Effets du déficit hydrique sur la phénologie du cambium, la xylogénèse et la densité du bois des semis d'épinettes noires soumis à différents régimes thermiques

En forêt boréale, la température est considérée comme un important facteur abiotique qui règle l'activité cambiale et la production des cellules, mais la sécheresse est aussi connue pour avoir un effet direct sur la largeur des cernes de croissance. Selon les dernières prévisions climatiques, une augmentation de la température de l'ordre de 2 à 6°C est attendue avec un effet plus prononcé aux latitudes élevées. Cela favorisera l'évaporation de l'eau et la récurrence des phénomènes de sécheresse. Ces changements climatiques pourraient avoir une incidence sur la croissance des jeunes plants ainsi que sur les mécanismes physiologiques qui y sont associés. Au Canada, une augmentation de la mortalité des arbres suite à la sécheresse a déjà été enregistrée dans les dernières décennies. Dans un tel contexte, il est donc primordial de comprendre comment et dans quelle mesure les jeunes plants réagiront aux stress environnementaux, comme les vagues de chaleur et la sécheresse, suite à leur mise en terre. Le but de cette thèse est d’étudier les effets de différents régimes thermiques et hydriques sur la croissance radiale et la formation du bois de l’épinette noire [Picea mariana (Mill.) BSP] en milieu contrôlé, où des conditions environnementales déterminées par des modèles de prévision climatique ont été appliquées. Des expériences ont eu lieu durant les saisons de croissance 2010 à 2012 dans les serres expérimentales de l’Université du Québec à Chicoutimi (Québec, Canada). Les plants ont été cultivés à des températures plus élevées pendant le jour et pendant la nuit par rapport au contrôle. Les épinettes ont également subi deux différents régimes hydriques, soit une irrigation en continu et une période ou des périodes de déficit hydrique durant le maximum de croissance. L'activité cambiale, la xylogénèse, les relations hydriques, les échanges gazeux, l'assimilation de CO2 et les réserves en amidon ont été déterminés chez les jeunes plants d'épinettes noires afin de pouvoir définir les limites physiologiques associées à la croissance sous les scénarios climatiques futurs. Les plants ont atteint un déficit hydrique sévère lorsqu’ils rejoignaient des valeurs proches de -3 MPa, correspondant au point d'entrée de l'air suite à une augmentation des embolies dans le xylème. Pendant le déficit hydrique, l'activité cambiale et la formation du bois ont été ralenties ou stoppées par le manque d’eau. Après la relance de l'irrigation, la reprise de l'activité cambiale s’étendait sur 2 à 4 semaines avec des reprises plus longues à des températures plus chaudes. Lors d’une augmentation de la température entre 2 et 6 °C, les plants non-irrigués présentaient un taux de mortalité de 5 à 20% respectivement alors que la mortalité était près de 0% à température ambiante. De plus, ce pourcentage de mortalité était beaucoup plus important lors d'une augmentation de la température de 6°C pendant le jour seulement. Les résultats ont permis d’élucider les mécanismes physiologiques qui sont impliqués dans la mortalité des jeunes plants comme l’inaccessibilité de l’eau et le manque de carbone. Chez les plants non-irrigués, une diminution de la densité du bois a été observée. Les sous-processus associés à la xylogénèse, soit la durée et les taux d’élargissement et de déposition de la paroi cellulaire, ont permis de comprendre la mécanique de formation du cerne et de la densité sous les différents scénarios climatiques testés. Durant le déficit hydrique, le taux d'expansion des cellules a diminué de 28% mais a été compensé par une augmentation de la durée de cette phase qui était 21% plus longue. Similairement, le taux de formation de la paroi a été de 37% inférieur chez les plants non-irrigués. Pendant le déficit hydrique, la durée de la formation de la paroi a été en moyenne plus courte de 22% et 24% à températures plus élevées. La modification des sous-processus durant la formation du bois a grandement contribué aux variations de la densité du bois observées sous les différentes conditions de stress hydrique et de température. Ces résultats ont permis de développer de façon plus approfondie la dynamique de la xylogénèse et de mettre en évidence un mécanisme de compensation minimisant les impacts des stress sur l’anatomie du bois. Chez les plants non-irrigués, l'effet du stress récurrent a conduit à une durée de contraction et d’expansion radiale plus longue dans les cycles diurnes de la tige, mesurés à l’aide de dendromètres automatiques. L'intensité et la durée du déficit hydrique, combinée à des températures nocturnes plus chaudes, ont une influence importante sur les variations radiales de la tige. Cette thèse a permis de mettre en évidence la sensibilité de l'épinette noire à la disponibilité hydrique ainsi que la grande plasticité dans les mécanismes de formation du cerne. Les processus de formation du bois sont déterminants pour la largeur du cerne (xylogénèse) et de sa structure (anatomie du bois), la première étant très plastique et la seconde très conservatrice

The contribution of carbon and water in modulating wood formation in black spruce saplings

Nonstructural carbohydrates (NSCs) play a crucial role in xylem formation and represent, with water, the main constraint to plant growth. We assessed the relationships between xylogenesis and NSCs in order to (1) verify the variance explained by NSCs and (2) determine the influence of intrinsic (tissue supplying carbon) and extrinsic (water availability and temperature) factors. During 2 years, wood formation was monitored in saplings of black spruce (Picea mariana) subjected to a dry period of about 1 month in June and exposed to different temperature treatments in a greenhouse. In parallel, NSC concentrations were determined by extracting the sugar compounds from two tissues (cambium and inner xylem), both potentially supplying carbon for wood formation. A mixed-effect model was used to assess and quantify the potential relationships. Total xylem cells, illustrating meristematic activity, were modeled as a function of water, sucrose, and D-pinitol (conditional r2 of 0.79). Water availability was ranked as the most important factor explaining total xylem cell production, while the contribution of carbon was lower. Cambium stopped dividing under water deficit, probably to limit the number of cells remaining in differentiation without an adequate amount of water. By contrast, carbon factors were ranked as most important in explaining the variation in living cells (conditional r2 of 0.49), highlighting the functional needs during xylem development, followed by the tissue supplying the NSCs (cambium) and water availability. This study precisely demonstrates the role of carbon and water in structural growth expressed as meristematic activity and tissue formation

Variation of stem radius in response to defoliation in boreal conifers

In the long term, defoliation strongly decreases tree growth and survival. Insect outbreaks are a typical cause of severe defoliation. Eastern spruce budworm (Choristoneura fumiferana Clem.) outbreaks are one of the most significant disturbances of Picea and Abies boreal forests. Nevertheless, in boreal conifers, a 2-year defoliation has been shown to quickly improve tree water status, protect the foliage and decrease growth loss. It suggests that defoliation effects are time-dependent and could switch from favorable in the short term to unfavorable when defoliation duration exceeds 5–10 years. A better understanding of the effect of defoliation on stem radius variation during the needle flushing time-window could help to elucidate the relationships between water use and tree growth during an outbreak in the medium term. This study aims to assess the effects of eastern spruce budworm (Choristoneura fumiferana Clem.) defoliation and bud phenology on stem radius variation in black spruce [Picea mariana (Mill.) B.S.P.] and balsam fir [Abies balsamea (L.) Mill.] in a natural stand in Quebec, Canada. We monitored host and insect phenology, new shoot defoliation, seasonal stem radius variation and daytime radius phases (contraction and expansion) from 2016 to 2019. We found that defoliation significantly increased stem growth at the beginning of needle flushing. Needles flushing influenced the amplitude and duration of daily stem expansion and contraction, except the amplitude of stem contraction. Over the whole growing season, defoliation increased the duration of stem contraction, which in turn decreased the duration of stem expansion. However, the change (increase/decrease) of the duration of contraction/expansion reflects a reduced ability of the potential recovery from defoliation. Black spruce showed significantly larger 24-h cycles of stem amplitude compared to balsam fir. However, both species showed similar physiological adjustments during mild stress, preventing water loss from stem storage zones to support the remaining needles’ transpiration. Finally, conifers react to defoliation during a 4-year period, modulating stem radius variation phases according to the severity of the defoliation

Soil microarthropod communities from Mediterranean forest ecosystems in Central Italy under different disturbances

The aim of this study is to assess soil quality in Mediterranean forests of Central Italy, from evergreen to deciduous, with different types of management (coppice vs. high forest vs. secondary old growth) and compaction impacts (machinery vs. recreational). Soil quality was evaluated studying soil microarthropod communities and applying a biological index (QBS-ar) based on the concept that the higher is the soil quality, the higher will be the number of microarthropod groups well adapted to the soil habitat. Our results confirm that hardwood soils are characterised by the highest biodiversity level among growth) and compaction impacts (machinery vs. recreational). terrestrial communities and by a well-structured and mature microarthropod community, which is typical of stable ecosystems (QBS value, >200). While silvicultural practices and forest composition do not seem to influence QBS-ar values or microarthropod community structure, the index is very efficient in detecting soil impacts (soil compaction due to logging activities). Several taxa (Protura, Diplura, Coleoptera adults, Pauropoda, Diplopoda, Symphyla, Chilopoda, Diptera larvae and Opiliones) react negatively to soil compaction and degradation (QBS value, <150). In particular, Protura, Diplura, Symphyla and Pauropoda, are taxonomic groups linked to undisturbed soil. This index could also be a useful tool in monitoring soil biodiversity in protected areas and in urban forestry to prevent the negative effects of trampling. QBS-ar is a candidate index for biomonitoring of soil microarthropod biodiversity across the landscape to provide guidance for the sustainable management of renewable resource and nature conservation

Stem cycle analyses help decipher the nonlinear response of trees to concurrent warming and drought

Key message High-resolution analysis of stem radius variation can quantify the impact of warming and drought on stem water balance and stem growth in black spruce [ Picea mariana (Mill.) B.S.P.)]. Drought affected plant water status and stem growth. However, warming affects the components of the circadian stem cycle differently if the impacts occur in the daytime or nighttime. The interactive effect of abiotic stresses had less impact on the circadian stem cycle than when the stresses occurred independently. Context Warming and recent droughts in boreal regions reflect the multiple dimensions of climate change. How these climate-related stresses will affect the stem growth of trees remains to be described. Plant water relations can detect the dynamics of stem depletion and replenishment under conditions of climate-forced stress. Aims This study aimed to verify the impacts of a combination of asynchronous warming (nighttime versus daytime warming) and drought on stem water balance and stem growth in black spruce [Picea mariana (Mill.) B.S.P.)]. Methods We investigated the water status and variations in stem radius of black spruce saplings growing in a controlled environment from May through August. We grew four-year-old saplings in warmer conditions either during the day (DW) or night (NW) at temperatures ca. 6 °C warmer than the ambient air temperature (CT). We then simulated a one-month drought in June. Automatic point dendrometers provided a high-resolution analysis of variations in stem radius, and we also monitored leaf water potentials and volumetric soil water content during the entire experimental period. Results We detected significant reductions in stem radius variation under water deficit conditions. In the daytime warming scenario, we observed a significant increase in the duration of contraction and a decrease in expansion of the stems. The amplitude of this contraction and expansion was reduced under the nighttime warming conditions. The main effect of warming was to enhance drought stress by accelerating soil water depletion. Changes in predawn water potential drove the duration of stem circadian cycles under conditions of daytime warming, whereas irreversible growth dynamics drove these cycles under nighttime warming conditions due to the midday water potential. The interaction of night/daytime asynchronous warming and drought reduced the amplitude rather than the duration of stem contraction and expansion. Conclusion Water deficit decreased stem growth during the growing season. Asymmetric warming (as a single independent treatment) affected the timing and magnitude of stem circadian cycles. Under daytime warming scenarios, the duration of contraction and expansion were regulated mainly by predawn water potential, inducing longer (shorter) durations of contraction (expansion). Under nighttime warming, the smaller amplitudes of stem contraction and expansion were associated with midday water potential. Therefore, the interaction of abiotic stresses had less of an impact on the circadian stem cycle components than when these stresses were applied independently

PhenoCaB : a new phenological model based on carbon balance in boreal conifers

Traditional phenological models use chilling and thermal forcing (temperature sum or degree-days) to predict budbreak. Because of the heightening impact of climate and other related biotic or abiotic stressors, a model with greater biological support is needed to better predict budbreak. Here, we present an original mechanistic model based on the physiological processes taking place before and during budbreak of conifers. As a general principle, we assume that phenology is driven by the carbon status of the plant, which is closely related to environmental variables and the annual cycle of dormancy-activity. The carbon balance of a branch was modelled from autumn to winter with cold acclimation and dormancy and from winter to spring when deacclimation and growth resumption occur. After being calibrated in a field experiment, the model was validated across a large area (> 34 000 km 2 ), covering multiple conifers stands in Québec (Canada) and across heated plots for the SPRUCE experiment in Minnesota (USA). The model accurately predicted the observed dates of budbreak in both Québec (±3.98 d) and Minnesota (±7.98 d). The site-independent calibration provides interesting insights on the physiological mechanisms underlying the dynamics of dormancy break and the resumption of vegetative growth in spring

Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis

BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London

Surgical site infection after gastrointestinal surgery in high-income, middle-income, and low-income countries: a prospective, international, multicentre cohort study

Background: Surgical site infection (SSI) is one of the most common infections associated with health care, but its importance as a global health priority is not fully understood. We quantified the burden of SSI after gastrointestinal surgery in countries in all parts of the world. Methods: This international, prospective, multicentre cohort study included consecutive patients undergoing elective or emergency gastrointestinal resection within 2-week time periods at any health-care facility in any country. Countries with participating centres were stratified into high-income, middle-income, and low-income groups according to the UN's Human Development Index (HDI). Data variables from the GlobalSurg 1 study and other studies that have been found to affect the likelihood of SSI were entered into risk adjustment models. The primary outcome measure was the 30-day SSI incidence (defined by US Centers for Disease Control and Prevention criteria for superficial and deep incisional SSI). Relationships with explanatory variables were examined using Bayesian multilevel logistic regression models. This trial is registered with ClinicalTrials.gov, number NCT02662231. Findings: Between Jan 4, 2016, and July 31, 2016, 13 265 records were submitted for analysis. 12 539 patients from 343 hospitals in 66 countries were included. 7339 (58·5%) patient were from high-HDI countries (193 hospitals in 30 countries), 3918 (31·2%) patients were from middle-HDI countries (82 hospitals in 18 countries), and 1282 (10·2%) patients were from low-HDI countries (68 hospitals in 18 countries). In total, 1538 (12·3%) patients had SSI within 30 days of surgery. The incidence of SSI varied between countries with high (691 [9·4%] of 7339 patients), middle (549 [14·0%] of 3918 patients), and low (298 [23·2%] of 1282) HDI (p < 0·001). The highest SSI incidence in each HDI group was after dirty surgery (102 [17·8%] of 574 patients in high-HDI countries; 74 [31·4%] of 236 patients in middle-HDI countries; 72 [39·8%] of 181 patients in low-HDI countries). Following risk factor adjustment, patients in low-HDI countries were at greatest risk of SSI (adjusted odds ratio 1·60, 95% credible interval 1·05–2·37; p=0·030). 132 (21·6%) of 610 patients with an SSI and a microbiology culture result had an infection that was resistant to the prophylactic antibiotic used. Resistant infections were detected in 49 (16·6%) of 295 patients in high-HDI countries, in 37 (19·8%) of 187 patients in middle-HDI countries, and in 46 (35·9%) of 128 patients in low-HDI countries (p < 0·001). Interpretation: Countries with a low HDI carry a disproportionately greater burden of SSI than countries with a middle or high HDI and might have higher rates of antibiotic resistance. In view of WHO recommendations on SSI prevention that highlight the absence of high-quality interventional research, urgent, pragmatic, randomised trials based in LMICs are needed to assess measures aiming to reduce this preventable complication