Spatio-temporal variability of stable isotopes (18 O and 2H) in soil and xylem waters under Mediterranean conditions.

Soil profiles and trees twigs were sampled in the Can Vila Mediterranean catchment (0.56 km2; Vallcebre Research catchments, NE Spain) to evaluate the spatial variability of the isotopic signature (18O and 2H) of xylem and bulk soil waters at the plot scale and between different locations within the catchment. During two one day sampling campaigns with different antecedent soil moisture conditions, soil samples (0-10, 10-20, 20-30, 40-50 and 90-100 cm) and xylem samples (3 trees per plot) were collected in six Scots pine stands distributed throughout the catchment. Moreover, the water stable isotopes analysed were collected in rainfall, groundwater and streamwater at the catchment outlet during and between the sampling campaigns. Water from soil and xylem samples was extracted by cryogenic vacuum distillation and isotope analyses were obtained by infrared spectroscopy. Stable isotopes ratios of bulk soil water and xylem water fell below the local meteoric water line (LMWL) in both sampling campaigns. In contrast, groundwater ratios fell along the LMWL, being well mixed with stream water. A marked vertical variation in soil water isotopes was observed for the dry campaign in all profiles, with enriched shallow horizons indicating evaporation. This variation was not observed for the wet campaign. Moreover, the spatial variation across the catchment was much greater for the dry campaign compared to the wet campaign. A marked variability in the xylem isotopic signature among trees of the same plot was observed for both sampling campaigns. Finally, in some plots and for both campaigns, the isotopic signature of xylem water was more evaporated than that of bulk soil water. There was no clear pattern relating the topographic index, as an indicator of saturation conditions of the sampling location within the catchment, with soil water isotopic signature. Nor was there a clear relationship found between the isotopic signature of pines¿ xylem and tree characteristics, such as DBH, height, or tree competition index

El prió de la tremolor ovina atípica no presenta limfotropisme

Dins el marc del programa de vigilància activa de les encefalopaties espongiformes transmissibles (EETs) a Catalunya, el laboratori Priocat del Centre de Recerca en Sanitat Animal (CReSA) ha evidenciat la inexistència de limfotropisme per part de la proteïna resistent associada a la tremolor ovina atípica. Uns resultats que s'extreuen del següent article, on es presenta de forma concomitant una encefalitis vírica -el que implica la formació de fol·licles limfoides a l'encèfal- i una malaltia priònica. Així, mentre els prions de la variant clàssica s'acumulen al teixit limfàtic, els de la variant atípica no ho fan ni tan sols estant presents al cervell.En el marco del programa de vigilancia activa de las encefalopatías espongiformes transmisibles (EETs) de Catalunya, el laboratorio Priocat del Centro de Investigación en Sanidad Animal (CReSA) ha evidenciado la inexistencia de limfotropismo por parte de la proteína resistente asociada al temblor ovino atípico. Unos resultados que se extraen del siguiente artículo, donde se presenta de forma concomitante una encefalitis vírica -lo que implica la formación de folículos linfoides en el encéfalo- y una enfermedad priónica. Así, mientras los priones de la variante clásica se acumulan en el tejido linfático, los de la variante atípica no lo hacen ni siquiera estando presentes en el cerebro.From the active TSE surveillance programme in Catalonia, the Priocat laboratory of animal health research centre (CReSA) has detected a noticeable lack of lymphotropsim as far as scrapie-associated prion protein deposition is concerned. Results described in the paper, where a viral encephalitis -which implies formation of lymphoid follicles in the brain- is concomitant to a prion disease. In the classical strains the prions show a strong affinity for the lymphoid tissue but not in this case, even when follicles are as close as in the same brain

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

Modification of rainfall stable isotopes by throughfall and stemflow. The case of Scots pine and downy oak forest under Mediterranean conditions.

In forested ecosystems the isotopic composition of rainfall that reaches the soil either as throughfall or stemflow is modified by processes that take place in the tree canopies. The known factors that can cause a change in the isotopic composition are evaporation, exchange between liquid and atmospheric vapor, and selective canopy storage for isotopically temporal varying rainfall. These processes are still poorly understood, but they have important implications on the heterogeneities of the input water at the catchment scale. Recent advances suggest that equilibrium exchange and selective canopy storage are the dominant processes, even though there is a lack of data to unambiguously identify them. Here, we present the results of an experiment focused on the characterization of the spatio-temporal variability of the isotopic composition of rainfall, throughfall and stemflow in order to identify the main factors affecting its modification. The study was carried out between May 2015 and June 2016 in a Downy oak (Quercus pubescens) forest and a Scots pine (Pinus sylvestris) forest located in the Vallcebre research catchments (NE Spain, 42º 12¿N, 1º 49¿E), under Mediterranean climate conditions. The sampling design for isotopic analysis of each stand consisted of one automatic sampler and 10 throughfall collectors distributed within the stand to collect throughfall, and 4 stemflow collectors to collect stemflow. Bulk rainfall was collected with automatic samplers and bulk collectors in two open areas near each forest stand. At each stand, isotopic sampling was combined with hydrometric measurements that consisted of 20 tipping buckets to measure throughfall and 7 stemflow rings connected to tipping buckets to measure stemflow. Moreover, rainfall depth was measured in the two open areas and meteorological variables in the two stands by means of towers located above canopies. In total 36 rainfall events were analyzed. Our results revealed a high heterogeneity on the isotopic composition of the open rainfall during the studied period. Enrichment occurred for 74% of the throughfall samples and for 91% of the stemflow samples. In general

Oxygen-18 and deuterium spatio-temporal variability in throughfall and stemflow in Scots pine and Downy oaks forests under Mediterranean climate

Rainfall partitioning processes can be better understood complementing classical hydrometric techniques with water isotopes. Oxygen-18 and deuterium can be used to shed some light on mechanisms of rainfall evaporation from the canopies, and their relationship with canopy and meteorological variables that are not completely understood. Several mechanisms have been described to explain the differences between event-scale bulk rainfall and throughfall isotopic compositions (i.e. evaporation, selective storage, exchange with ambient vapor, residual moisture), and their relation to factors like the amount of water held in the forest canopy, rainfall intensity, time interval between rainfall events, or meteorological conditions. However, there are much fewer studies examining the spatio-temporal variability of isotopic composition in both throughfall and stemflow along rainfall events. This study aims to characterize the water stable isotopes spatio-temporal variability in throughfall and stemflow in a Downy oak (Quercus pubescens) and a Scots pine (Pinus sylvestris) forests located in the Vallcebre research catchments (NE Spain, 42º 12¿N, 1º 49¿E), under Mediterranean climate conditions. The isotopic sampling design of each stand consisted of one automatic sampler to sample the temporal variability of throughfall signature every 5 mm of rainfall, 10 throughfall collectors distributed within the stand to sample the spatial variability and 4 stemflow collectors. Bulk rainfall was collected with automatic samplers and bulk collectors in two open areas near each forest plot. At each stand isotopic sampling was combined with hydrometric measurements that consisted of 20 tipping buckets to measure throughfall spatial variability and 7 stemflow rings connected to tipping buckets to measure stemflow depth. Moreover, rainfall depth was measured in the open areas and meteorological variables in two towers located above canopies. The study started on May 2015 and is still in progress. Up to now, a total of 1235 samples, corresponding to 27 rainfall events, have been collected in the two stands and are being analysed by infrared spectroscopy. First available results show the complexity of rainfall partitioning process and its spatial and temporal variability, as well as the high diversity of responses, depending on rainfall characteristics, canopy structure and meteorological conditions

A dual stable-isotope approach to analyse the linkages between tree water fluxes and soil water pools in a Mediterranean mountain catchment

This work uses a dual isotope-based approach (18O, 2H) to examine the mixing of water in the soil and the linkages between tree water fluxes and soil water pools in a Mediterranean mountain catchment (Vallcebre Research Catchments, NE Spain, 42º 12¿N, 1º 49¿E). Since May 2015, water-isotopes have been monitored in rainfall, throughfall and stemflow below a Scots pine stand and in stream water at the Can Vila (0.56 km2) catchment outlet. Moreover, fortnightly (From May to December 2015) soil samples (10, 20, 30, 50 and 100 cm), xylem samples (3 Scots pines) and mobile soil water samples in low-suction lysimeters (20, 50 and 100 cm) and in a piezometer (150-300 cm deep) were collected at the same stand. Water from soil and xylem samples was extracted by cryogenic vacuum distillation and isotope analyses were obtained by infrared spectroscopy. All this information has been combined with continuous measurement of meteorological, soil moisture and water potential, piezometric levels and hydrological variables at the stand and catchment scales. Stable isotopes ratios of bound soil water fell below the local meteoric water line (LMWL), with more evaporative enrichment in the shallow horizons. On the contrary, mobile soil water (low suction lysimeters) and groundwater fell along the LMWL, well mixed with stream water. The differences observed between these two water pools remained similar during the whole study period. Stable isotopes ratios indicate that Scots pine trees use shallow bound soil water during the whole study period. No marked changes in depth of water uptake were observed, presumably due to the availability of water in the shallow horizons, even during the summer months

Ecohydrological separation in a Mediterranean mountain environment (Vallcebre research catchments, NE Spain)

Until very recently, a general paradigm in hydrology was that water is well mixed in the soil, and therefore groundwater, stream water and plant transpiration are all sourced by this well mixed pool. However, recent works (Brooks et al., 2010; Goldsmith et al., 2012) have shown the existence of different water pools in the soil, where tightly bound water, potentially used by plants, does not mix with mobile water that potentially contributes to groundwater and streamflow. This new ¿two water worlds hypothesis¿ of ecohydrological separation of water between streams and trees should however be verified in areas with different climates and land covers (McDonnell, 2014). With this objective, we examine this hypothesis in the Vallcebre Research Catchments (NE Spain, 42º 12¿N, 1º 49¿E) using the dual isotope-based approach combined with meteorological and hydrometric monitoring. Since May 2015, stable water-isotopes have been monitored in rainfall (2 locations), in throughfall and stemflow below Scots pines as well as in stream water at the Can Vila (0.56 km2) catchment outlet. Moreover, three spatially distributed sampling campaigns in different antecedent soil moisture conditions have been performed (May, August and November 2015) within the catchment. During the sampling campaigns soil samples (10, 20, 30, 50 and 100 cm) and xylem samples (3 Scots pines) were collected at 8 locations, with different topographic indices.Water in soil and xylem samples was extracted by cryogenic vacuum distillation. This information was complemented with mobile soil water sampled in 3 lysimetric profiles (20, 50 and 100 cm) and in 13 piezometers (150-300 cm deep) distributed within the catchment. These campaigns were combined with a similar regular sampling, every 15 days (From May to December 2015) at one of the 8 locations. All the isotopic information, obtained by infrared spectroscopy, has been combined with continuous measurement of meteorological, soil moisture and potential, piezometric levels and hydrological variables at plot and catchment scales. These results will provide some new insights on the relevance of the two water worlds concept in a highly seasonal Mediterranean climate

Particulate matter fluxes in throughfall and stemflow under oak and pine stands

The atmospheric particulate deposition (APD) is one source of nutrients for forest ecosystems. Forest canopies offer large deposition surfaces that can enhance the amount of particles reaching the soil as throughfall or as stemflow. However, the influence of the forest canopy on APD is still poorly known. In this study, we aim to compare the fluxes of APD reaching the soil in an open field and below the canopy (via throughfall and stemflow) in Pinus sylvestris L. (Scots pine) and Quercus pubescens Willd. (downy oak) stands located in the Vallcebre research catchments (NE Spain, 42o 12¿N, 1o 49¿E). After every rainfall, samples of each water flux were collected and filtered (0.45 m pore size cellulose filters) to determine the particulate matter fluxes. In addition, filters corresponding to 7 rainfall events were selected to analyse the morphometric characteristics of particulates using a confocal microscopy. The APD annual rates were: 66 kg ha¿¿1 year¿¿1 in the open field, 82 kg ha¿¿1 year¿¿1in throughfall for both species and 2.8 and 1.2 kg ha¿¿1 year¿¿1in stemflow for pines and oaks respectively. At the event scale, APD in throughfall increased with increasing rainfall volume and in stemflow with increasing funnelling ratio. The flux of particulate matter in throughfall was strongly linked with the presence or absence of foliage; being higher for oaks during the dormant season. On the other hand, rainfall intensity and the time lag between rainfalls were important factors determining the number of particles below the canopy. These results show the importance of throughfall and stemflow regarding to the transfer of particulate matter to the soil. Despite APD in stemflow per surface area was small, this flux represents a hotspot of particulate matter that reaches the base of the trunks, and is therefore of special interest to understand forest soils biogeochemical cycles

Soil water content spatial and temporal variability around a tree. The role of stemflow.

Rainfall redistributed in forest ecosystems by throughfall and stemflow implies a large degree of temporal and spatial soil water inputs variability, affecting both tree water uptake and deep percolation. In this sense, stemflow, although being a small proportion of rainfall compared to throughfall, is recognized an important hotspot concentrating water close to root systems. This work studies the soil water content spatial and temporal variability around a Scots pine tree growing within a forest in a Mediterranean mountainous area (Vallcebre Research catchments, NE Spain). The main objective is to investigate the role of stemflow as a hotspot input of water into the soil. The monitoring started on December 2017 and consists of a set of 68 automatic TDR probes covering a Scots pine tree canopy projected area of about 13 m2. Stemflow production of the monitored tree was formerly studied. Two complementary designs, trying to cope with the soil water content variability in surface and depth, were stablished: (a) 40 TDR probes (0-30 cm depth) were distributed in 8 radii around the tree at 10, 20, 50, 100 and 200 cm from the trunk. (b) Eight soil profiles (0-90cm), were located in four of the radii formerly described, one at 10cm and the other at 100 cm from the trunk. This work presents the first results describing the soil water content variability around the tree and its dependency on tree structure, distance to tree trunk, stemflow and throughfall inputs, as well as meteorological and soil characteristics