Heat Waves Effect On Grassland Ecosystem Carbon Flux In North China

As a consequence of global Climate change, intensity and frequency of extreme climatic events such as heat waves(HW) has been increasing worldwide. The studies have designed two experiments to construct a clear picture of how a HW effect the ecosystem particularly with respect to the dynamics of carbon, water, and vegetation on the grassland of north China. First experiment used a combination of continuous 60-year meteorological data with 6-year eddy covariance measurements of carbon dioxide(CO2) flux to analysis the HW’s effect on grassland ecosystem of Songnen Meadow Steppe. The second experiment was a simulation of human activities(by mowing the different height of plant) and HW’s influence on the typical grassland ecosystem of Duolun.(1)The number of HWs on the Songnen meadow steppe began to increase in the 1980 s, and the occurance of HW has increased more dramatically than ever before since the 2010s(2) HWs usually happen along with a drought. A 5-day long HW reduced SWC(soil water content) by 30%, and the SWP(soil water potential) increased by 38% as a result. In addition, mowing will accelerate the water’s evaporation rate. And the temperature of the soil still remained at a high level after the HW’s gone, unlike the air temperature which recovered soon when HW ended.(3) HWs reduced the carbon exchange of the grassland ecosystem. The natural HW of Songnen meadow steppe make NEE(Net Ecosystem Carbon Exchange) decreased by 50%. However, the simulated HW on Duolun typical grassland reduced NEE by 40% on year 2013 and 28% on 2014. In conclusion, HW can cause grassland ecosystem NEE decrease by 25%~50%, Re(Ecosystem Respiration) decrease by 5%~20% and GEP(Gross Ecosystem Production) decrease by 15%~30%.(4)The harmful impact that HW’s made on grassland ecosystem is mainly result from drought. High temperature alone can not make serious damage on ecosystem, but the combination of drought and high temperature result from HW will cause huge damage on ecosystem. When a HW lasts a few days, water from the ecosystem has evaporated to dry because of the high temperature, meanwhile, the WUE(Water Use Efficiency) of ecosystem decreased too, a 5-day long HW make WUE decrease by 26%.(5) When a HW ended, the negative influences of the HW will not ended immediately, it lasted a long time(nearly 20 days). In addition, these bad influences on ecosystem caused by HW may last a few growing seasons. HWs often occured during the period of plant growth peak, and HW restricted plants’ growing. And the most effective way to balance the negative influences of a HW is precipitation. Plants grow more rapidly by water complement, by this way, ecosystem will balance the impact of HW quickly.(6) Mowing and grazing are the main ways of how people utilize grassland ecosystem in northern China. In this study, we mowed the plant by different height to simulate these two human activites. At first year, slight mowing made no significant impact on the ecosystem, but still reduced the ecosystem’s tolerance to extreme climate. However, a joint action of continuous mowing and extreme weather conditions will destroy the balance between water and heat throughout the ecosystem. At the second year of the experiment, this joint action had changed the hydrothermal balance of the grassland ecosystem significantly, the ecosystems had become vulnerable. In addition, severe mowing will cause the same damage effect on grassland ecosystem; with more fiercely and more quickly.