Coupling between stomatal conductance and photosynthesis of subtropical tree seedlings under warming and progressive drought

Climate warming intensifies drought stress, threatening the carbon sink capacity of forest ecosystems. The coupling between stomatal conductance ( g s ) and photosynthesis ( A ) is crucial for predicting carbon cycling, yet its stability under combined warming and drought remains uncertain. We conducted a controlled experiment using open-top chambers to simulate warming (+1°C) combined with a progressive drought on seedlings of three dominant subtropical tree species ( Castanopsis carlesii , Schima superba , and Cunninghamia lanceolata ) in China. The responses of leaf gas exchange and chlorophyll fluorescence to these stressors were examined. Our results show that warming did not increase photosynthetic sensitivity to progressive drought across species. Despite significant drought-induced declines in photosynthesis, g s and A remained tightly coupled. Leaf photosynthesis was mainly limited by reduced CO 2 diffusion (decrease in both g s and mesophyll conductance); meanwhile, there was no clear indication of photochemical damage. Species diverged in stomatal regulation: C. carlesii exhibited “profligate” stomatal behavior (higher slope parameters ( m 1 ) from the unified stomatal optimization model), while S. superba and C. lanceolata were conservative (lower m 1 ). This study shows that these subtropical trees can maintain leaf-level A - g s coupling during moderate warming and progressive drought, and the use of species-specific stomatal parameters is recommended when modeling carbon and water flux in subtropical forests.