Warming Alters Non-Structural Carbohydrate Dynamics and Source–Sink Regulation in Two Temperate Tree Species

Understanding how trees coordinate carbon acquisition and allocation under warming is essential for predicting forest–carbon dynamics; however, whether warming induces common or species-specific source–sink regulation strategies remains unresolved. We conducted a field warming experiment (+2 °C and +4 °C above ambient) to investigate photosynthetic responses, organ-specific non-structural carbohydrate (NSC) dynamics, and carbon allocation strategies in two temperate tree species, Fraxinus mandschurica and Juglans mandshurica. Moderate warming (+2 °C) initially enhanced carbon assimilation in both species, whereas severe warming (+4 °C) progressively inhibited photosynthetic performance during the later growing season. In F. mandschurica, +2 °C warming promoted carbon storage by increasing starch accumulation across leaves, stems, and roots, with starch concentrations 2.5–38.0% higher than those in control plants from July to September. These enhanced NSC reserves were positively associated with photosynthetic performance and survival, indicating an effective source-driven carbon storage strategy. In contrast, J. mandshurica exhibited a distinct carbon allocation pattern under warming, characterized by preferential soluble sugar accumulation in leaves and reduced carbon investment in storage tissues. Specifically, leaf soluble sugar-to-starch ratios increased by 6.0–162.3%, whereas root soluble sugar content decreased by up to 55.7% under warming, suggesting a shift toward short-term carbon regulation rather than long-term storage. Structural equation modeling further demonstrated that photosynthetic performance and leaf NSC pools were central regulators of warming responses, but their effects on downstream carbon allocation differed between species. Our results reveal that warming does not induce a uniform carbon response among temperate trees; instead, species-specific source–sink regulation strategies determine carbon resilience under elevated temperatures. These findings improve understanding of how coexisting tree species may diverge in their adaptive capacity under future climate warming.