Mechanisms Of Particulate Matters Capture Capacity And Response Of Leaf Functional Traits For Urban Tree Species

Air particulate matter pollution has become an increasingly serious problem with the rapid development of industrialization and urbanization.Such pollution has always been a matter of great concern because of the adverse effect of particulate matter on humans and plants,and does great harm to the environment as well.Despite the remarkable achievements in air pollution control and the obvious improvements in air quality in recent years,however,due to the continuous accumulation and transformation of pollutants in the short term,the regional air pollution control task is still very serious.Forests can effectively improve air quality by reducing atmospheric particulate matters.The purification of forests is recognized as an effective ecological management approach.Too many particles deposited on the leaves can affect the normal growth of trees.Therefore,the ability to remove particulate matter and pollution tolerance are both necessary factors to be considered in the selection and optimization of urban tree species.The functional traits of leaves not only affect the growth of trees,but also directly reflect the changes of the environments.In the present study,the monitoring sites of different pollution intensity were selected.Based on leave functional traits,the methods of the simulation of aerosol reoccurrence,the observation of scanning electron microscope and atomic force microscope,and the analysis of physiological indexes of leaves were applied.The dynamic of the particulate matter removing effects of forests was analyzed under the influence of weather condition,pollution intensity,growing season and dormant season.The influence of growth and pollution-driven changes in leaf micromorphology on the particulate matter capture of leaves was clarified.The sensitivity and ecological adaptation of leaf functional traits under air pollution stress was analyzed.The main results were as follows:（1）The particulate matter removing effects of forest covered areas in sunny,cloudy and rainy days were stronger than that in extrem dusty days.In sunny,cloudy and rainy days,the concentration of particulate matter in forest covered areas which kept lower than that in non-forest covered areas were for longer time than that in extrem dusty days,as well as their reduction efficiency were higher.With the increase of pollution intensity,the particulate matter removing in forest covered areas became weaker.In the process of light pollution,the duration of maintaining low particulate concentration in the forest covered areas were longer and the reduction efficiency were higher than during heavy pollution.The particulate matter removing effects in the growing season was stronger than that in the dormant season.In the growing season,the duration of lower particulate concentration in the forest covered areas were longer than that in the dormant season,as well as the reduction efficiency was lower.In the process of dust storm,the combination of strong wind and abundant exogenous particulate matters during the outbreak period leads to very low particulate matter removing effects of forests.At the end of the dust storm,the decrease of exogenous particulate matters and the enhancement of turbulence in the forests promoted the diffusion,and the concentration of particulate matter in the forest covered area was lower than that in the non-forest covered area.（2）By comparing the particulate matter removing effects of forest covered area,the following results can be obtained:1)forests were more effective in reducing fine particulate matter such as PM₁₀.Except for the process of sunny and extrme dusty days,the reduction efficiency of PM₁₀ was higher than that of PM_2.5.2)the PM_2.5reduction efficiency of forest covered area was highest on sunny days,followed by light pollution periods,cloudy days,rainy days,dust storm,heavy pollution periods during dormant season,and heavy pollution periods during growing season.The PM₁₀reduction efficiency of forest covered area was highest in light pollution periods,followed by cloudy days,rainy days,heavy pollution periods during growing season,sunny days,heavy pollution periods during dormant season,and dust storm.Compared with other processes,the particulate matter removing effects of forest covered area in dust storm was closely related to exogenous particulate matters and wind speed.（3）The ability of old leaves to capture TSP,PM₁₀,PM_2.5,and PM₁ for the three evergreen trees（Platycladus orientalis,Pinus tabuliformis,and Taxus cuspidata var.）was greater than that for new leaves.Moreover,there were extremely significant differences between the capture capacity of different particle sizes for new leaves and old leaves（p<0.001）.TSP and PM₁₀ captured by new leaves of P.orientalis was the most.PM_2.5 and PM₁ captured by new leaves of T.cuspidata var.was the most.The particulate matter captured by Pinus tabuliformis was the least.The amount of TSP and PM₁₀ captured by five tree species（P.orientalis,P.tabuliformis,Sophora japonica,Populus tomentosa and Ginkgo biloba）was higher in Chaoyang park than in Beijing botanical garden.The amount of PM_2.5 and PM₁ captured by diffenent tree species were inconsistent.Pollution intensity extremely significantly affected the capture capacity of TSP,PM₁₀,PM_2.5 and PM₁ on leaves（p<0.05）.（4）The change of leaf surface micromorphology caused by leaf age and pollution intensity changed their roughness and consequently affected their particle capture ability.Due to the increase in wettability of the damaged leaf wax layer,the amount of particulate captured on the adaxial surface of the old leaves increased significantly.Compared with the Beijing botanical garden,the changes of surface micromorphology such as stomata,waxy layer,trichomes and surface texture of the leaves in the Chaoyang park increased the roughness Rq,and the adaxial surface increased the most obviously,therefore,the ability of particulate matter captured by leaves were significantly increased.In the dust storm,the particles captured on leaves increased significantly.In particular,the particulate matter capture ability for deciduous trees with newly spreading leaves was more significant in spring.In the dust storm,the leaves surface with more grooves where there were more particles captured.（5）With the increase of pollution intensity,stomatal density of leaves increased,stomatal index decreased,guard cell length did not change much,trichomes became harder and longer,and leaf morphological characteristics had adaptive changes.The leaves surface roughness increased as the pollution intensification.Under the stress of air pollution,the physiological traits of leaves of six tree species（P.orientalis,P.tabuliformis,Cedrus deodara,S.japonica,P.tomentosa and G.biloba）were affected by different degrees.The chlorophyll in leaves were degraded,the chlorophyll a and b contents were decreased.The cell membrane structure of leaves were destroyed and the cell membrane permeability increased.Carbohydrate synthesis in leaves increased,and the content of soluble sugar,polysaccharide,glucose,fructose were all accumulated in the leaves.The ascorbic acid content of leaves were increased.Leaves adapted to and mitigated the damage of pollutants to cells by regulating physiological and biochemical levels,and enhanced their resistance to stress to ensure the normal metabolic activities of the cell.Based on comprehensive multi-physiological indexes analysis,the air pollution tolerance ability from strong to weak were S.japonica,G.biloba,P.tabuliformis,P.tomentosa,C.deodara,P.orientalis.