The Impact of Understory Presence on Soil CO2 Flux Dynamics in Temperate Forests

Understory with forest floor, which constitutes the canopy structure in temperate forest, plays a significant role in carbon cycle through various mechanisms. It alters soil environmental conditions and microbial dynamics, both of which contribute to soil CO2 fluxes. By increasing carbon input to the forest floor, the understory promotes the formation of thicker litter layers, which in turn modify soil CO2 flux through direct and indirect pathways. Despite its importance, field-based studies examining the effects of understory or litter layers on soil CO2 flux, as well as how these effects vary across seasonal patterns, remain limited. In this study, we conducted both the understory experiment and the litter experiment. The understory experiment aimed to investigate changes in soil CO2 flux and various environmental factors associated with the presence or absence of understory. For this, we setup two treatment sites: one with understory (CU) and the other without understory (CO) in a temperate deciduous forest (Mt. Nam) located in Seoul, South Korea. The litter experiment was designed to evaluate the influence of litter on CO2 flux, soil temperature, and soil water content by manipulating litter depths: no litter (NL), normal litter (Con; 5.5 cm), and double litter (DL; 11 cm) in a temperate urban forest located close to the undestory experiment. Soil CO2 flux, soil temperature, and soil water content were periodically monitored in both experiments. Soil CO2 flux was consistently higher in CU than in CO across all seasons, with increases ranging from 41.9% to 130%. The largest difference was observed in winter (Dec.–Feb.), where soil CO2 flux was 0.22 g C m⁻² day⁻¹ in CO and 0.51 g C m⁻² day⁻¹ in CU. This significant difference in CU was attributed to higher soil temperature (by 0.5 C, Dec.–Feb.).Additionally, soil water content in CU was higher than in CO in all seasons except autumn (Sep.–Nov.), which could be related to deep litter layer. These results from the understory experiment can be attributed to the significantly deeper litter layer in CU compared to CO (p < 0.01). Deep litter layer could reduce the sensitivity of soil temperature to atmospheric temperature fluctuations and decrease soil water evaporation. Supporting this finding, the litter experiment demonstrated that daily mean soil water content was highest in DL followed by Con and NL. Consistently, the litter experiment showed that litter layer contributed 12.6–22.0% to the total CO2 flux from the forest floor. Intergrating the findings from both experiments, we found that the presence of understory increases annual soil CO2 flux by 60.1%. Additionally, increased litter depth influenced by understory could locally enhance the CO2 flux, contributing to the spatial dynamics of carbon cycle in the temperate forest ecosystem. Overall, our results suggest that such changes can significantly influence the dynamics of soil CO2 flux within the temperate forest carbon cycle.