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The 'Grain-for-Green' program implemented on the Loess Plateau in China has dramatically changed land use types, and subsequently enhanced the spatiotemporal variability of soil organic carbon (SOC) in the watersheds. However, the spatiotemporal variability of SOC for different topographic and land use types within small watersheds has not been adequately explored following the implementation of the 'Grain-for-Green' program. In this study, we determined the spatiotemporal variability of SOC content using the data collected in 1993, 2002, 2005, and 2012 and measured in 2018 and identified its driving factors for different topographic (tableland, sloping land, and gully) and land use types in the Wangdonggou watershed on the Loess Plateau. The spatial patterns of SOC content differed among tableland, sloping land, and gully, with higher spatial variability in gully than sloping land and tableland. The SOC content in the 0-20 cm soil layer in 2018 increased by 8.58%, 26.4%, and 22.2%, compared to 2002, for tableland, sloping land, and gully, respectively. Woodland and grassland had a great potential to sequester and stabilise carbon. The vegetation cover was a relatively dominant factor affecting SOC content throughout the watershed. Our results indicate a close relationship between SOC content and topographic, vegetation, and edaphic variables. This information is critical for understanding SOC dynamics at the watershed scale for sustainable ecological restoration. Keywords: ecological restoration, land use type, spatial distribution of SOC, temporal change of SOC, topographic type., Introduction Watershed soil organic carbon (SOC) dynamics are linked to nutrient cycling (Zhao et al. 2017), sediment transport (Haregeweyn et al. 2008; Hancock et al. 2019), land use changes (Gelaw [...]

Large-scale afforestation on the Loess Plateau has been subjected to more frequent and severe drought due to climate change. However, there is a lack of definite evidence elucidating the adjustments in water supply-demand relationships under prolonged drought. In this study, we investigated the responses of plant characteristics related to plant water supply and demand in 2-year-old Robinia pseudoacacia seedlings to soil water availability (SWA). We systemically analyzed the acclimation of plant water use to prolonged drought. A logistic function was used to fit relationships between normalized net photosynthetic rate (P n ), transpiration rate (T r ), stomatal conductance (G s ), soil-root hydraulic conductance (K sr ), root-leaf hydraulic conductance (K rl ), and whole plant hydraulic conductance (K plant ) with SWA. The results revealed significant difference in responses of these parameters to SWA (p  < .001). The sensitivities of P n , T r , and G s to drought increased from 60-day to 120-day drought, contributing to reduce water consumption. Meanwhile, the relative stable sensitivities of hydraulic conductance (K sr , K sr , and K plant ) promoted a consistent water supply at different ecological levels. Additionally, coordination between G s with hydraulic conductance helped maintain normal physiological activities under drought. These findings enhance our understanding of adjustments in plant water use in response to prolonged drought in Robinia pseudoacacia. Study Implications: Our findings have implications for better understanding the acclimation of Robinia pseudoacacia to prolonged drought. First, our results suggest decreased water demand (water consumption) by Robinia pseudoacacia under prolonged drought because the sensitivities of net photosynthetic rate (P n ), transpiration rate (T r ), and stomatal conductance (G s ) to drought increased from 60-day to 120-day drought treatments. Second, our study indicates a relative stable water supply (soil-root, root-leaf, and whole plant hydraulic conductance; K sr , K rl , and K plant ) of Robinia pseudoacacia at different ecological levels under prolonged drought. Finally, our research has implications for the coordination of stomatal and hydraulic regulation. [ABSTRACT FROM AUTHOR]

Rainfall partitioning is a key process in forest ecosystems that affects the forest hydrological cycle and ecological services. However, little is known about the rainfall partitioning in Pinus tabuliformis (PT), Robinia pseudoacacia (RP), and Pinus tabuliformis×Robinia pseudoacacia (PT × RP) forests, which are widely distributed on the Chinese Loess Plateau (CLP) for controlling soil and water losses. The objectives of this study were to compare the rainfall partitioning and conduct interception modelling in PT, RP, and PT × RP forests, using data from the Ansai and Changwu sites on the CLP. The sites feature similar weather conductions and both pure and mixed forests of the same age. The tree density of PT × RP was moderate at the Ansai site but higher at the Changwu site compared to PT and RP. We quantitatively measured the throughfall (Tf), stemflow (Sf), and interception loss (Ic) for the forests at the two sites during the growing season (May–October, 2022), and simulated Ic using the Revised Gash Analytical Model (RGAM) and Liu model. The results showed that measured Ic for PT × RP (26.0% of the total gross rainfall, Pg) was larger than for RP (22.0% of Pg) but smaller than for PT (27.8% of Pg) at the Ansai site, and vice versa for the Tf and Sf (RP > PT × RP > PT), while the measured Ic for PT × RP (33.7% of Pg) at the Changwu site was larger than both for PT (31.1% of Pg) and RP (19.5% of Pg). The Tf and Sf for PT × RP was smaller than for PT and RP. Given the similar weather conditions, differences in Ic for the pure and mixed forests at each site were attributed to canopy storage capacity, wet canopy evaporation rate, and tree density. Additionally, the RGAM and the Liu model effectively simulated and captured the differences in Ic for PT, RP, and PT × RP. However, the RGAM performed better with a relative error less than −7.3%, Nash‐Sutcliffe model efficiency ranging from 0.82 to 0.95, and root mean square error less than 0.5 mm. According to the RGAM, evaporation from the wet canopy during rainfall events and after rainfall cessation were the two most influential components. The results of this study suggest that mixed forest can optimize rainfall partitioning compared to pure forests, but a reasonable density should be adopted to increase rainwater replenishment to the forest floor. [ABSTRACT FROM AUTHOR]

To examine the spatiotemporal characteristics of the distribution of fine-root area and its relationship with stand age and environmental factors in black locust (Robinia pseudoacacia L.) on the Chinese Loess Plateau, black locust stands were selected at four sites along a precipitation gradient. Four stands of different ages and a transect along the hillslope were also selected at one of the sites. With increasing stand age, fine-root area at the tree level increased exponentially, and the rooting pattern tended to be deeper for trees up to 15 years old and then shallower thereafter. The temporal changes of fine-root distribution could be quantified using stand age and soil nutrients. At the hillslope scale, fine-root area index (FRAI) was lower while the rooting pattern was deeper in the middle slope than in the upper and lower slopes, and the fine-root distribution could be quantified using elevation and soil properties. At the regional scale, FRAI decreased substantially while exhibiting similar rooting patterns with decreasing soil water and nutrient availability along the precipitation gradient. Humidity index represented the regional environmental variation and could be used to quantify FRAI. These findings will be helpful for improving quantification of fine roots and enhancing the accuracy of ecohydrological models. Key words: fine-root area, rooting pattern, stand age, hillslope, precipitation gradient. Afin d'etudier les caracteristiques spatio-temporelles de la repartition de la surface des racines fines et sa relation avec l'age dupeuplement et les facteurs environnementaux chez le robinier faux-acacia (Robinia pseudoacacia L.) sur le plateau de Loess en Chine, des peuplements de robinier faux-acacia ont ete choisis a quatre endroits le long d'un gradient de precipitation. Quatre peuplements d'ages differents et un transect situe le long d'une pente ont aussi ete choisis a l'un des endroits. A l'echelle de l'arbre, la surface des racines fines augmentait exponentiellement avec l'augmentation de l'age du peuplement et l'enracinement avait tendance a etre plus profond jusqu'a l'age de 15 ans, apres quoi il devenait moins profond. Les variations temporelles de la repartition des racines fines pouvaient etre quantifiees avec l'age du peuplement et les nutriments du sol. A l'echelle de la pente, l'indice de surface racinaire des racines fines (ISRF) etait plus faible alors que le profil d'enracinement etait plus profond au milieu qu'en haut ou en bas de la pente. La repartition des racines fines pouvait etre quantifiee avec l'altitude et les proprietes du sol. A l'echelle regionale, l'ISRF diminuait substantiellement tout en presentant des profils d'enracinement similaires avec la diminution de la disponibilite de l'eau et des nutriments du sol le long du radient de precipitation. L'indice d'humidite representait la variation environnementale regionale et pouvait etre utilise pour quantifier l'ISRF. Ces resultats seront utiles pour ameliorer la quantification des racines fines et la precision des modeles ecohydrologiques. [Traduit par la Redaction] Mots-cles: surface des racines fines, profil d'enracinement, age du peuplement, pente, gradient de precipitation., Introduction Fine roots (diameter Fine-root distribution is believed to vary temporally and spatially, being affected by the root-to-shoot ratio and stand age and demonstrating plasticity in response to environmental factors. [...]