Stem hydraulic capacitance decreases with drought stress : implications for modelling tree hydraulics in the Mediterranean oak Quercus ilex

Hydraulic modelling is a primary tool to predict plant performance in future drier scenarios. However, as most tree models are validated under non-stress conditions they may fail when water becomes limiting. To simulate tree hydraulic functioning under moist and dry conditions, the current version of a water flow and storage mechanistic model was further developed by implementing equations that describe variation in xylem hydraulic resistance (RX) and stem hydraulic capacitance (CS) with predawn water potential (ΨPD). The model was applied in a Mediterranean forest experiencing intense summer drought, where six Quercus ilex trees were instrumented to monitor stem diameter variations and sap flow, concurrently with measurements of predawn and midday leaf water potential. Best model performance was observed when CS was allowed to decrease with decreasing ΨPD. Hydraulic capacitance decreased from 62 to 25 kg m-3 MPa-1 across the growing season. In parallel, tree transpiration decreased to a greater extent than the capacitive water release and the contribution of stored water to transpiration increased from 2.0% to 5.1%. Our results demonstrate the importance of stored water and seasonality in CS for tree hydraulic functioning, and they suggest that CS should be considered to predict the drought-response of trees with models

Arable weeds in Luberon natural regional Park are they bad weeds ?

La protection des messicoles (plantes inféodées aux champs de céréales) fait l’objet d’un paradoxe. Alors que les conservateurs d’espaces (Parc naturel régional du Luberon) et d’espèces (Conservatoire botanique national de Porquerolles) les protègent pour leur forte valeur patrimoniale ; les exploitants agricoles, quant à eux, les considèrent comme des mauvaises herbes dont la prolifération peut être responsable d’une chute de rendement de l’espèce cultivée. Cependant, peu d’expérimentations ont été réalisées sur les capacités compétitives réelles des messicoles alors qu’elles sont en voie de régression un peu partout en France suite à l’application massive d’herbicides. C’est pourquoi, les capacités compétitives de 14 messicoles encore présentes dans les champs de céréales du Parc naturel régional du Luberon ont été interprétées à partir de la mesure de leurs caractéristiques morphologiques en conditions de compétition avec le blé d’hiver (Triticum aestivum var. Darius). Nos résultats montrent que les messicoles caractérisées par des biomasses importantes ou avec une forte contribution de biomasse aux tiges, entraînent une réduction significative de la biomasse totale du blé et pour l’épi, une réduction de sa hauteur, de sa biomasse et du nombre de graines qu’il contient. Ces résultats sont ensuite discutés dans un objectif de conservation des espèces messicoles car, contrairement aux idées reçues, certaines d’entre elles, n’entrent pas en compétition avec le blé.Arable weed species are the origin of a paradox : while conservation agencies (Luberon Nature Park and Porquerolles Botanical Reserve) protect them for their historical and cultural interest, farmers commonly perceived them as bad weeds responsible for a decrease in crop yield. Arable weed species substantially decline everywhere in France due to fertiliser application and very few experiments are carried out on the real competitive capacity of arable weed species. Therefore, the competitive capacity of 14 arable weed species, still growing in cereal fields in the Luberon Nature Park, was measured using their morphological traits while being cultivated together with winter wheat (Triticum aestivum var. Darius). Results show that arable weed species, characterised by a high biomass or a high shoot biomass, have a significant negative effect on the total biomass, the ear height, the ear biomass and the number of seeds of wheat plants. These results are discussed within the context of the biological conservation of arable weeds, because, contrary to common knowledge, some of them do not out-compete wheat plants

Технология моделирования штормовых нагонов и ветрового волнения в Азовском море на неструктурированных сетках

Представлена технология численного моделирования штормовых нагонов и ветровых волн в Азовском море, объединяющая модель циркуляции вод ADCIRC и модель ветрового волнения SWAN. Обе модели реализованы на неструктурированной сетке и адаптированы для параллельных вычислений. Приведены результаты верификации численного алгоритма и анализ его чувствительности к вариациям входных параметров.Представлена технологія чисельного моделювання штормових нагонів і вітрових хвиль в Азовському морі, що об'єднує модель циркуляції вод ADCIRC і модель вітрового хвилювання SWAN. Обидві моделі реалізовані на неструктурованій сітці і адаптовані для паралельних обчислень. Наведено результати верифікації чисельного алгоритму і аналіз його чутливості до варіацій вхідних параметрів.The technology of numerical modeling of storm surge and wind waves in the Sea of Azov, unifying model of the ADCIRC ocean circulation model and SWAN wind waves model. Both models are implemented on unstructured mesh and adapted for parallel computing. The results numerical algorithm verification and analysis of its sensitivity to variations in input parameters are given

Stem CO2 efflux and its contribution to ecosystem CO2 efflux decrease with drought in a Mediterranean forest stand

tThe rate of metabolic processes demanding energy in tree stems changes in relation with prevailing cli-matic conditions. Tree water availability can affect stem respiration through impacts on growth, phloemtransport or maintenance of diverse cellular processes, but little is known on this topic. Here we moni-tored seasonal changes in stem CO2efflux (Fs), radial growth, sap flow and non-structural carbohydrates intrees of Quercus ilex in a Mediterranean forest stand subjected since 2003 to either partial (33%) through-fall exclusion (E) or unchanged throughfall (C). Fsincreased exponentially during the day by an effectof temperature, although sap flow attenuated the increase in Fsduring the day time. Over the year, Fsalso increased exponentially with increasing temperatures, but Fscomputed at a standard temperatureof 15?C (F15s) varied by almost 4-fold among dates. F15swas the highest after periods of stem growth anddecreased as tree water availability decreased, similarly in C and E treatments. The decline in F15swas notlinked to a depletion of soluble sugars, which increased when water stress was higher. The proportionof ecosystem respiration attributed to the stems was highest following stem growth (23.3%) and lowestduring the peak of drought (6.5%). High within-year variability in F15smakes unadvisable to pool annualdata of Fsvs. temperature to model Fsat short time scales (hours to months) in Mediterranean-type for-est ecosystems. We demonstrate that water availability is an important factor governing stem CO2effluxand suggest that trees in Mediterranean environments acclimate to seasonal drought by reducing stemrespiration. Stem respiratory rates do not seem to change after a long-term increase in drought intensity,however

Few multiyear precipitation-reduction experiments find a shift in the productivity-precipitation relationship

Well-defined productivity–precipitation relationships of ecosystems are needed as benchmarks for the validation of land models used for future projections. The productivity–precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates interannual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation–reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyse the effects of dry treatments in eleven multiyear precipitation–manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity–precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water loss or nutrient loss. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation–reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid

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

Global transpiration data from sap flow measurements : the SAPFLUXNET database

Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land-atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The "sapfluxnetr" R package - designed to access, visualize, and process SAPFLUXNET data - is available from CRAN.Peer reviewe

The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe

Interpreting canopy development and physiology using the EUROPhen camera network at flux sites

Peer reviewe