This research conducted two experiments comparing the responses of photosynthetic traits of
various shade-tolerant plant species to naturally enriched CO2 concentrations near the forest floor
in the Nanjenshan Forest, southern Taiwan. The objective of this study was to investigate how
seedlings of shade-tolerant species can maintain positive net photosynthetic rates (Pn) while those
of shade-intolerant species cannot in understory low-light environments. In the first experiment,
we measured the photosynthetic light responses of six species with different levels of shade
tolerance under CO2 concentrations of 360-420 ppm. Results showed that as the CO2 concentration
was elevated, the Pn and apparent quantum use efficiency increased while light compensation
points (LCP) and dark respiration rates (Rd) decreased in most species, with significantly more
changes in shade-tolerant seedlings. In the second experiment, we provided 350-430 ppm of CO2
concentrations to seedlings of 40 woody and herbaceous species for photosynthesis by pumping
the air at 5 and 100 cm above the forest floor. Results showed that under a low light of ≤ 12
μmol photon m-2 s-1, seedlings with higher shade-tolerant levels had higher photosynthetic CO2
use efficiency (the slope of regression lines between Pn and ambient CO2 concentrations, CaUE).
Seedlings of 19 shade-tolerant species showed positive Pn values at 400 ppm of CO2 (A400), while
seedlings of 10 shade-intolerant species showed negative A400 values. Furthermore, in the lowlight
environments of the understory, the CO2 compensation point (the CO2 concentration above
which Pn shifts from a negative to a positive value, CaCP) of shade-tolerant species were 105 ppm
less than that of shade-intolerant species. This is an innovative finding. Seedlings of shade-tolerant
species could maintain positive Pn values within the range of elevated CO2 concentrations due
to soil respiration. In conclusion, though the photosynthesis of understory woody or herbaceous
species is limited by insufficient light availability, the seedlings of these shade-tolerant species
could benefit from higher CO2 concentrations near the forest floor through significantly decreasing
their LCP, Rd, and CaCP and increasing CaUE to maintain positive carbon gain.