Study On Growth Response And Mechanism Of Constructive Species At Subalpine Treeline To Elevated CO₂ And Temperature In Western Sichuan, China

The atmosphere CO2 concentration and other greenhouse gas have steadily risen globally after the industrial revolution, and result in the increased temperature due to the global greenhouse effects. The influences of these changes and other environmental problems on natural ecosystem and human society have become a major topic of discussion in global climate change studies. Ecotone at subalpine treeline is highly sensitive to disturbance and environmental change due to high environmental heterogeneity and harsh condition. Many species are at the limits of their distributional ranges at ecotone. So, ecotone can be a suitable platform for climate change research. Subalpine transition zone of forest and grass at Western Sichuan Province is in eastern edge of Qinghai-Tibetan Plateau. It is one of the major concerns in global climate change studies, with sharp environment gradient. The study on effects of climate change on plant growth would be helpful for understanding the change of community structure and components in the future climate.In this study, fir(Abies faxoniana) and four herbs with native soil from subalpine tree line ecotone were transferred into enclosed-top chambers for elevated CO2 and temperature treatments. The objective of this study was to determine the effects of CO2 and temperature on growth, physiology and morphology under competitive condition. At the same time, the possible mechanism of growth is discussed in terms of physiology and morphology. With research on fir and 4 herbs under elevated CO2 and temperature, we hope to detect the difference in effects of elevated CO2 and temperature on different species. Therefore, our results intent to provide theoretical evidence for predicting developments of community and basic data for carbon cycle model in future climate.The results and conclusions are as follows:1) Effects of elevated CO2 aud temperature on biomass of community Biomass of community was 645.95g/m2,515.83g/m2 and 584.19g/m2 respectively under elevated CO2, elevated temperature and the combination of elevated CO2 and temperature, which decreased significantly compared to the control (769.48g/m2). Further analysis showed that the different responses of species to elevated CO2 and temperature resulted in change of community components. Decreases in biomass of Deyeuxia scabrescen and Carex kansuensis, which contributed larger proportion to community biomass, offset increases in biomass of fir, Fragaria orientali and Cardamine tangutorum. The difference in biomass response of species to elevated CO2 and temperature, even reverse, resulted in change of community components and structure.2) Responses of Arbor and 4 herbs to elevated CO2 and temperature in growth, physiology and morphologyElevated CO2, elevated temperature and the combination of elevated CO2 and temperature stimulated the growth of fir. Furthermore, high temperature accelerated the stimulation of elevated CO2 on growth. Except for Deyeuxia scabrescen under elevated CO2, elevated CO2, elevated temperature and the combination decreased the biomass of Deyeuxia scabrescen and Carex kansuensis. On the contrary, biomass of Fragaria orientali and Cardamine tangutorum increased under elevated CO2, elevated temperature and the combination, except for Fragaria orientali under elevated temperature. The above meant that elevated CO2 and temperature was favorable for the growth of fir and some of herbs, but disadvantageous to the other herbs, which suggested that in low temperature ecotone, dominant and subordinate herbs in subalpine ecotone might shift relative to each other in the climate change.Elevated CO2 and the combination of elevated CO2 and temperature increased the photosynthesis and water use efficiency of arbor and 4 herbs. Elevated temperature increased stomata conductance and transpiration of arbor and 4 herbs. So, water use efficiency of fir, Deyeuxia scabrescen and Carex kansuensis decreased due to no change in photosynthesis, while increased for Fragaria orientali and Cardamine tangutorum due to increased photosynthesis. Increased water use efficiency of some species under the changed climate would alleviate dry stress caused by elevated temperature or changed precipitation, which might improve the competitive ability. Photosynthetic acclimation was observed in arbor and 4 herbs under elevated CO2, elevated temperature and the combination of elevated CO2 and temperature. In addition, the magnitude of reduction in photosynthetic parameter of fir was smaller than that of 4 herbs. Analysis on Pn-PAR and Pn-Ci curve showed that apparent quantum yield and carboxylated efficiency of arbor and 4 herbs decreased under elevated CO2, elevated temperature and the combination. This meant a decline in electron transport, Rubisco content and activity of fir and herbs grown at elevated CO2 and temperature. Increased CO2 compensated point and light compensated point meant that plant decreased their capacity to use the low CO2 concentration and light.Elevated CO2, elevated temperature and the combination of elevated CO2 and temperature increased N absorption of fir, but decreased or had no influence on herbs. N use efficiency of fir was not influenced by elevated CO2 and elevated temperature, but enhanced for herbs under elevated CO2, elevated temperature and the combination of elevated CO2 and temperature. High N use efficiency of herbs might alleviate N insufficiency. The above suggested that strategy in absorption and use of nutrient among species was different under elevated CO2 and temperature.Crown morphology of fir was influenced by elevated CO2, elevated temperature and the combination of elevated CO2 and temperature, with larger crown volume and leaf area, changed branch angle, length and distribution. Elevated CO2 and elevated temperature both increased root to shoot ratio (RSR). The above changes in morphology were favorable for fir to intercept light and absorb belowground resources. On the contrary, RSR and total leaf area of Deyeuxia scabrescen and Carex kansuensis decreased at elevated CO2, elevated temperature and the combination of elevated CO2 and temperature, which was disadvantageous for capturing resources. So, morphology of plants could regulate the potential to exploit resources.Specific root length of arbor and herbs increased at elevated CO2, while N absorption per root length decreased, which suggested that in order to exploit more resources, plants used more investment to occupy larger space. This might be related to less N availability of soil at elevated CO2. Elevated temperature and the combination of elevated CO2 and temperature increased specific leaf area of fir, but decreased N absorption per root length. The reverse was true for herbs. The above showed that elevated temperature had different effects on efficiency of investment and acquisition between fir and herbs.3) The mechanism of growth response to elevated CO2 and temperature was different among speciesThe mechanism of growth response in arbor and herbs to elevated CO2 and temperature was different. The stimulated growth of fir at elevated CO2 was resulted from larger occupied space, increased total absorbed resources, photosynthesis and water use efficiency, but the physiological and morphological traits related to N were disadvantageous to the growth of fir. The combination of elevated CO2 and temperature stimulated the growth of fir through increased total resources and high use efficiency of resources. Growth of Deyeuxia scabrescen and Carex kansuensis was constrained at elevated CO2 and the combination of elevated CO2 and temperature due to morphology. The increased growth of fir at elevated temperature was resulted from larger occupied space and increased total absorbed resources, but not from increasing use efficiency of resources. Except for Fragaria orientali, herbs growth were constrained at elevated temperature due to the low use efficiency of resources and morphology. The above differences in mechanism of growth response between fir and herbs suggested that the physiological and morphological parameter used in carbon cycle model should be adjusted when predicting the global carbon balance at the different conditions.