Abstract: Three light intensities (100%, 56.2%, and 12.5%) were installed to simulate the light regimes of opening field (cutting blank), forest gap, and understory, respectively, aimed to understand the effects of different light regimes on the seedling growth, photosynthetic characteristics, and biomass accumulation and allocation of alien species Alnus formosana and native species A. cremastogyne. Low light regime limited the seedling growth of the two alder species, while the light regime of forest gap was more favorable for the growth, in comparison with that of the opening field. Regardless of the light regimes, A. formosana seedlings had higher specific leaf area (SLA), relative growth rate (RGR), leaf area, leaf length, leaf width, plant height, and basal diameter, but smaller leaf number, leaf area ratio (LAR), and petiole length. Under low light regime, A. formosana seedlings had higher maximum net photosynthetic rate (P_{n max}), light saturation point (LSP), and apparent quantum yield (AQY), but smaller light compensation point (LCP) and dark respiration rate (R_day). With the decrease of light intensity, A. formosana seedlings had much higher root mass ratio (RMR) and much lower leaf mass ratio (LMR), implying that more carbon was allocated and stored to the roots rather than new leaves, whereas the A. cremastogyne seedlings were in adverse, i.e., more carbon was allocated to the above-ground parts, which might increase the risk of animal feeding and mechanical damage.