Effects Of Climate Change On Phenology,Productivity And Carbon Balance In Alpine Grasslands

Climate is a driver of terrestrial ecosystem structure and function.The Qinghai–Tibetan Plateau has recently been subject to a significant increase in temperature as a consequence of generalized global warming.Climate change will have a series of effects on phenology,community structure,productivity and carbon cycling in species,community and ecosystem level.To explore the effects of climate change on aforementioned processes,the relationships between aforementioned processes,and the different responses of alpine meadow and cultivated grassland to climate change,we conducted five-years of warming and spring water enhancement experiment in central Qinghai-Tibetan Plateau.We measured plant phenology,foliar functional trait,community composition and structure,plant productivity,soil carbon and nitrogen,ecosystem CO₂ exchange,and litter decomposition rate.The results showed that spring soil temperature and soil water water content are two main drivers of green up date and reproductive phenology in alpine meadow.Under high spring water condition,warming induced advance of green up date while warming induced delay of green up under low spring soil water.Warming has shortend the growth season length in the year with drought spring.The irrigation induced advance of green up date in drought spring rather than that in moist spring.Both warming and spring water enhancement had significant positive effects on species height and leaf area and the effect of warming was higher than the effect of spring water enhancement.Warming had no significant effects on leaf carbon and nitrogen content while spring water enhancement changed leaf carbon and nitrogen.Warming induced reduction in dominance of sedges and increase in proportion of grasses and forbs.The proportion of sedges had a negative correlation with species diversity in alpine meadow.In alpine meadow and cultivated grassland,short-term warming induced increase in community cover,height,and productivity.But the positive effects of warming would be weaken or reversed under long-term warming.Spring water enhancement also had a positive effect on plant productivity,especially in the year with low spring precipitation.Warming increased the productivity stability of both alpine meadow and cultivated grassland.Spring green up date had a slight effect on productivity and diversity while the environmental factors had a direct effect and species compositon had an indirect effect.Warming had a positive effect on carbon exchange of alpine meadow while a negative effect on cultivated grassland.After reclaimation,the soil carbon emission increased and accelerated carbon loss which would be intensified under warming.Overall,under phonological changing condition after warming,plant height and leaf area would increase in a short time and result in increase of productivity and net carbon uptake in alpine meadow.The decrease of sedge and increase of other species would maintain high species diversity in alpine meadow.But under long-term warming,response intensity of alpine meadow to warming would be weakened.The ability of cultivated grassland to tackling climate change is lower than alpine meadow.Warming has much of negative impacts on cunltivated grassland,especially results in decrease of net carbon uptake and soil carbon,which would have a big impact on this ecosystem.