Effects Of Elevated CO₂ Concentration And Community Density On Uptake And Allocation Of Carbon And Nitrogen In Red Birch (Betula Albosinensis Burk.) Seedlings

The increased CO₂ concentration is one of the most important problems amongglobal changes. The increase of CO₂ will also cause other environmental problems,such as global wanning, etc. So the effects of elevated CO₂ on plant have drawnsights of many scientists in the research field of global change. Red birch (Betulaalbosinensis) usually emerges as the pioneer species in initial stage and asconstructive species in later stages of forest community succession of the darkconiferous forests in Western Sichuan, China. It's response to elevated CO₂ maydetermine the succession process of the community where it lives in. By controllingCO₂ at the ambient and twice as the ambient level (ambient+350 umol mol^-1) usingenclosed-top chambers (ETC), possible effects of elevated CO₂ on carbon fixation andallocation under two plantation densities are investigated. The effects of elevated CO₂on competition within canopy of red birch seedlings are also observed in the presentpaper. We hope to make sure of the effects of elevated CO₂ on the representativespecies, red birch. And so that, our results could provide a strong theoretical evidenceand scientific direction for forest management and afforestation under a future, CO₂elevated world. The results are as fowllows:1. The effects of elevated CO₂ on growth and the different responses of wood andbark of red birch seedlings(1) Elevated CO₂ increases the growth of seedling biomass, seedling height andbasal diameter of red birch. It also changed the biomass allocation in red birchseedlings. The ratio of root and main stem to all biomass is increased and the ratio ofleaf is decreased. (2) Tree bark and wood show different response degree but similarresponse direction to elevated CO₂. 2. Negative effects of planting density(1) The increase of planting density showes negative effects on the individualgrowth of seedling biomass, seedling height and basal diameter of red birch. It alsoeliminates the positive effects of elevated CO₂ on growth of red birch seedlings. (2)Community biomass is increased by the elevated planting density, which means thatthe high density red birch community could fix more CO₂ than the low density one.These results show that planting density plays an important role in determiningbiomass and carbon fixation ability of red birch community. Thus, exploring properplanting density becomes economically important for the future, CO₂ elevated word.3. The effects of elevated CO₂ on crown architecture and competition withincanopy of red birch seedlings(1) Crown width, crown depth, crown surface area and crown volume are allincreased under the influence of elevated CO₂. (2) Leaf number per unit area ofprojected crown area (LDcpa) and per unit volume of crown volume (LDcv) are lowerunder elevated CO₂. This is resulted from the stimulated growth of tree crown features.(3) The decrease of LDcpa and LDcv indicate that plants will respond forwardly toreduce the possible increase of competition resulted from stimulated growth ofindividual plant and collectives in conditions of elevated CO₂.4. The effects of elevated CO₂ on nutrition accumulation and allocation of red birchseedlings(1) Contents of N and P decrease due to the prompt increase of biomass of plantorgans caused by elevated CO₂. However, their accumulations increase under elevatedCO₂. (2) Elevated CO₂ increases the allocation of N, P to main stem but reduced itsallocation to leaf for that dry weight of the former increased but the dry weight of thelater decreased. (3) Using efficiencies of N, P (NUE and PUE) and their accumulationrates (NAcR and PAcR) are found to increase under elevated CO₂. Soil nutritioncontents are always the limiting factors for plant growth at subalpine and alpineregion. The increased NUE and PUE are helpful to eliminate the nutrition limitationin this area in the future world, when CO₂ concentration doubles the ambient. 5. The effects of elevated CO₂ on carbon balance of red birch communities(1) Net photosynthetic rates (Pn), dark respiration rates (Rd) and growth are allstimulated by elevated CO₂. (2) Content soil organic carbon increases sharply at theprimary stage of experiments and then the increasing rates decrease to a low level atlater stages. (3) Soil respiration rates increase significantly with the elevation of CO₂concentration. (4) The daily carbon fixations of whole community are heightened byelevated CO₂. The results (1)-(4) suggest that, the community being studied aresensitive to current climate change; the studied community, as a sink of atmosphericCO₂, is pool-sink alternative between seasons. (5) The carbon fixations are increasedalong the increase of planting densities.6. The effects of elevated CO₂ on physiological features of leaf senescences of redbirch seedlings at the later stage of growing seasonElevated CO₂ helps to postpone the leaf senescences of red birch at the end of thegrowth season. CO₂ enrichment increases the photosynthetic rates, contents of solubleproteins and photosynthetic pigments. And meanwhile contents of malondialdehyde(MDA) decreases and activities of superoxide dismutase (SOD) and catalase (CAT)are both increased. These results suggest that the senescences of red birch leaves aredelayed by elevated CO₂, which keep the photosynthetic rates at relatively high levels.Our results lend supports to hypothesis and results on stimulated photosynthetic ratesand growth from both other researchers and the present paper.