Spatiotemporal Characteristics Of Negative Air Ions In Typical Forests Ecosystem In Southeast Tibet

In this study,the typical coniferous forest,broad-leaved forest,and artificial mixed forest in southeastern Tibet were taken as the research objects,and the urban unforested land was used as a control.Single factor variance and multiple comparative analyses were used to analyze characteristics and differences of the spatiotemporal dynamics of negative and positive air ions and the microclimate were analyzed,and the air quality was evaluated by using the Ampere Air Ion Evaluation Coefficient and the Forest Air Ion Evaluation Index.Finally,relying on data of National Forest Ecosystem Observation&Research Station of Nyingchi Tibet,path analysis and correlation were performed.The influencing factors of air negative ions were analyzed and researched in order to provide theoretical support for the forest ecological benefit assessment in the area,provide data support for the establishment of forest oxygen bar,and provide a basis for the planning and development of ecotourism in Tibet.Main results were as follows:（1）The negative air ion concentration fluctuated greatly during the day in each season,and they all showed a"single peak"type.It was relatively stable from midnight to early morning.The negative air ion concentration during the day was higher than at night.The peak appeared around 15:00 and the valley appeared around 9:00.Except for the Quercus aquifolioides forest,the Abies georgei var.smithii forest,the artificial mixed forest and the urban area showed a"single peak"type,with peak values from June to August.The Q.aquifolioides forest was"U"shaped,and the valley value appeared in July.The negative air ion concentration of A.georgei var.smithii forest,artificial mixed forest and the urban area showed that in the spring,summer,autumn and winter seasons,summer and autumn seasons>winter and spring seasons,while the Q.aquifolioides forest showed higher autumn and winter seasons than spring and summer seasons.In the comparison of the dry and wet seasons,except that the negative air ion concentration in the Q.aquifolioides forest was dry season>wet season,the others showed that the rainy season was higher than the dry season.In spring,autumn,and winter,the negative air ion concentration in typical stands and control points was shown as the Q.aquifolioides forest>the A.georgei var.smithii forest>the artificial mixed forest>the urban area,and in summer,it was expressed as the A.georgei var.smithii forest>the Q.aquifolioides forest,artificial mixed forest>the urban area.In each season,the forest area was higher than that in the no-forest area.The daily change of the positive air ion concentration was similar to that of the negative air ion,and the peak value still appeared around 15:00.The daily fluctuation range was smaller than that of the negative air ion,but the fluctuation frequency was higher than the negative air ion.The monthly dynamic and seasonal regularities of the positive air ion in various typical forest stands were not completely consistent,and fluctuate in each month of the year,which were higher in spring and summer.The positive air ion concentration of various typical forests was generally higher in of the A.georgei var.smithii forest,the artificial mixed forest than that the Q.aquifolioides forest and urban area.Simultaneously,the positive air ion concentration also showed that the forested area was higher than the no-forest area.（2）According to the temperature and humidity statistics of typical forests and the control point,it was found that the air temperature and relative humidity of the three typical forests showed day and night variations of"cosine"and"sine",respectively."And inverted"U".The annual temperature and humidity changes of the three typical stands and control points were all“single peak”,with the peak time around July.The temperature and humidity of each typical stand and control point in different seasons are as follows:summer>autumn>spring>winter.The temperature of the three typical stands was significantly lower than the control point,and the relative humidity was significantly higher than the control point.The forest played a significant role in cooling and humidifying.（3）Ampere air ion evaluation coefficients of the three typical forests in each month of the year were all above 1.00,and the air quality had reached the cleanest level,the air quality was better every month of the year.However,the air quality in all seasons was better in autumn and winter than in spring and summer.The control point reached the cleanest level only in May and August.The ampere air ion evaluation coefficient from May to October was above 0.90,which belonged to the clean range.The dry season air quality was the worst.In general,the air quality of typical stands and control points in different seasons in the spring,summer,and autumn seasons in different seasons was:the A.georgei var.smithii forest>the artificial mixed forest>the Q.aquifolioides forest>the urban area,that is,the coniferous forest>the mixed conifer and the broadleaf forest>the broad-leaved forest>No woodland.In winter,it appeared as the coniferous forest>the broad-leaved forest>the coniferous-broad mixed forest>the no-forest area.The frequency of air ion coefficients of three typical forests>0.50gradually decreased throughout the year,but their forest air ion evaluation index were all>2.40,and their air quality reached GradeⅠ.The quality of forest air ions throughout the year was similar to that of ampere air ions,and was also shown to be higher in autumn and winter than in spring and summer.The concentration of the negative air ion in urban areas was significantly lower than that of three typical forest stands.From November to April of the following year,the concentration of the negative air ion was less than 1 000 ion·cm^-3,and in other months were also significantly lower than those of three typical forest stands.The highest value was 1250 ion·cm^-3.The forest air ion evaluation index was all less than 2.40,all of which were below Grade I.Generally,February to August belong to Grade II air quality,and other months were grade III air quality.In general,the air quality of three typical stands and the control point in each season of spring,summer,autumn,and winter in different seasons was as follows:the A.georgei var.smithii forest>the Q.aquifolioides forest>artificial mixed forest>the urban area,that is,coniferous forest>broad Leaf forest>coniferous and broad-leaved mixed forest>the no-forest area.（4）According to the path analysis,air temperature and ultraviolet radiation had a greater impact on the negative air concentration.Other factors exert their effects mainly through air temperature and ultraviolet radiation,while relative humidity was its main limiting factor.Meteorological factors had a large residual path coefficient for the negative air ion（0.694）.There was a very significant positive correlation among the particulate matter（P<0.01）,and a significant negative correlation between the particulate matter and the negative air ion concentration（P<0.01）.