Abstract：Deep-seated fluid activity and the recently proposed “exudative” metallogenic model for sandstone-type uranium deposits are closely associated with reducing geological environments. Therefore, a systematic understanding of uranium speciation, mobility, and controlling mechanisms under reducing conditions is essential for clarifying uranium transport in deep fluids and for improving genetic interpretations of uranium mineralization. In this study, the occurrence forms and migration potential of uranium in several representative reducing systems enriched in organic ligands, silica, sulfur, halogens, and carbonate are comprehensively reviewed. The results indicate that uranium in reducing environments does not necessarily occur exclusively as insoluble mineral phases. Instead, its speciation is strongly controlled by pH, redox conditions, ligand composition, ligand concentration, temperature, and colloidal interactions. Under acidic reducing conditions, uranium may occur as U(IV) -SO₄^2- complexes, U(IV) -F^- complexes, U(IV) -Cl^- complexes, and U(IV) -organic ligand complexes. In contrast, under neutral to alkaline reducing conditions, uranium is more likely to exist as uranyl carbonate complexes, Ca²⁺-U(VI) -CO₃^2- complexes, U(IV) -Si colloids, UO₂ colloids, or as uranium adsorbed onto organic matter-related carriers. The mobility of these uranium species varies markedly among different geochemical systems. Under ambient geological conditions, uranyl carbonate complexes, Ca²⁺-U(VI) - CO₃^2- complexes, UF₃⁺, UF₃²⁺ , UF₄⁰ (aq), U(IV)-Cit₃^3- complexes, and U(IV)-Si colloids may exhibit relatively strong mobility. Among them, carbonate and calcium-uranyl-carbonate complexes can significantly enhance uranium solubility under weakly alkaline conditions, whereas U(IV)-Si colloids may remain dispersed and mobile owing to their nanoscale particle size and favorable surface charge characteristics. Under high-temperature geological conditions, UCl04 (aq) may become an important mobile uranium species, particularly in acidic, high-salinity reducing fluids. Ligand concentration is a key factor governing the stability and migration capacity of complexed uranium species, and it also influences the formation and persistence of U(IV)-Si colloids. Meanwhile, positively charged mineral colloids may restrict the migration of U(IV) -Si colloids through colloid-colloid interactions, thereby reducing uranium mobility. The effect of temperature on uranium speciation and mobility has been investigated only in a limited number of systems, and its role remains insufficiently constrained. Overall, previous studies have mainly focused on room-temperature and atmospheric-pressure systems, or on single hightemperature experimental conditions. However, the geochemical behavior of uranium species under coupled lowto medium-temperature and medium- to high-pressure conditions, as well as under medium- to high-temperature and medium- to high-pressure conditions, remains poorly understood. Future research should therefore emphasize laboratory simulations of reducing environments under low- to medium-temperature and medium- to highpressure conditions, and progressively extend such investigations to medium- to high-temperature and mediumto high-pressure systems. These efforts will provide important constraints on the occurrence forms, migration mechanisms, and evolutionary behavior of uranium species in deep reducing ore-forming environments.

Under the context of global warming and rising atmospheric CO₂ levels, increases in vegetation leaf area and longer growing seasons both contribute to a clear greening trend. However, whether these two factors show a trade-off or work together remains unclear. In this study, we used multi-source remote sensing data to analyze the patterns of maximum leaf area index (LAImax) and growing season length (LOS) in deciduous broadleaved forests (DBF) across China from 2003 to 2020. The results indicated that: ① An increase in leaf area is negatively correlated with the extension of the growing season. In Northeast China, deciduous broadleaf forests show relatively small increases in leaf area but more significant extensions of the growing season. Conversely, in the northern transitional zone, the opposite pattern is observed, suggesting a trade-off between these two strategies. ② Tree height plays an important role in explaining this trade-off. In the northern transitional zone, where dwarf trees dominate, forests tend to adopt a strategy of increasing leaf area with only small changes in growing season length. By contrast, in Northeast China, where tall trees are more common, forests are more likely to extend the growing season while showing limited leaf area change. This spatial difference reflects the contrasting physiological adaptations of trees with different heights. ③ These contrasting strategies are jointly shaped by climate factors and vegetation traits. Rising atmospheric CO₂ is more likely to promote leaf area increase in dwarf trees, whereas higher surface temperature has a stronger effect on growing season extension in tall trees. In addition, dwarf trees generally have lower initial leaf area and a higher leaf-to-sapwood area ratio, which favors leaf area increase, while tall trees tend to extend the growing season because of their larger basal leaf area and lower leaf-to-sapwood ratio. ④ The two strategies have different ecosystem consequences. Dwarf trees can enhance photosynthesis and ecosystem productivity by increasing leaf area. In contrast, tall trees mainly adapt by extending the growing season, but their lower photosynthetic efficiency may reduce ecosystem productivity.This expansion provides additional context to the ecological dynamics, emphasizing how specific strategies driven by tree height and climate factors interact to shape vegetation function across regions.