use of the random model. It is also shown that the increase in leaf area index that was observed when neglecting one or two rings (PCA) was caused by an important error in the estimation of the slope of the function K(θ). We quantified this bias which depends on the leaf angle distribution within the canopy. Errors made on K function by the PCA are often compensated by an arbitrary omission of one or two rings. The consequences of neglecting these rings are discussed, together with the respective interest of both techniques. (© Inra/Elsevier, Paris.)Estimation des surfaces et angles foliaires dans une hêtraie par deux techniques indirectes : la photographie hémisphérique et le Plant Canopy Analyzer. Nous avons mesuré la fraction de trouées dans deux gaulis de hêtres en utilisant deux techniques différentes : la photographie hémisphérique et le Plant Canopy Analyzer de Li-Cor (LAI-2000). Le nombre moyen de contacts dans plusieurs directions zénithales (fonction K(θ)) a été déterminé, puis l'indice foliaire a été calculé en utilisant la méthode proposée par Lang [14]. Nous avons effectué ce calcul pour : 1) le PCA (Li-Cor), en utilisant trois, quatre ou cinq anneaux, 2) les photographies hémisphériques subdivisées en anneaux concentriques puis en secteurs azimutaux de 360, 90, 45 ou 22,5°. Les résultats ont été comparés à une estimation semi-directe de l'indice foliaire basée sur des relations allométriques à l'échelle de l'arbre. Les deux techniques indirectes fournissent la même estimation de la fraction de trouées dans chaque anneau. Lorsque les données sont traitées avec le modèle aléatoire, le PCA et les photographies donnent la même valeur d'indice foliaire, laquelle est nettement plus faible que celle estimée par allométrie (-25 %). Lorsque les photographies sont traitées en prenant en compte l'hétérogénéité directionnelle de la fraction de trouées dans chaque couronne (traitement par secteurs azimutaux de 22,5°), l'estimation de l'indice foliaire est meilleure, sans toutefois atteindre la valeur obtenue par allométrie (-10 %). Notre étude confirme que la sous-estimation fréquemment observée en utilisant le PCA avec cinq anneaux s'explique par l'utilisation inadéquate du modèle aléatoire. Sur un plan théorique, nous montrons que l'omission d'un ou des deux anneaux inférieurs, lors du traitement des données PCA, amène un biais dans l'estimation de la pente de la fonction K(θ). L'erreur commise dépend de la distribution d'inclinaison foliaire. Dans le cas très fréquent d'une distribution planophile du feuillage, l'erreur commise par l'omission arbitraire d'un ou plusieurs anneaux est globalement compensée par la sous-estimation d'indice foliaire due à l'utilisation du modèle aléatoire. Nous discutons des conséquences du mode d'exploitation des données, et de l'intérêt respectif des deux techniques utilisées. (© Inra/Elsevier, Paris.

Spatial and seasonal variations in stem respiration of beech trees (Fagus sylvatica)

Stem respiration of adult beech (Fagus sylvatica L.) trees was measured in the field in eastern France at several levels in the crown and along the stem. Strong variations in respiration rates throughout the season and within the trees were mainly caused by gradients in stem temperature, growth rates and distribution of living cells. The higher respiration rates, were measured in the upper crown. During the non-growing season, maintenance respiration ranged between 7.2 and 528 $\mu$mol m$^{-3}$ s$^{-1}$ at breast height and in the upper crown, respectively. $Q_{10}$ increased along the stem from 1.3 at breast height to 2.0 in the upper crown. There was a linear relationship between [N] and the percentage of living cells in the wood, but respiration increased strongly with [N]. Growth respiration accounted for 45-76% of annual stem respiration, and the growth respiration coefficient was close to 0.2 g C respired g$^{-1}$ C fixed.Variations spatiales et saisonnières de la respiration ligneuse chez le Hêtre (Fagus sylvatica). La respiration ligneuse a été mesurée de façon continue sur des Hêtres (Fagus sylvatica L.) adultes dans une forêt de l'est de la France, à trois niveaux dans la couronne de branches en 1997 et à deux voire trois niveaux le long du tronc en 1998. Les fortes variations du taux de respiration observées au cours de la saison et au sein de l'arbre étaient essentiellement causées par des gradients de température, de taux de croissance et de distribution des cellules vivantes. Les plus fortes valeurs de respiration correspondaient au sommet de la couronne. Pendant la période de non-croissance, la respiration d'entretien variait entre 7,2 et 528 $\mu$mol m$^{-3}$ s$^{-1}$ à 1,3 m et au sommet de la couronne, respectivement. Le $Q_{10}$ augmentait aussi le long du tronc de 1,3 à 2,0 pour ces mêmes positions. Il existait une relation linéaire entre la concentration en azote, [N], dans le bois et le pourcentage de cellules vivantes. La respiration d'entretien augmentait fortement avec [N]. La respiration de croissance représentait 45 à 76 % de la respiration annuelle des troncs, et le coût de synthèse du bois était de 0,2 g C respiré g$^{-1}$ C fixé dans le tissu

Leaf phenology sensitivity to temperature in European trees: Do within-species populations exhibit similar responses?

Consequences of climate warming on tree phenology are readily observable, but little is known about the differences in phenological sensitivity to temperature between species and between populations within a species. The aim of the present study is to compare phenological sensitivities to temperature of seven woody species between each other and within-species between two geographical areas using both altitudinal and temporal gradients (Abies alba, Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Ilex aquifolium and Quercus petraea). The timing of leaf unfolding was monitored (i) over 2 years along two altitudinal gradients in the Pyrénées mountains (six species), and (ii) over 22 years in Fontainebleau forest (four species). Three species were present in both areas which allowed us to compare their phenological sensitivity to temperature over altitudinal and temporal gradients. Along altitudinal gradients, we observed for all species an advance in leaf unfolding with decreasing elevation, ranging from 11 to 34 days 1000 m−1 for beech and oak, respectively. Across the temporal gradient, we found significant advances in leaf unfolding for oak (−0.42 days year−1) and ash (−0.78 days year−1) since 1976, whereas no significant advance was observed for beech and hornbeam. For both gradients and for all species, significant correlations were found between leaf unfolding dates and temperature, except for beech in the temporal study. Moreover, we highlighted that phenological sensitivity to temperature was very similar between the two geographically separated populations (Pyrénées and Fontainebleau forests). Thus, oak had the strongest sensitivity (−7.48 and −7.26 days °C−1 in altitudinal and temporal gradient, respectively) and beech had the lowest (−2.09 and −2.03 days °C−1). Our results suggest that population sensitivity to global warming might be stable for a given species, in spite of its possible local adaptation

A survey of proximal methods for monitoring leaf phenology in temperate deciduous forests

International audienceTree phenology is a major driver of forest–atmosphere mass and energy exchanges. Yet, tree phenology has rarely been monitored in a consistent way throughout the life of a flux-tower site. Here, we used seasonal time series of ground-based NDVI (Normalized Difference Vegetation Index), RGB camera GCC (greenness chromatic coordinate), broadband NDVI, LAI (leaf area index), fAPAR (fraction of absorbed photosynthetic active radiation), CC (canopy closure), fRvis (fraction of reflected radiation) and GPP (gross primary productivity) to predict six phenological markers detecting the start, middle and end of budburst and of leaf senescence in a temperate deciduous forest using an asymmetric double sigmoid function (ADS) fitted to the time series. We compared them to observations of budburst and leaf senescence achieved by field phenologists over a 13-year period. GCC, NDVI and CC captured the interannual variability of spring phenology very well (R2>0.80) and provided the best estimates of the observed budburst dates, with a mean absolute deviation (MAD) of less than 4 d. For the CC and GCC methods, mid-amplitude (50 %) threshold dates during spring phenological transition agreed well with the observed phenological dates. For the NDVI-based method, on average, the mean observed date coincides with the date when NDVI reaches 25 % of its amplitude of annual variation. For the other methods, MAD ranges from 6 to 17 d. The ADS method used to derive the phenological markers provides the most biased estimates for the GPP and GCC. During the leaf senescence stage, NDVI- and CC-derived dates correlated significantly with observed dates (R2=0.63 and 0.80 for NDVI and CC, respectively), with an MAD of less than 7 d. Our results show that proximal-sensing methods can be used to derive robust phenological metrics. They can be used to retrieve long-term phenological series at eddy covariance (EC) flux measurement sites and help interpret the interannual variability and trends of mass and energy exchanges