Abstract

Background: Climate change, primarily driven by rising atmospheric CO2 and temperatures, affects plant phenology, a key indicator of ecosystem response. Phenological shifts in dominant forest species such as oaks can alter carbon assimilation, community dynamics, and forest resilience. However, experimental studies in Korea on multiple native Quercus species under climate manipulation are limited. Results: Using a multi-year greenhouse experiment, we quantified the phenological responses of six native deciduous oak species (Quercus aliena, Quercus serrata, Quercus variabilis, Quercus acutissima, Quercus mongolica, and Quercus dentata) under elevated CO2 (750–800 ppm) and temperature (+2°C). All species exhibited earlier budburst and leaf unfolding, with an average advancement of 12–20 days. Autumn leaf coloration and abscission were generally delayed, leading to a mean growing season extension of 20.3 days. The responsiveness varied across species; Q. mongolica, Q. serrata, and Q. variabilis were most sensitive, whereas Q. dentata showed inconsistent or reversed trends. Regression analysis revealed that accumulated temperature significantly predicted phenological timing in the control group, but correlations weakened or disappeared under treatment conditions, indicating saturation effects. Conclusions: Elevated CO2 and temperature extend the growing season in temperate oaks, but species-specific thresholds and saturation responses complicate phenological prediction. These findings emphasize the need for adaptive, non-linear modeling and species- targeted strategies in forest management under climate change.