| Since the industrial revolution,with the continuous growth of economy and population,the concentration of greenhouse gases in the atmosphere has gradually increased,causing the greenhouse effect and leading to global warming.The impact of climate change on ecosystems and human society has gradually intensified,which received more and more concerns,and extensive attention from the international community.The species geographical distribution is an important spatial feature and the result of the long-term interaction between species and environment.On the macro scale,among many factors that affect the species geographical distribution,climate plays the most important role.The greenhouse effect caused by human activities increased global temperature continuously and further affected the pattern of precipitation.The frequency of extreme climate events also increased.These inevitably affected the species geographical distribution,changed species composition and richness distribution pattern,had an impact on biodiversity,and ultimately influenced the structure and function of ecosystems.Currently,species distribution models(SDMs)are mainly used to study the species geographical distribution and its response to climate change.Among large numbers of SDMs,the MaxEnt model based on the principle of maximum entropy only needs the presence data of species distribution when simulating and predicting,and good results can be obtained when there is only a small amount of data.It is considered to be one of the most reliable and commonly used models among many SDMs.Oak(Quercus L.)was selected as the research subject.There are 35 species of oak in China,which are distributed throughout the country.Oak is a major composition,and indeed one of the dominant species,of forests in China,and plays a key role in maintaining ecosystem stability and biodiversity.At present,a large number of studies on Oak have been carried out all over the world.But few studies on the changes in the richness distribution pattern of Quercus L.and population dynamics under climate change have been reported.Based on a large amount of accurate distribution data of 35 species and climate data,the potential richness distribution and spatial change of Quercus L.in China were simulated and predicted using the MaxEnt model under present climatic conditions and three different emission scenarios for the years 2050 and 2070.The main conclusions are as follows:1.Using the MaxEnt model and ArcGIS,based on the distribution data of 35 species and current climate data,the potential distribution of Quercus L.and its richness in China under current climate conditions were simulated,and the richness distribution pattern and characteristics were explored.According to the actual richness distribution of Quercus L.in China obtained from the species distribution data,Quercus L.is mainly distributed in the mountainous areas of southwestern China.Although it is widely distributed,the overall richness of Quercus L.exhibits a fragmented distribution pattern.The potential richness distribution pattern of Quercus L.in China simulated by the MaxEnt model is basically consistent with the actual richness distribution pattern.The region from Hengduan Mountains to Qinling Mountains and Daba Mountains is still the hot spot for the distribution of Quercus L.According to the AUC evaluation results,the accuracy of simulation results for all species reached a good or above level,which proved that the MaxEnt model could be used to simulate and predict the potential distribution of Quercus L.in China.2.Using the MaxEnt model and ArcGIS,the dominant variables affecting the distribution of Quercus L.and its richness in China were selected,and the threshold of dominant variables was analyzed.Then,the relationship between the richness distribution of Quercus L.and bioclimatic variables was discussed.According to the results,annual precipitation and temperature seasonality were the dominant variables affecting the distribution of Quercus L.and its richness in China,which showed that the combined effect of heat and water conditions played an important role in shaping the distribution of Quercus L.and its richness in China.Drought and temperature instability may threaten the distribution of Quercus L.,leading to changes and reductions in distribution ranges of some species,and eventually changing the richness distribution pattern of Quercus L.3.Using the MaxEnt model and ArcGIS,based on the distribution data of 35 species and future climate data,the potential distribution of Quercus L.and its richness in China were predicted under the influence of climate change.The richness distribution pattern and changes of Quercus L.in China were also explored.According to the results,climate change could affect the richness distribution pattern of Quercus L.,and this impact would gradually increase as climate change intensified.This was caused by changes and contractions of some species distribution ranges which were affected by climate change.And as the temperature continued to rise,these species would migrate to high altitudes or high latitudes.At the same time,it was found that mountains were the hot spots for the richness distribution of Quercus L.Under the influence of climate change,the area extending from the eastern Himalayas to the Qinling Mountains and Daba Mountains was still the core area of the distribution of Quercus L.,which proved that mountains could play a key role in maintaining species richness under climate change.4.Combining simulation and prediction results with the relationship between the richness distribution of Quercus L.and bioclimatic variables,the extinction risk of Quercus L.under climate change was assessed,and the key areas for the protection of Quercus L.were identified.The results showed that with the emission scenarios intensifying,the number of area-contracting species and the reduced area of their distribution ranges would both increase.However,in different future periods and emission scenarios,the distribution range of each species would contract within 30%,which meant that the distribution of Quercus L.in China would basically remain stable in the future,and there was no risk of extinction.The areas where Quercus L.was concentrated and its species richness could be maintained at a high level under the influence of climate change were identified as the key area for the protection of Quercus L.The area that was highly vulnerable to climate change(that was,high species lost rate and high species richness reduction),and the high-altitude area were also the key areas for the protection of Quercus L.in China.The effective protection measures under climate change were put forward for Quercus L.and its key areas.The results have the important practical significance for biodiversity conservation,and also have a certain enlightenment for the sustainable utilization and management of forests in China.The innovations of this thesis are:first,by analyzing the area changes,boundary changes and center migration of Quercus L.and its richness distribution in China,the dynamics of Quercus L.and its richness distribution over time under the influence of climate change were explored.This is a refinement and supplement to previous studies which only focused on distribution patterns.Secondly,multiple periods(2050 and 2070),multiple emission scenarios(RCP2.6,RCP4.5 and RCP8.5)and multiple Global Climate Models were considered comprehensively.Possible future potential climate changes were taken into account as much as possible,reducing the uncertainty.Thirdly,the extinction risk assessment of Quercus L.and the identification of key areas for the protection will help to effectively protect and rationally utilize Quercus L.in China on time and space scales.Based on the species distribution data and climate data,the potential richness distribution pattern of Quercus L.in China were simulated and predicted using the MaxEnt model,and the relationship between richness distribution of Quercus L.and bioclimatic variables were explored.The extinction risk of Quercus L.under climate change was assessed,and the key areas for the protection of Quercus L.were identified.This research focused on the population dynamic changes of vegetation and its response to global climate change.Simulating and predicting potential changes in the distributions of Quercus L.and its richness under climate change will help to better understand the biogeographic characteristics and spatial distribution patterns of Quercus L.,and provide theoretical guidance for adaptive management and sustainable utilization of Quercus L.under the background of global change.It is also of great significance for further exploration of biodiversity conservation. |
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