Abstract: To investigate the photosynthetic responses of needles with different ages (current-year and annual-year) in Pinus koraiensis to sunflecks along the vertical gradient of canopy, we conducted an experiment at the Changbai Mountain Forest Ecosystem Positioning Station utilizing a canopy tower crane platform. We selected current-year and annual-year needles from the upper (mean height: 23.26 m), middle (16.55 m), and lower (11.15 m) layers of the canopy of dominant species P. koraiensis, to simulate photosynthetically active radiation (PAR) abruptly increasing from a shaded state (50 μmol·m^-2·s^-1) to a sunfleck state (1200 μmol·m^-2·s^-1) and then decreasing back to 50 μmol·m^-2·s^-1 by using Li-6400 portable photosynthesis system. We measured the physiological response of needles during photosynthetic induction and photosynthetic recovery, as well as stomatal dynamic process. The results showed that leaf age and canopy height had significant effects on photosynthetic induction. During photosynthetic induction, the highest maximum reaction rate (RS_max) was recorded in current-year needles at the lower canopy (18.41 nmol·m^-2·s^-1) and in annual-year needles at the upper canopy (17.39 nmol·m^-2·s^-1). During photosynthetic recovery, current-year needles at the lower canopy showed the highest RS_max(2.93 nmol·m^-2·s^-1), while annual-year needles at the upper canopy exhibited the highest RS_max(0.66 nmol·m^-2·s^-1). This result indicated complementary strategies in sunfleck response between needle ages across canopy layers. There was a significant vertical gradient in stomatal kinetics. In the stomatal opening stage, the RS_max for both current-year and annual-year needles was significantly higher at the upper canopy compared to the lower canopy (145.5% faster and 104.4% faster, respectively). During the stomatal closing stage, the RS_max was lower for both needle ages at the upper canopy compared to the lower canopy (40.2% slower and 34.5% slower, respectively). These stomatal dynamics indicated that upper-canopy needles utilized sunflecks more effectively. Collectively, the complementary strategies in sunfleck utilization across needle ages and canopy heights would contribute to maximizing the overall photosynthetic carbon assimilation capacity of the broad-leaved Korean pine forest canopy.

Key words: broad-leaved Korean pine forest, carbon sink, canopy height, leaf age, photosynthetic induction, stomatal dynamics