Plant Community Composition And Nitroge-phosphorus Stoichiometry Alon A Slope Aspect Gradients In A Alpine Meadow

Slope aspect is one of the key factors in determining plant community composition on terrestrial ecosystem of the Qinghai-Tibet Plateau Mountain. Slope aspect greatly influence the coming solar radiation, which determine soil temperature, soil evaporation, soil water content and other ecological factors. South-facing slope (sunny slope) absorbs more solar radiation, hence it is hotter and drier, while north-facing slope (shady slope) absorbs less solar radiation, so it is wetter and colder. Thus, vegetation responded with different structure and compositon in different slope aspects. Most studies on the large scale have shown that the distributions of light, heat, water affect vegetation structure and community composition on terrestrial ecosystem, many studies did on aspect impects-less on high altitude. Here we analyzed the variation characteristic of soil factors and plant community composition along north-south-facing slope gradient, studied the response of plant leaf N and P stoichiometry characteristic to slope aspect, and discussed the nitrogen, phosphorus stoichiometry differences in different functional groups and the physiological changes of species at north-and south-facing slope on a alpine meadow of the eastern Tibetan Plateau.The main results of the study showed:1. From the south-facing slopes to north-facing slope, the change of environmental factors was larger, daily average soil temperature and light intensity were south-facing slope> west-facing slope> north-facing slope, soil water content was north-facing slope> west-facing slope> south-facing slope. Soil total phosphorus and organic matter were greater in north-facing slope than south-facing slope. Soil total nitrogen, available nitrogen and available phosphorus didn’t show a significant change in the aspect gradients.2. Significant changes in plant composition occurred in the different slope aspects where the south-facing slope was dominated mainly by grasses (Poaceae and Cyperaceae) and the north-facing slope was dominated by mainly forbs (non-legumes forbs). Species diversity and richness changes significantly in the different slope aspects, it showed north-facing slope> west-facing slope> south-facing slope.3. Nitrogen and phosphorus stoichiometry in plant leaves also changed dramatically from the south-facing slopes to north-facing slope, in the south-facing slope species average leaf nitrogen and phosphorus content were significantly less than the north-facing slope, but Leaf N:P ratio was significantly higher in south-facing slope than the north-facing slope. In addition, we also found that the nitrogen and phosphorus stoichiometry in the plant functional groups also changed, average plant leaf nitrogen and phosphorus content of grasses was significantly lower than the forbs, and the legume leaf nitrogen content and N:P ratio in different aspects higher than that of grasses and forbs.4. The limiting elements changed from the south-facing slope to north-facing slope. In the south-facing slope the N:P ratio of plant leaf was greater dramaticlly than16, while the north-facing slope was less than14. Therefore, according to the limit elements stoichiometry ratio theory, south-facing slope was phosphorus limitation, while north-facing slope was nitrogen limitation.5. From the south-facing slopes to north-facing slope, the plant growth rate was significantly negatively correlated with N:P, while it was significantly positively correlated with phosphorus content of plant leaves, which supported the growth rate hypothesis. Nitrogen and phosphorus content in plant leaves and N:P ratio showed a significantly decline trend with increasing of altitude, which supported the biogeochemical hypothesis.6. Along the north-facing slope to south-facing slopes, soil water content declined and soil temperature, light intensity increased, the trend of proline and soluble sugar content of plant species were increased. The order of proline and soluble sugar content:Oxytropis kansuensis> Stellera chamaejweisme> Otentilla fragarioid.es> Kobresia humilis> Potentilla fruticosa, while the trend of plant chlorophyll content were declined, and the order plant chlorophyll content:Oxytropis kansuensis<Kobresia humilis<Otentilla fragarioides<Leontopodium leontopodioides<Potentilla fruticosa<Stellera chamaejweisme, these indicated that with the increasing of adversity stress, plants show against external stress ability through their own cell osmotic regulation role. According to these results, we draw the following conclusions:1. Soil water content was the key factor determining vegetation composition and species diversity on the north-and south-facing slope gradients in alpine meadow.2. Plant leaf nitrogen and phosphorus stoichiometry was not consistent along the south-facing slope to north-facing slope gradients, the nitrogen and phosphorus content of the plant leaves on north-facing slope were significantly higher than south-facing, N:P ratio was significantly less than the south-facing slope. These suggested that these stoichiometry changes were controlled by soil water content and soil temperature.3. Along the north-facing slope to south-facing slope gradients, soil water content declined and soil temperature, light intensity increased. With the increasing of stress, plant species of proline and soluble sugar content was significantly increased, while chlorophyll content decreased, and this was physiological adaptation of plant to stress, it shown that the stress resistance of plants was stronger in south-facing slope.