Effects Of Global Change Factors On Plant Community In A Semi-arid Grassland On The Loess Plateau

Global warming,changes of rainfall patterns and enhanced nitrogen deposition have significantly changed the global environment since the Industrial Revolution.Grasslands occupy 51×106 km2 and are the largest component of terrestrial ecosystems.Many grasslands are vulnerable and sensitive to environmental changes.The Loess Plateau in Northwest China is one of the most important components of terrestrial ecosystem in China.It included an area of about 62×104 km2 and 60%of which belongs to the semi-arid climate area.Vegetations on the Loess Plateau play critical roles in soil retention and mitigating the soil erosion by wind and water.However,these systems are highly sensitive to environment changes,potentially compromising their functions under future climate change scenarios.Yet,there is still limited information about the direction and magnitude of Loess grassland responses to predicted climate change in the future.This dissertation research aims to examine how plant communities respond to global warming,altered precipitation and nitrogen addition in an attempt to quantify the magnitude of the responses and identify the potential mechanisms.We initiated a long-term field experiment in May 2015 on a mountain top of Yunwu Mountain National Park,Guyuan,Ningxia to examine the responses of grassland plants and microbes to global change factors.The experiment manipulated air temperature(control and warming by open top chamber),N inputs(ambient and N addition of 12 kg N m2 per year),and precipitation alterations(ambient,precipitation reduction-ambient-3 0%,and precipitation increase-ambient+30%).Dynamics of plant community and functions in response to manipulated global change factors was assessed by quantifying the composition and biomass of the plant community,and ecosystem C fluxes(including net ecosystem CO2 exchange(NEE),ecosystem respiration(ER),gross ecosystem productivity(GEP)and soil respiration(SR).Major results are summarized below:1.Effects of global change factors on community structureEffects of global change factors on species richness was significantly(i.e.P<0.05,applied to the whole dissertation)affected by the timing of the treatments.In 2015,treatments had no effect on plant species richness.In the second and third year,warming and nitrogen addition significantly decreased species richness.Altered precipitation had no effect on species richness across the three years.However,species richness was significantly lower in precipitation reduction than increased precipitation plots.Warming significantly decreased the Shannon-Wiener index in 2016 and 2017.Yet,none of the global change factors had significant effect on Simpson index and Pielou index.Global change factors altered community biodiversity mainly by changing species number of forbs.Effects of warming,altered precipitation and nitrogen addition on the plant community structure also varied among different years.Warming significantly decreased abundance of forbs in 2016 and reduced the abundances of both forbs and grasses in 2017.Precipitation reduction decreased abundance of grasses in 2015 and both forbs and grasses in 2016.Nitrogen addition reduced the abundance of grasses in 2016 and grasses and semi-shrubs in 2017.In contrast,warming increased grass coverage in 2015,but decreased forb coverage in all the three experimental years.Increased precipitation increased grass coverage in 2015 and semi-shrub coverage in 2016,Precipitation reduction decreased forb coverage in 2016.Nitrogen addition increased forb coverage,but decreased semi-shrub coverage in 2015.SEM analysis showed soil moisture was the factor that was mostly related to community structure and that experimental treatments affected the plant community structure mainly through modifying soil moisture.2.Effects of global change factors on plant aboveground biomass and its temporal stabilitySignificant effects of the three global change factors on aboveground plant biomass occurred in the third experimental year,i.e.2017.Warming significantly decreased community aboveground biomass by 24.61%but nitrogen addition significantly increased it by 22.61%.Aboveground community biomass was significantly higher in precipitation increase than precipitation reduction plots.Also,warming decreased aboveground biomass of non-dominant species by 31.22%and 54.42%in 2016 and 2017,respectively.Similarly,precipitation reduction decreased aboveground biomass of non-dominant species by 40.35%in 2016.In contrast,increased precipitation enhanced aboveground biomass of dominant semi-shrub by 44.24%and 70.65%in 2015 and 2016.Nitrogen addition increased aboveground biomass of dominant grasses by 66.51%in 2017.None of warming,altered precipitation and nitrogen addition had a significant effect on temporal stability of aboveground community biomass.There was,however a significant interaction between nitrogen addition and warming or altered precipitation were found.Warming tended to increase stability without nitrogen addition,but marginally(i.e.0.05<P<0.1,applied to the whole dissertation)decreased it with nitrogen addition.Increased precipitation had no effect on stability,but significantly increased it in nitrogen addition plots.Both warming and nitrogen addition significantly decreased stability of non-dominant species.There was no significant relationship between community stability and species richness(P>0.05).However,community stability was closely related to species synchronization and stability of aboveground biomass of dominant species.3.Effects of global change factors on ecosystem carbon fluxesWarming and altered precipitation significantly affected ecosystem carbon fluxes in the mid of growth season.In July and August of 2016,warming significantly affected ER,GEP and SR,but had no effect on NEE.Precipitation reduction significantly decreased NEE,ER and GEP in June,July and August.Nitrogen addition significantly increased SR in May.While precipitation reduction significantly decreased mean values of NEE,ER and GEP by 34.71%,20.00%and 26.71%,respectively,increased precipitation significantly enhanced mean values of SR by 17.10%.All parameters of ecosystem carbon fluxes were closely related to aboveground biomass of dominant grasses and semi-shrubs.Together,our results showed warming,altered precipitation and nitrogen addition significantly changed plant community structure of a semi-arid grassland on the Loess Plateau.All the three factors affected plant community by changing soil moisture.Dominant species,especially dominant semi-shrub,were shown to be more resistant to climate change in our short-term study.Because they play a primary role in maintaining ecosystem services,over the longer term,continued attention should be directed towards monitoring the impact of global change factors on these species.