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

Variabilité saisonnière et interannuelle de la croissance du chêne vert méditerranéen et vulnérabilité au changement climatique

Tree secondary growth is responsible for woody biomass accumulation and is a major component of carbon storage in forest ecosystems. Environmental constraints on secondary growth in Mediterranean ecosystems must, however, be described in more to details to better understand how they will be modified by climate change. This dissertation aims at studying the functional responses of Mediterranean holm oak (Quercus ilex) to seasonal and inter-annual climate variations through the study of carbon allocation to secondary growth. Different experimental approaches, at spatial scales ranging from tree rings to the ecosystem and at temporal scales from the day to several decades, were used to identify the main environmental constraints (water availability, temperature warming, competition) to secondary growth and carbon isotopic composition of tree rings. The phenology of stem growth shows evidence for a direct environmental control on annual growth by winter temperature and summer drought that is more limiting than the carbon supply from photosynthesis. Climate change from 1968 to 2013 resulted in earlier water limitation on secondary growth, which was compensated by earlier growth onset, due to warmer winter temperature, and higher water use efficiency, due to increased atmospheric CO2 concentration. Thinning reduced tree mortality and increased stem growth, so thinning management in old holm oak coppices could prepare the ecosystem to better withstand the increasing drought forecasted for the Mediterranean region.La croissance secondaire est à l'origine de l'accumulation de biomasse pérenne par les arbres et détermine en partie la capacité des écosystèmes forestiers à stocker du carbone. Cependant, les contraintes environnementales sur la croissance en milieu méditerranéen sont encore mal décrites et nous ne savons pas comment les changements climatiques futurs vont les modifier. L'objectif de la thèse est de déterminer, principalement à partir de l'étude de l'allocation du carbone à la croissance secondaire, les réponses fonctionnelles saisonnières et interannuelles du chêne vert (Quercus ilex L.) aux variations climatiques en région méditerranéenne. L'utilisation de différentes approches expérimentales, à des échelles spatiales allant du cerne à l'écosystème et à des échelles temporelles allant de la journée à plusieurs dizaines d'années, a permis de mettre en évidence l'effet de différentes contraintes environnementales (disponibilité en eau, réchauffement de la température, et densité du peuplement) sur la croissance secondaire et la composition isotopique du cerne. L'étude de la phénologie de la croissance montre que celle-ci est contrôlée directement par les températures hivernales et le déficit hydrique, plus que par la disponibilité en éléments carbonés issus de la photosynthèse. De 1968 à 2013, les changements climatiques ont entrainé une contrainte hydrique de plus en plus précoce qui s'est trouvée compensée par un début de croissance initié plus tôt dans l'année, sous l'effet du réchauffement des températures hivernales, et une meilleure efficacité d'utilisation de l'eau, sous l'effet de l'augmentation de la concentration en CO2 atmosphérique. La réduction de la mortalité et l'augmentation de la croissance observée dans des parcelles éclaircies montre que cette pratique sylvicole permet de préparer les taillis de chêne vert à l'intensification de la sècheresse prévue pour la région méditerranéenne

Seasonal and inter-annual growth variations and vulnerability to climate change in Mediterranean Quercus ilex

La croissance secondaire est à l'origine de l'accumulation de biomasse pérenne par les arbres et détermine en partie la capacité des écosystèmes forestiers à stocker du carbone. Cependant, les contraintes environnementales sur la croissance en milieu méditerranéen sont encore mal décrites et nous ne savons pas comment les changements climatiques futurs vont les modifier. L'objectif de la thèse est de déterminer, principalement à partir de l'étude de l'allocation du carbone à la croissance secondaire, les réponses fonctionnelles saisonnières et interannuelles du chêne vert (Quercus ilex L.) aux variations climatiques en région méditerranéenne. L'utilisation de différentes approches expérimentales, à des échelles spatiales allant du cerne à l'écosystème et à des échelles temporelles allant de la journée à plusieurs dizaines d'années, a permis de mettre en évidence l'effet de différentes contraintes environnementales (disponibilité en eau, réchauffement de la température, et densité du peuplement) sur la croissance secondaire et la composition isotopique du cerne. L'étude de la phénologie de la croissance montre que celle-ci est contrôlée directement par les températures hivernales et le déficit hydrique, plus que par la disponibilité en éléments carbonés issus de la photosynthèse. De 1968 à 2013, les changements climatiques ont entrainé une contrainte hydrique de plus en plus précoce qui s'est trouvée compensée par un début de croissance initié plus tôt dans l'année, sous l'effet du réchauffement des températures hivernales, et une meilleure efficacité d'utilisation de l'eau, sous l'effet de l'augmentation de la concentration en CO2 atmosphérique. La réduction de la mortalité et l'augmentation de la croissance observée dans des parcelles éclaircies montre que cette pratique sylvicole permet de préparer les taillis de chêne vert à l'intensification de la sècheresse prévue pour la région méditerranéenne.Tree secondary growth is responsible for woody biomass accumulation and is a major component of carbon storage in forest ecosystems. Environmental constraints on secondary growth in Mediterranean ecosystems must, however, be described in more to details to better understand how they will be modified by climate change. This dissertation aims at studying the functional responses of Mediterranean holm oak (Quercus ilex) to seasonal and inter-annual climate variations through the study of carbon allocation to secondary growth. Different experimental approaches, at spatial scales ranging from tree rings to the ecosystem and at temporal scales from the day to several decades, were used to identify the main environmental constraints (water availability, temperature warming, competition) to secondary growth and carbon isotopic composition of tree rings. The phenology of stem growth shows evidence for a direct environmental control on annual growth by winter temperature and summer drought that is more limiting than the carbon supply from photosynthesis. Climate change from 1968 to 2013 resulted in earlier water limitation on secondary growth, which was compensated by earlier growth onset, due to warmer winter temperature, and higher water use efficiency, due to increased atmospheric CO2 concentration. Thinning reduced tree mortality and increased stem growth, so thinning management in old holm oak coppices could prepare the ecosystem to better withstand the increasing drought forecasted for the Mediterranean region

Les taillis méditerranéens de chênes verts face aux changements climatiques : éclaircir pour améliorer leur résistance à la sécheresse

National audienc

An investigation of the viscoelastic and mechano-sorptive creep behaviour of reinforced timber elements

The reinforcement of timber using fibre reinforced polymer (FRP) rods or plates is widely accepted as an effective method of increasing the strength and stiffness of members. Across Europe, this technology has been used, not only in new structures, but in the upgrading and repair of existing structures. When retrofitting these structures, changes in use of the building or, indeed, changes in building regulations often require a higher load capacity than that of the existing members. The additional capacity requirements can be successfully achieved in a timely and cost-effective manner through the use of FRP reinforcement. More widespread use of this technology has been hampered by the lack of a harmonised standard governing their design. Currently, design rules for FRP reinforcement are not included in Eurocode 5, which is the European standard governing the design of timber structures. The reasons for this are partly due to a lack of knowledge, particularly related to the long-term performance. This body of research aims to help researchers and design engineers to better understand the long-term or creep performance of FRP reinforced timber elements and to contribute to the development of design approaches, which account for the influence of reinforcement on this behaviour. This requires an examination of the main mechanical responses that are commonly observed in timber members under long-term loading, namely, the elastic, viscoelastic, mechano-sorptive and swelling/shrinkage responses. An experimental programme was designed to identify and characterise these individual responses on matched groups of glued laminated beams having statistically equal bending stiffness. This experimental programme involved the simultaneous loading of an equal proportion of unreinforced and reinforced beams to a common bending stress on the compression face in constant and variable climates for a period of 75 weeks. In the constant climate condition, the unreinforced and reinforced beams were subject to elastic and viscoelastic creep behaviour. In the variable climate condition, the beams were subject to elastic and viscoelastic creep behaviour and also, as a result of the variable climate condition, were subject to hygro-mechanical behaviour, which includes mechano-sorptive and swelling/shrinkage behaviour. The mid-span vertical deflection and longitudinal strain on the tension and compression faces were continuously monitored. No statistically significant difference was found between the creep deflection of unreinforced and reinforced beams in the constant climate condition. A key conclusion is that the viscoelastic creep deflection is governed by the stress level in the timber and is independent of the reinforcement. In the variable climate, the total deflection dramatically increased due to the variable moisture content conditions. In this case, there was a statically significant difference between the creep deflection of the unreinforced and reinforced beams with a reduction in deflection due to the reinforcement. By analysing the strain measurements from beams in the constant and variable climates together with separate swelling/shrinkage measurements on matched groups, it was possible to separate and characterise each individual strain component. The test methodology was specifically designed for this purpose. Significantly, the mechano-sorptive creep component has been separately quantified unlike in previous work where the mechano-sorptive and swelling/shrinkage effects have been combined. This separation has led to a significant finding in relation the influence of these components on the response. No statistically significant difference in the mechano-sorptive strain behaviour due to the reinforcement was observed. It was found that the main difference in the hygro-mechanical response is due to the different swelling/shrinkage behaviour of unreinforced and reinforced beams. The reinforcement restrains the swelling/shrinkage behaviour in the timber resulting in less relative creep deflection in reinforced members. This means that the influence of the reinforcement on hygro-mechanical response may be characterised through short-term swelling/shrinkage tests. A hygro-mechanical creep model was developed to predict the interaction between stress and moisture content change in timber elements when simultaneously loaded under a constant dead load and subject to a variable relative humidity condition with time. User-defined UMAT and DFLUX subroutines were utilised to define the material behaviour and variable relative humidity boundary conditions, respectively. Material characterisation tests were performed on Irish-grown Sitka spruce and BFRP rod reinforcement to provide the necessary material data for the numerical models. The numerical deflection and strain results have shown good agreement when compared to the experimental creep test results. Using this calibrated model, numerical studies to investigate a wide variety of reinforcement configurations and beam geometry are possible without having to undertake time-consuming and expensive experimental studies. To illustrate this, a parametric study was performed to examine the effect of different types of FRP reinforcement on the creep behaviour of reinforced Irish Sitka spruce beams over a 10 year period. The results of this parametric study showed that this modelling tool can be used to predict the long-term relative creep behaviour of such beams, and it can easily be adapted to account for different timber and reinforcement types and geometry once the relevant material characteristics are available. Application of the model to a wide variety of reinforced beam configurations will enable the development of reliable modification factors for implementation in the design of FRP reinforced beams.2017-09-1

How drought severity constrains gross primary production(GPP) and its partitioning among carbon pools in a <em>Quercus ilex</em> coppice?

International audienceThe partitioning of photosynthates toward biomass compartments plays a crucial role in the carbon (C) sink function of forests. Few studies have examined how carbon is allocated toward plant compartments in drought-prone forests. We analyzed the fate of gross primary production (GPP) in relation to yearly water deficit in an old evergreen Mediterranean Quercus ilex coppice severely affected by water limitations. Carbon fluxes between the ecosystem and the atmosphere were measured with an eddy co-variance flux tower running continuously since 2001. Discrete measurements of litterfall, stem growth and f(APAR) allowed us to derive annual productions of leaves, wood, flowers and acorns, and an isometric relationship between stem and belowground biomass has been used to estimate perennial belowground growth. By combining eddy covariance fluxes with annual net primary productions (NPP), we managed to close a C budget and derive values of autotrophic, heterotrophic respirations and carbon-use efficiency (CUE; the ratio between NPP and GPP). Average values of yearly net ecosystem production (NEP), GPP and R-eco were 282, 1259 and 977 g C m(-2). The corresponding aboveground net primary production (ANPP) components were 142.5, 26.4 and 69.6 g C m(-2) for leaves, reproductive effort (flowers and fruits) and stems, respectively. NEP, GPP and R-eco were affected by annual water deficit. Partitioning to the different plant compartments was also impacted by drought, with a hierarchy of responses going from the most affected - the stem growth - to the least affected - the leaf production. The average CUE was 0.40, which is well in the range for Mediterranean-type forest ecosystems. CUE tended to decrease less drastically in response to drought than GPP and NPP did, probably due to drought acclimation of autotrophic respiration. Overall, our results provide a baseline for modeling the inter-annual variations of carbon fluxes and allocation in this widespread Mediterranean ecosystem, and they highlight the value of maintaining continuous experimental measurements over the long term

What drives carbon allocation to stem and fine roots in a mature coppice of <em>Quercus ilex</em> in the Mediterranean? A data model analysis

International audienc

What drives carbon allocation to stem and fine roots in a mature coppice of <em>Quercus ilex</em> in the Mediterranean? A data model analysis

International audienc