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

Boosting Adsorption Isosteric Heat for Improved Gravimetric and Volumetric Hydrogen Uptake in Porous Carbon by N‑Doping

Porous carbon materials (PCMs) hold great promise as hydrogen storage materials due to their high capacity but are limited by adsorbing H2 at either cryogenic temperature or very high H2 pressure due to their weak van der Waals forces with the H2 molecules. In this study, N-doped hierarchical porous carbon (NHPC) materials were prepared by a simple one-step chemical activation method. Experimental results reveal that N-doping significantly enhances the interaction between H2 and the PCMs, which is demonstrated by increased adsorption isosteric heat (Qst) and H2 storage capacity per specific surface area (SSA). At lower H2 coverage, the Qst increases from 7.45 kJ/mol (NHPC-0) to 7.95 kJ/mol (NHPC-2 and NHPC-3), which aligns with the enhanced gravimetric H2 uptake per SSA. At higher H2 coverage (77 K, 50 bar-H2), there is a notable enhancement in the volumetric H2 uptake per SSA for NHPC-3 (11.41 g·L–1/m2·g–1) compared to that for NHPC-0 (8.49 g·L–1/m2·g–1) as the N content increases. Furthermore, N-doping can increase the packing density, thereby improving the volumetric H2 storage capacity of NHPC-x. The enhancement is strikingly demonstrated by NHPC-2, which achieves a volumetric H2 uptake of 26.96 g/L (SSA = 2458.44 m2/g) at 77 K and 50 bar. This is almost the same as that for NHPC-0, despite a 21% reduction in SSA, which is 26.47 g/L (SSA = 3116.58 m2/g) at the same condition. This work contributes to a deeper understanding of the effect of heteroatom doping on the H2 storage performance in PCMs

Synergetic Effect of Silver Nanocrystals Applied in PbS Colloidal Quantum Dots for High-Performance Infrared Photodetectors

PbS colloidal quantum dot (CQD) photodetectors hold great potential for near-infrared detection due to their extremely high sensitivity and low-cost solution processing. In this paper we report that incorporation of 0.5% to 1% (by weight) Ag nanocrystals (NCs) into the PbS CQDs film could simultaneously enhance the photocurrent and suppress dark current and hence significantly boost device detectivity. A set of control experiments suggested that Ag NCs, once added to the PbS CQD film, could trap photogenerated electrons from neighboring PbS CQDs, extend carrier lifetime, and increase photocurrent. We further built a sensitive flexible photodetector using the optimized composite on stone paper, achieving an estimated detectivity as high as 1.5 × 10¹⁰ Jones. The synergetic effect found in our PbS CQD/Ag NC composite photodetectors is expected to be extendable to other binary NC systems for various applications

Ynamides as Racemization-Free Coupling Reagents for Amide and Peptide Synthesis

A highly efficient, two-step, one-pot synthetic strategy for amides and peptides was developed by employing ynamides as novel coupling reagents under extremely mild reaction conditions. The ynamides not only are effective for simple amide and dipeptide synthesis but can also be used for peptide segment condensation. Importantly, no racemization was detected during the activation of chiral carboxylic acids. Excellent amidation selectivity toward amino groups in the presence of −OH, −SH, −CONH₂, ArNH₂, and the NH of indole was observed, making the protection of these functional groups unnecessary in amide and peptide synthesis

Low-Temperature-Processed Amorphous Bi₂S₃ Film as an Inorganic Electron Transport Layer for Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells have attracted great attention due to their unique properties and rapid increased power conversion efficiency. Currently, PC₆₁BM is widely used as the electron transport layer (ETL) for inverted hybrid perovksite solar cells. Here we propose and demonstrate that Bi₂S₃, a ribboned compound with intrinsic high mobility and stability, could be used as the ETL for perovksite solar cells. Through a simple thermal evaporation with the substrate kept at room temperature, we successfully produced a compact and smooth amorphous Bi₂S₃ ETL with high conductivity. Our NiO/CH₃NH₃PbI₃/Bi₂S₃ solar cell achieved a device efficiency of 13%, which is comparable with our counterpart device using PC₆₁BM as the ETL. Moreover, our device showed much improved ambient storage stability due to the hydrophobic and hermetic encapsulation of the perovskite layer by the Bi₂S₃ ETL. We believe thermally evaporated Bi₂S₃ is a promising ETL for inverted hybrid perovskite solar cells and worthy of further exploration

Investigation of the Interaction between Perovskite Films with Moisture via in Situ Electrical Resistance Measurement

Organometal halide perovskites have recently emerged as outstanding semiconductors for solid-state optoelectronic devices. Their sensitivity to moisture is one of the biggest barriers to commercialization. In order to identify the effect of moisture in the degradation process, here we combined the in situ electrical resistance measurement with time-resolved X-ray diffraction analysis to investigate the interaction of CH₃NH₃PbI_3−<i>x</i>Cl_<i>x</i> perovskite films with moisture. Upon short-time exposure, the resistance of the perovskite films decreased and it could be fully recovered, which were ascribed to a mere chemisorption of water molecules, followed by the reversible hydration into CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>·H₂O. Upon long-time exposure, however, the resistance became irreversible due to the decomposition into PbI₂. The results demonstrated the formation of monohydrated intermediate phase when the perovskites interacted with moisture. The role of moisture in accelerating the thermal degradation at 85 °C was also demonstrated. Furthermore, our study suggested that the perovskite films with fewer defects may be more inherently resistant to moisture

Strontium-Doped Low-Temperature-Processed CsPbI₂Br Perovskite Solar Cells

Cesium (Cs) metal halide perovskites for photovoltaics have gained research interest due to their better thermal stability compared to their organic–inorganic counterparts. However, demonstration of highly efficient Cs-based perovskite solar cells requires high annealing temperature, which limits their use in multijunction devices. In this work, low-temperature-processed cesium lead (Pb) halide perovskite solar cells are demonstrated. We have also successfully incorporated the less toxic strontium (Sr) at a low concentration that partially substitutes Pb in CsPb_1–<i>x</i>Sr_<i>x</i>I₂Br. The crystallinity, morphology, absorption, photoluminescence, and elemental composition of this low-temperature-processed CsPb_1–<i>x</i>Sr_<i>x</i>I₂Br are studied. It is found that the surface of the perovskite film is enriched with Sr, providing a passivating effect. At the optimal concentration (<i>x</i> = 0.02), a mesoscopic perovskite solar cell using CsPb_0.98Sr_0.02I₂Br achieves a stabilized efficiency at 10.8%. This work shows the potential of inorganic perovskite, stimulating further development of this material

Sulfoxide-Functional Nanoarchitectonics of Mesoporous Sulfur-Doped C₃N₅ for Photocatalytic Hydrogen Evolution

While carbon nitrides have emerged as leading materials in photocatalysis over the past two decades, innovative and facile approaches for porosity engineering (to enhance effective surface area) and atomistic heteroatom doping (to boost catalytic activity) are presently being hunted. We herein report the first synthesis of mesoporous sulfur-doped C3N5 (mesoporous sulfur-doped carbon nitrides (MSCNs)) with sulfoxide-functionalization via pyrolysis of 5-amino-1,3,4-thiadiazole-2-thiol, utilizing nanoporous silica templates with 2D and 3D porous structures (KIT-6 and SBA-15). Morphological and physicochemical properties of MSCNs have been systematically evaluated. We demonstrate that highly ordered mesoporous structural features with high effective surface areas, sulfur doping, and generated defects significantly dampen exciton recombination. In addition, adequate doping and functionalization yielding a sufficient number of catalytically active sites constitute the favorable set of conditions, eventually resulting in a remarkable hydrogen generation rate of 1370 μmol g–1 h–1 and effective pollutant remediation (>97% degradation rate in 150 min). Spectroscopic investigations and density functional theory calculations reveal that the sulfoxide functionalities generate efficient charge-transfer pathways on the catalyst’s surface, thereby catalyzing the reaction and impeding charge carrier recombination. The implications of this research offer insights into the development of surface/interface engineering and atomistic doping for enhanced photocatalysis, which will inspire superior futuristic catalytic design