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Response Of Vegetation Productivity And Distribution To Climate Change

Posted on:2021-02-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:W Z ChenFull Text:PDF
GTID:1360330614473080Subject:Quaternary geology
Abstract/Summary:PDF Full Text Request
Quaternary global climate has been changing dramatically,which is characterized by multiple glacial-interglacial cycles and current global warming.During the Last Glacial Maximum??21 ka,LGM?,the sea level decreased and the ice sheet expanded.In the middle Holocene??6 ka,MH?,the climate was much warmer.As a result,the African summer monsoon and rainfall were intensified,which is known as the African humid period.After the industrial revolution,the global air temperature has been increasing rapidly,together with the elevated CO2 concentration.Climate change is closely associated with terrestrial ecosystems.Terrestrial vegetation and land surface processes have been affected by the late Quaternary climate change.Climate change inevitably causes changes in the ecological environment.China currently is facing an increasing environmental problem,which needs to be solved urgently.As the most important part of the terrestrial ecosystem,vegetation plays an important role in wind and sand fixation,soil and water conservation,and climate regulation.Therefore,it is essential to unveil the coupling relationship between climate change and vegetation.On the one hand,it will help us understand the response of vegetation distribution and productivity to climate change in the past;on the other hand,it is the basis for our prediction and management of ecosystems under future climate change.However,most previous studies either focused on the impact of climate change in a certain period or only looked into a single factor.It limits our understanding of the terrestrial vegetation's response to past short-and long-term climate changes.Therefore,we have selected three study periods:the LGM,the MH and the modern climate.We used a variety of data and methods such as meteorological station observations,remote sensing inversion of surface parameters,paleo-data and land surface models to explore the effects of various factors?e.g.CO2,climate,fire and soil properties?on the distribution and productivity of terrestrial vegetation.The research includes two main parts:the simulation and reconstruction of vegetation in the LGM and MH with a dynamic global vegetation model?DGVM?,and the analysis of the impact of climate change on modern vegetation productivity in China.This paper has the following four conclusions:1.Global vegetation distributions during LGM and Pre-industrial?PI?periods were reconstructed,and the results revealed the impacts of difference CO2 levels,climate changes and fire on the global vegetation distribution.We simulated the distribution of vegetation under three CO2 levels for two climate states,the LGM and PI climate with fire activated or deactivated using the ORCHIDEE-MICT DGVM.The results show that CO2 change has a larger effect than climate on fractional shifts of tropical and temperate trees and C4 grass in low latitudes,while climate dominates the cover changes of boreal trees and C3 grass in high latitudes.The larger gross primary productivity?GPP?enhancements for tropical trees compared to that for temperate and boreal trees explain their different sensitivities of vegetation cover to CO2 change.The CO2 change alters the vegetation cover and mean annual precipitation?MAP?relationship but shows less impacts on the tipping MAP?Pm?for saturated GPP of each tree type.The remarkable Pm decreases for tropical trees under elevated CO2imply that a higher tree cover is expected in semi-humid and semi-arid regions regardless of human activities and other limitations.Furthermore,this enhancement of tree cover could be amplified because fires would be suppressed by excluding grass and fuel load.2.Vegetation distribution in the mid-Holocene North Africa was reconstructed.We estimated the rainfall and carbon pool,and explored the soil feedbacks on vegetation coverage.With a better characterization of soil process in the land surface model ORCHIDEE-MICT,we investigated the amount of precipitation needed to sustain the vegetation coverage in the mid-Holocene Sahara.We added each process to the model step by step in order to separate the impact of each soil factor on vegetation dynamics and the terrestrial water cycle.These processes include the increased water holding capacity of soil due to the soil organic matters,the increased soil water infiltration because of the presence of plant root and reduced bare soil evaporation resulted from the litter cover.Compound effect of these factors sustains the Green Sahara?GS?under a lower precipitation threshold??400 mm/yr?than previous model estimates,highlighting the overall positive soil impacts on vegetation.Our result further reveals that the desertification of the Sahara from the mid-Holocene to pre-industrial period produced a large carbon source of?58 Pg C due to the removal of carbon in vegetation,soil and litter pools of the GS.The released carbon to atmosphere potentially has an impact on climate.3.The frequency of climate extremes in Hubei Province showed an increasing trend,and climate extremes had an impact on the interannual changes in vegetation productivity.This study comprehensively analyzed the spatio-temporal variabilities of 21temperature and precipitation indices during 1961-2015 across Hubei Province in central China based on daily temperature and precipitation data from 52 meteorological stations.To quantify the sensitivity of the vegetation to climate indices in the study area,we correlated climate indices with three vegetation indicators:leaf area index?LAI?,normalized difference vegetation index?NDVI?and GPP.The results indicate that warm-related indices exert considerable increasing trends.For instance,summer days has increased 20 days during the past five decades.Spatially,extreme precipitation has increased in the eastern regions of the study area and decreased in the western regions.In addition,the trends of 18 indices during 1982-2015 are larger than those during 1961-2015,indicating accelerated climate changes in Hubei Province.Correlation analyses reveal that warm anomalies of the Atlantic Multidecadal Oscillation resulted in extreme warm conditions and extreme precipitation in the study area.Stepwise linear regression analyses identify three temperature indices and three precipitation indices,which are most relevant with the three ecosystem variables at the site scale.Further multiple regressions demonstrate the main negative factors of frost days,warm spell duration,extremely heavy precipitation and consecutive dry days on the terrestrial ecosystem in Hubei Province.4.An innovative three-dimensional data analysis method was applied to study the extreme events in GPP and the climatic drivers in China by using 13 sets of GPP data.Results show that vegetation in Northeast China and North China was prominently affected by extreme drought events while the vegetation in South China was more vulnerable to cold spell and heat wave.Our study presents the first attempt to analyze spatio-temporally contiguous GPP extreme events at the national scale and sub-regions in China.Using 13 sets of GPP data,we explored the spatiotemporal characteristics and driving factors of the negative event of vegetation productivity in China from 1982 to 2015.The results show that vegetation in Northeast and North China is most vulnerable to extreme events,especially in mountainous areas.Over the past three decades,45%and 68%of GPP deficits in China occurred in summer in TRENDY models and Yao-GPP,respectively.Low precipitation is associated with most extreme events among the nine studied climatic factors in China in TRENDY models.Both power law distribution analysis and sensitivity analysis highlight the impact of drought on large GPP negative anomalies.Vegetation in southern China is more vulnerable to temperature extremes?i.e.cold spell and heat wave?than in northern China,which has been less clarified before.In summary,this paper reconstructed the global vegetation distribution during the LGM and the vegetation distribution in North Africa during the MH with a dynamic vegetation model.Moreover,we separated the impacts of different factors on terrestrial vegetation at different time scales.In addition,this work emphasized the potential feedbacks of surface processes on climate change.The results have some implications for the present and the future.We believe that tree coverage in semi-humid and semi-arid areas is expected to increase with elevated CO2 concentration in absence of human activity and other limitations.The coupling effect of CO2 and climate change and fire may cause significant changes in future tree coverage in arid tropics.Extreme climate in Hubei Province has shown an increasing trend for the past five decades,which has affected the interannual variability of vegetation productivity.During the past three decades,the vegetation in Northeast China and North China has been greatly affected by extreme events.The extreme events in northern and southern China are sensitive to drought and extreme temperature,respectively.Therefore,policymakers could use typical management strategy for different region to mitigate the negative impact of climate changes.This study is of great significance to reveal the close relationship between climate change,vegetation and land surface processes.It has implication to understand the impact of past climate change and to cope with future climate change.
Keywords/Search Tags:Late Quaternary, Global change, Dynamic vegetation model, Climate extremes, Terrestrial carbon cycle
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