Scaling from single-point sap velocity measurements to stand transpiration in a multispecies deciduous forest: Uncertainty sources, stand structure effect, and future scenarios

9 páginas.-- 5 figuras.-- 2 tablas.-- 58 referencias[EN] A major challenge in studies estimating stand water use in mixed-species forests is how to effectively scale data from individual trees to the stand. This is the case for forest ecosystems in the northeastern USA where differences in water use among species and across different size classes have not been extensively studied, despite their relevance for a wide range of ecosystem services. Our objectives were to assess the importance of different sources of variability on transpiration upscaling and explore the potential impacts of future shifts in species composition on the forest water budget. We measured sap velocity in five tree species (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton, and Betula papyrifera Marsh.) in a mature stand and a young stand in New Hampshire, USA. Our results showed that the greatest potential source of error was radial variability and that tree size was more important than species in determining sap velocity. Total sapwood area was demonstrated to exert a strong controlling influence on transpiration, varying depending on tree size and species. We conclude that the effect of potential species shifts on transpiration will depend on the sap velocity, determined not only by radial variation and tree size, but also by the sapwood area distribution in the stand.[FR] Les études dont le but est d'estimer l'utilisation de l'eau a` l'échelle du peuplement dans les forêts mélangées font face a` un défi majeur : comment passer efficacement de l'échelle des arbres individuels a` l'échelle du peuplement. C'est le cas pour les écosystèmes forestiers dans le nord-est des États-Unis où les différences dans l'utilisation de l'eau entre les espèces et parmi les différentes catégories de taille n'ont pas fait l'objet d'études approfondies malgré leur pertinence pour une vaste gamme de services de l'écosystème. Nos objectifs consistaient a` évaluer l'importance des différentes sources de variation sur l'extrapolation de la transpiration et a` explorer les impacts potentiels des changements futurs dans la composition en espèces sur le bilan hydrique de la forêt. Nous avons mesuré la vitesse de la sève chez cinq espèces d'arbre (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton et Betula papyrifera Marsh.) dans un peuplement mature et dans un jeune peuplement au New Hampshire (É.-U.). Nos résultats ont montré que la plus grande source potentielle d'erreur était la variation radiale et que la vitesse de la sève était davantage déterminée par la taille des arbres que par l'espèce. La surface totale de bois d'aubier avait un effet très déterminant sur la transpiration qui variait selon la taille et l'espèce d'arbre. Nous concluons que l'effet des changements potentiels dans la composition en espèces sur la transpiration dépendra de la vitesse de la sève qui est principalement déterminée par la variation radiale et la taille des arbres mais aussi de la distribution de la surface de bois d'aubier dans le peuplement.This work was funded by the University of New Hampshire and the New Hampshire Agricultural Experiment Station. The Bartlett Experimental Forest is operated by the USDA Forest Service Northern Research Station. S. Mcgraw, P. Pellissier, C. Breton, S. Alvarado-Barrientos, R. Snyder, and Z. Aldag assisted in the field and in the lab. The 2011 stand inventory was led by S. Goswami. Tree heights were measured and compiled by C. Blodgett, T. Fahey, and L. Liu. A. Richardson shared meteorology and solar radiation data from the Bartlett Amerflux tower. The stands used in this experiment are maintained and monitored by the MELNHE project under the direction of R. Yanai and M. Fisk, with funding from NSF grants DEB 0235650 and DEB 0949324Peer reviewe

Optically Pumped Polaritons in Perovskite Light-Emitting Diodes

Lead halide perovskites have achieved significant progress in light-emitting diodes (LEDs) with high efficiency in the past few decades. They are also ideal candidates for reaching the strong exciton–photon coupling regime due to their large exciton binding energy and oscillator strength. The generation and control of exotic phenomena in perovskite electroluminescent microcavities, such as electrically pumped polariton lasing and polariton LEDs, operating in the strong coupling regime at room temperature, is still a great challenge. Here, we demonstrate room-temperature strong coupling in a perovskite LED structure. The best device shows a current efficiency of 4.5 cd/A and an external quantum efficiency of 1.4% while exhibiting anticrossing behavior via optical pumping. Our approach represents a new strategy to explore ultrafast LEDs as well as electrically pumped perovskite lasing

Thermal Evaporation and Characterization of Sb₂Se₃ Thin Film for Substrate Sb₂Se₃/CdS Solar Cells

Sb₂Se₃ is a promising absorber material for photovoltaic cells because of its optimum band gap, strong optical absorption, simple phase and composition, and earth-abundant and nontoxic constituents. However, this material is rarely explored for photovoltaic application. Here we report Sb₂Se₃ solar cells fabricated from thermal evaporation. The rationale to choose thermal evaporation for Sb₂Se₃ film deposition was first discussed, followed by detailed characterization of Sb₂Se₃ film deposited onto FTO with different substrate temperatures. We then studied the optical absorption, photosensitivity, and band position of Sb₂Se₃ film, and finally a prototype photovoltaic device FTO/Sb₂Se₃/CdS/ZnO/ZnO:Al/Au was constructed, achieving an encouraging 2.1% solar conversion efficiency