Understanding belowground carbon dynamics is essential for predicting the carbon balance of forest ecosystems. This study aimed to investigate links between soil CO₂ efflux (R_S), soil physicochemical properties, and fine-root morphology across four middle-aged plantations of different species (Robinia pseudoacacia, Quercus mongolica, Pinus koraiensis, and Metasequoia glyptostroboides) in Mt. Ansan, Seoul, Republic of Korea. Seasonal measurements of R_S, soil temperature (T_S), and soil water content (SWC) were conducted, and soils and fine roots (≤2.0 mm) were analyzed for physicochemical properties and morphological traits, with a focus on very-fine roots (≤0.5 mm). The results showed that R_S was positively correlated with T_S (r = 0.77) and negatively with SWC (r = −0.33). R_S normalized at 25 °C (R₂₅), differed significantly among plantations, and exhibited strong positive correlations with electrical conductivity (r = 0.81), as well as with total nitrogen and carbon concentrations and clay content. Among fine root traits, the length, surface area, and volume of very-fine roots exhibited the strongest associations with R₂₅, underscoring their pivotal role in regulating belowground respiration. These findings suggest that species-specific fine root strategies and soil conditions jointly control R_S dynamics, particularly under warmer conditions, and highlight very-fine root traits as key indicators of soil carbon flux in forest ecosystems.

Climate change is intensifying extreme climatic events such as warming and drought. This study investigated the physiological and growth responses of Quercus variabilis, a major broadleaf plantation species in South Korea, to warming and drought under open-field conditions. From July to August 2024, one-year-old seedlings were exposed to factorial treatments of temperature (ambient: TC; +3 °C: T3; +5 °C: T5) and precipitation (ambient: PC; drought: DR). Gas exchange was measured twice (early: 12 July; late: 16 August) during the treatment period. In the early phase of the experiment, net photosynthetic rate (P_n) was 11.4% lower in DR than in PC, whereas differences were no longer significant in the late phase. Stomatal conductance (g_s) was significantly affected by the interaction between temperature and precipitation. These results suggest that the decline in photosynthesis was driven by non-stomatal limitations such as photosystem II impairment or reduced Rubisco activity, despite maintained or enhanced stomatal conductance. Consequently, intrinsic water-use efficiency (iWUE) during the late phase was 24.3% lower in T5 than in TC. Root collar diameter (RCD) was more sensitive to drought than height growth. Shoot (stem + leaf) biomass was 23.7% higher in T5 than in T3, and root biomass was 20.5% higher in T5 than in TC. However, the root-to-shoot (R/S) ratio did not differ significantly among temperature and precipitation treatments. These findings suggest that Q. variabilis seedlings exhibit physiological plasticity and maintain relatively stable biomass allocation under short-term warming and drought conditions.