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The VSC-HVDC system can damp the inter-area low-frequency oscillation effectively. To enhance the ability of the VSC-HVDC supplementary control, this article proposes a unified low-frequency oscillation controller design using the robustness theory. The unified control strategy uses both the active power control and reactive power control loop of the VSC to expand the supplementary control dimensions. To design the controller, the TLS-ESPRIT identification method is used to obtain the system small signal model and the oscillation characteristic. Based on the model identified out, the linear matrix inequality method based robust control theory is applied for the controller design, and the robust controllers for active power control and reactive power control loop are both designed to improve the control effect. Finally, the simulation results show that the controller can reach better control effect and the robustness can also be guaranteed.

Human activities and industrialization have significantly increased soil nutrients, such as nitrogen (N) and phosphorus (P), profoundly impacting the composition and structure of plant community, as well as the ecosystem functions, especially in nutrient-limited ecosystems. However, as the key propagule pool of perennial grasslands, how belowground bud bank and its relationship with aboveground vegetation respond to short-term changes in soil nutrients was still unclear. In this study, we conducted a short-term (2021-2022) soil fertilization experiment with N addition (10 g N m-2 yr-1) and P addition (5 g N m-2 yr-1) in the meadow steppe of Inner Mongolia, China, to explore the responses of belowground bud bank, aboveground shoot population and their relationships (represented by the ratio of bud to shoot density-meristem limitation index (MLI)) for the whole community and three plant functional groups (perennial rhizomatous grasses-PR, perennial bunchgrasses-PB, and perennial forbs-PF) to nutrient addition. The short-term nutrient addition had no significant influences on belowground bud density, aboveground shoot density, and MLI of the whole plant community. Plant functional groups showed different responses to soil fertilization. Specifically, N addition significantly increased the bud density and shoot density of PR, especially in combination with P addition. N addition reduced the shoot density of PF but had no influence on its bud density and MLI. Nutrient addition had significant effects on the three indicators of PB. Our study indicates that the belowground bud bank and its relationship with aboveground vegetation in temperate meadow steppe are insensitive to short-term soil fertilization, but plant functional groups exhibit specific responses in terms of population regeneration, which implies that plant community composition and ecosystem functions will be changed under the ongoing global change. [ABSTRACT FROM AUTHOR]

Drylands are characterized by unique ecosystem types, sparse vegetation, fragile environments, and vital ecosystem services. The accurate mapping of dryland ecosystems is essential for their protection and restoration, but previous approaches primarily relied on modifying land use data derived from remote sensing, lacking the direct utilization of latest remote sensing technologies and methods to map ecosystems, especially failing to effectively identify key ecosystems with sparse vegetation. This study attempts to integrate Google Earth Engine (GEE), random forest (RF) algorithm, multi-source remote sensing data (spectral, radar, terrain, texture), feature optimization, and image segmentation to develop a fine-scale mapping method for an ecologically critical area in northern China. The results showed the following: (1) Incorporating multi-source remote sensing data significantly improved the overall classification accuracy of dryland ecosystems, with radar features contributing the most, followed by terrain and texture features. (2) Optimizing the features set can enhance the classification accuracy, with overall accuracy reaching 91.34% and kappa coefficient 0.90. (3) User's accuracies exceeded 90% for forest, cropland, and water, and were slightly lower for steppe and shrub-steppe but were still above 85%, demonstrating the efficacy of the GEE and RF algorithm to map sparse vegetation and other dryland ecosystems. Accurate dryland ecosystems mapping requires accounting for regional heterogeneity and optimizing sample data and feature selection based on field surveys to precisely depict ecosystem patterns in complex regions. This study precisely mapped dryland ecosystems in a typical dryland region, and provides baseline data for ecological protection and restoration policies in this region, as well as a methodological reference for ecosystem mapping in similar regions. [ABSTRACT FROM AUTHOR]

Trade-offs in growth and reproduction are essential parts of the adaptive strategies of clonal plants. How rhizomatous psammophytes respond to aeolian processes (sand burial and wind erosion) by means of trade-offs is supposed to be especially important for their colonization on the active sand dune. Despite partial documentation of the responses of rhizomatous species to aeolian processes, how these clonal species respond to aeolian processes by means of potential trade-offs in growth and reproduction still remains unclear. In this study, we employed field investigation and biomass modeling to evaluate the trade-offs between vegetative and reproductive growth as well as between the number and size of ramets of Phragmites communis in response to sand burial and wind erosion. Sand burial enhanced the accumulation of seed biomass and reproductive effort. Wind erosion reduced reproductive effort but had no significant influence on seed biomass. Sand burial increased the biomass of ramets, while wind erosion increased ramet population density and accelerated ramet maturation. Our results demonstrate that rhizomatous psammophytes adjust their growth strategies in response to aeolian processes, i.e. reproductive growth and ramet size increase in response to sand burial, while vegetative growth and ramet numbers increase in response to wind erosion. [ABSTRACT FROM AUTHOR]