Effects Of Elevated CO₂ Concentration On Physiological Characteristics And Growth Of Birch (Betula Albosinensis Burk.) Seedlings

The steady increae of atmospheric CO₂ concentration ([CO₂]) has been inevitablefact. Models predict that the atmospheric [CO₂] will increase to about 700μmol mol^-1 atthe end of the twenty-first century. As trees constitute a majoor carbon reservoir - 85%of total plant carbon is found in forest, and their ability to sequester carbon is a keydeterminant of future global change problems caused by increases in atmospheric CO₂.In addition to the role of forests in the global carbon cycle, inceased growth could be ofeconomic benefit, for example, offsetting deleterious effects of climatic changes. Betulaalbosinensis (Burk.) usually emerges as the pioneer species in initial stage and asconstructive species in later stages of forest community succession of mountain forestarea, and also is one of important tree species for afforestation in logged area, insouthwesten China. In this experinment, Betula albosinensis seedling (one-year-old)was used as the model plant. B. albosinensis seedlings were grown under two all-day[CO₂], ambient (about 350μmol·mol^-1) and elevated [CO₂] (about 700μmol·mol^-1), andtwo planting densities of 28 plants per m² and 84 plants per m². The objectives were tocharacterize birch mature leaf photosynthesis, growth, mass accumulation andallocation responses to long-tem elevated growth [CO₂] under the influences ofneighbouring plants, and to assess whether elevated [CO₂] regulated birch mature leafphotosynthetic capacity, in terms of leaf nitrogen concentration (leaf [N]), activity ofribulose bisphosphate carboxygenase (Rubisco), Rubisco photosynthetic efficiency, andtotal nonstructural carbohydrates (TNC) concentration, and also to provide a strongreference to predict the productivity of subalpine forests under the future global changes. reference to predict the productivity of subalpine forests under the future global changes.The results are as follows:1) B. albosinensis seedlings exposed to elevated [CO₂] for 120 days, photosyntheticacclimation phenomena occurred. At two planting densities, leaves of birch seedlingsgrown under elevated [CO₂] had lower net photosynthetic rate (A), stomatalconductance (g_s), transpiration (E), apparent quantum yield (AQY) and carboxylatedefficiency (CE) and higher water use efficiency (WUE), compared to those ofB.albosinensis seedlings grown under ambient [CO₂]. Based on the leaf area, leaf [N],Rubisco activity and photosynthetic pigments concentrations of B. albosinensisseedlings grown under elevated [CO₂] were significantly lower than those grown underambient [CO₂]. The ratio of chlorophyll a to chlorophyll b concentration was notaffected by elevated [CO₂]. Under elevated [CO₂], the TNC concentration per unit leafarea significantly increased, resulting in significant decrease in specific leaf area. Thusleaf photosynthetic capacity of B. albosinensis seedlings would perform worse underrising atmospheric [CO₂] and the influences of neighbouring plants.2) Under elevated [CO₂], the relative growth rate (RGR) of B. albosinensisseedlings height, basal diameter, a leaf area and branch length significantly increased,especially at the initial stage of exposure to elevated [CO₂], and a leaf area and leafnumbers per B. albosinensis seedling also significantly increased. Thus the total leafarea per B. albosinensis seedling was significantly increased under elevated [CO₂].3) As the increase of RGR and total leaf area, biomass of B. albosinensis seedlinggrown elevated [CO₂] was higher, compared to that of B.albosinensis seedlings grownat ambient [CO₂]. Elevated [CO₂] changed the biomass allocation pattern of B.albosinensis seedling.At two planting densities, B. albosinensis seedlings grown elevated [CO₂] hadlower leaf weight to total weight ratio (LWR), leaf area to total weight ratio (LAR) andleaf weight to non-leaf weight ratio (W_source/W_sink), but higher root weight to shootweight ratio (R/S), compared to those of B.albosinensis seedlings grown at ambient[CO₂]. Under elevated [CO₂], roots biomass to total biomass ratio was signigicantlyincreased, leaves biomass to total biomass ratio was significantly decreased. The main In conclusion, our results supported the hypothesis that the decline inphotosynthetic capacity of C₃ plants will appear after long-term exposure to elevated[CO₂], accompanying with the significant decrease in Rubisco activity, leaf Nconcontration, photosynthetic pigments concentration, and significant increase in totalnon-structural carbohydrates concentration. Our results also have shown that theincrease of biomass of B. albosinensis seedlings should be attributed to initialstimulation on RGR and total leaf area resulted from elevated [CO₂]. Under elevated[CO₂], the extra carbon sequestered by B.albosinensis seedlings transferred intounder-ground part because of increase in root biomass and R/S.