21 research outputs found
A study of karst hydrosystem recharge at the parcel scale, using modeling and correlation analysis - Low noise underground laboratory of Rustrel site
La caractĂ©risation des flux dâeaux qui rechargent rĂ©ellement les hydrosystĂšmes souterrains reste un frein Ă la comprĂ©hension du fonctionnement hydrogĂ©ologique des milieux souterrains. Lors dâĂ©vĂ©nements pluvieux, quelle part de lâeau est Ă©vapo-transpirĂ©e ? Quelle part est temporairement stockĂ©e dans le sol ? Ces incertitudes sont particuliĂšrement fortes dans le cas de la recharge des milieux hĂ©tĂ©rogĂšnes tel que le karst. En gĂ©nĂ©ral, les calculs de recharge des hydrosystĂšmes karstiques se basent sur une reprĂ©sentation simplifiĂ©e de lâĂ©vapotranspiration qui considĂšre seulement le climat et pas le fonctionnement de la vĂ©gĂ©tation. Dans cette Ă©tude, un modĂšle de vĂ©gĂ©tation permettant de simuler les transferts dâeaux entre le sol et lâatmosphĂšre en contexte forestier (le modĂšle CASTANEA), a Ă©tĂ© appliquĂ© Ă une parcelle de ChĂȘne vert. Lâinfiltration efficace (un indicateur de la recharge) estimĂ© avec CASTANEA a Ă©tĂ© comparĂ©e Ă celle estimĂ©e par des approches classiques ainsi quâĂ des sĂ©ries long terme de flux dâeaux souterraines (9 annĂ©es). Les rĂ©sultats de cette analyse rĂ©vĂšlent que lâinfiltration efficace modĂ©lisĂ©e Ă partir dâun modĂšle de vĂ©gĂ©tation comme CASTANEA est plus satisfaisante que les approches classiques ne tenant pas compte du fonctionnement de la vĂ©gĂ©tation. Ce travail ouvre des perspectives intĂ©ressantes pour mieux tenir compte du fonctionnement de la vĂ©gĂ©tation et de lâusage du sol sur la recharge des hydrosystĂšmes karstiques.Assessing the recharge of underground hydrosystems remains an obstacle to understand their hydrologeological functioning. During a rain event, which part of the rain is evapotranspired ? And how much is temporarily stored within the soil ? These questions are particularly relevant in heterogeneous media such as karst hydrosystems. Currently, the models used to compute recharge of karst hydrosystems, rely on simplistic formulations of evapotranspiration that do not account for vegetation functioning. In this study, we used the vegetation process based model CASTANEA, which is designed to compute water transfer between soil, plant and atmosphere. We computed effective infiltration (an index of recharge) with CASTANEA and with other classical approach (based on precipitation minus ETP), and for a welldocumented holm oak site in Provence. Our results provide evidences that effective infiltration computed with CASTANEA yield more satisfactory correlation with measured outflow than simulations based on the classical approach. Our results provide a promising way to improve the simulation of karst hydrosystem recharge
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
How do leaf and ecosystem measures of water-use efficiency compare?
The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant wateruse efficiency (WUE), is a useful indicator of vegetation function. WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques. We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs. Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE
Le plan de reboisement a besoin de temps pour lâacquisition du matĂ©riel vĂ©gĂ©tal adaptĂ©
International audienc
: Comprendre et modéliser le fonctionnement hydrique des arbres
International audienceFace au changement climatique, le risque de sĂ©cheresse est un risque majeur auquel les forĂȘts sont confrontĂ©es. L'ouvrage prĂ©sente les connaissances de bases du fonctionnement hydrique des arbres, les indicateurs observables des effets de la sĂ©cheresse et les modĂšles capables de simuler le fonctionnement des forĂȘts en fonction du climat et de son Ă©volution
The role of deep vadose zone water in tree transpiration during drought periods in karst settings - Insights from isotopic tracing and leaf water potential
Karst environments are unusual because their dry, stony and shallow soils seem to be unfavorable to vegetation, and yet they are often covered with forests. How can trees survive in these environments? Where do they find the water that allows them to survive? This study uses midday and predawn water potentials and xylem water isotopes of branches to assess tree water status and the origin of transpired water. Monitoring was conducted during the summers of 2014 and 2015 in two dissimilar plots of Mediterranean forest located in karst environments. The results show that the three monitored tree species (Abies alba Mill, Fagus sylvatica L, and Quercus ilex L.) use deep water resources present in the karst vadose zone (unsaturated zone) more intensively during drier years. Quercus ilex, a species well- adapted to water stress, which grows at the drier site, uses the deep water resource very early in the summer season. Conversely, the two other species exploit the deep water resource only during severe drought. These results open up new perspectives to a better understanding of ecohydrological equilibrium and to improved water balance modeling in karst forest settings
Functional diversity reduces the risk of hydraulic failure in tree mixtures through hydraulic disconnection
International audienceForest ecosystems are increasingly threatened by anthropogenic pressures, especially by the increase in drought frequency and intensity. Tree species mixtures could improve resilience to diverse global anthropogenic pressures. However, there is still little consensus on how tree diversity affects water stress. Although some studies suggest that mixing species with different drought response strategies could be beneficial, the underlying mechanisms have seldom been identified. By combining a greenhouse experiment and a soil-plant-atmosphere hydraulic model, we explored whether mixing a drought avoidant (Pinus halepensis) and a drought tolerant (Quercus ilex) tree species could reduce plant water stress (defined as the risk of hydraulic failure) during extreme drought, compared to their respective monocultures. Our experiment showed that mixing species with divergent drought response strategies had a neutral effect on the drought-avoidant species and a positive effect on the drought-tolerant species. The model simulations further suggested that the beneficial effect of mixture on plant water stress during extreme drought was related to changes in the hydraulic connection of the plant from both the soil and the atmosphere. The ability of the drought-avoidant species to disconnect from the soil and the atmosphere limits its exposure to water stress, whereas the ability of the drought-tolerant species to increase its hydraulic connection to the soil lowers its hydraulic risk. This study brings a new insight on the mechanisms and traits combinations improving drought resistance in diversified forests and plantations, with important implications for forest management under climate change
Functional diversity reduces the risk of hydraulic failure in tree mixtures through hydraulic disconnection
International audienceForest ecosystems are increasingly threatened by anthropogenic pressures, especially by the increase in drought frequency and intensity. Tree species mixtures could improve resilience to diverse global anthropogenic pressures. However, there is still little consensus on how tree diversity affects water stress. Although some studies suggest that mixing species with different drought response strategies could be beneficial, the underlying mechanisms have seldom been identified. By combining a greenhouse experiment and a soil-plant-atmosphere hydraulic model, we explored whether mixing a drought avoidant (Pinus halepensis) and a drought tolerant (Quercus ilex) tree species could reduce plant water stress (defined as the risk of hydraulic failure) during extreme drought, compared to their respective monocultures. Our experiment showed that mixing species with divergent drought response strategies had a neutral effect on the drought-avoidant species and a positive effect on the drought-tolerant species. The model simulations further suggested that the beneficial effect of mixture on plant water stress during extreme drought was related to changes in the hydraulic connection of the plant from both the soil and the atmosphere. The ability of the drought-avoidant species to disconnect from the soil and the atmosphere limits its exposure to water stress, whereas the ability of the drought-tolerant species to increase its hydraulic connection to the soil lowers its hydraulic risk. This study brings a new insight on the mechanisms and traits combinations improving drought resistance in diversified forests and plantations, with important implications for forest management under climate change