| Urban forests play an essential ecological function in dust retention.However,the interspecies differences in paticulate matter(PM)quantity,composition,and the potential improvement of urban forest dust retention were not yet well defined in the northern high latitude area.In this study,we conducted a field survey on urban forest resources in Harbin.After mastering the basic information of Harbin urban forest,such as tree species composition,population allocation structure,dominant tree species,etc,PM from the leaf surface and wax layer of common greening tree species in Harbin,respectively in the heating period and nonheating period,were used for finding differences in PM adsorption and its compositional traits by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectrum(IR),inductively coupled plasma emission spectroscopy(ICP),the scanning electron microscope(SEM)and other technical means.At the same time,the relationship between PM quantity,composition,and leaf morphological characteristics was decoupled.Furthermore,the relationship model between tree and leaf morphological characteristics and dust retention function was established,and the management scenarios of different forest species were set up to simulate and analyze the potential improvement on dust retention capability of the urban forest by adjusting the tree configuration.The results showed as follows:1)There were 98 species of woody plants in urban forests,belonging to 54 genera and 26 families.Rosaceae,Salicaceae,and Pinaceae were the dominant families,including 18 genera and 41 species,accounting for 33.3% of the whole genus and 41.8% of the total species,respectively.Populus,Ulmus,and Acer were the dominant genera,including 19 species,accounting for 19.3% of the total species.There were 32 genera with only one species,accounting for 32.7% of the total.So,the composition and structure of tree species in the Harbin urban forest were simple,and primarily concentrated in a few families or genera.According to the investigation,There were 51 common trees in Harbin’s urban forest,of which 25 species were arbor trees,accounting for 29.43% of the total amount;14 species were small trees,accounting for 9.15% of the total amount;12 species were shrubs,accounting for 61.41% of the total amount.Grouped by life forms,there were 41 deciduous trees,accounting for 95.6% of the total amount;10 species of evergreen trees,accounting for 4.4% of the total.In general,poplar(Populus spp.)and willow(Salix spp.)were the dominant species,and the configuration of deciduous trees in Harbin urban forest was prevalent,and evergreen trees were relatively low.So,there was much space for adjustment and optimization to improve urban forest PM retention capability.2)During the non-heating period(May-early October),the coniferous and broad-leaved tree species jointly play the role of dust retention.The amount of PM on the coniferous tree species leaf was 2.7-3.4 times that of broad-leaved trees.Of them,the content of C and Fe elements,the functional group of C-O&O-H and Si-O-Si stretching,and Pb,Cu,Ni,and Cd heavy metals of the coniferous tree species was 2.7-3.4 times,1.1-1.2 times,1.2-2.1 times,and 3.2-4.1 times that of broad-leaved trees,respectively;however,the adsorption of N,O,Si,K,and Ca mineral elements and functional groups of O-H&N-H stretching,C-H stretching,COO-stretching&C-H bending,and C-H bending of broad-leaved trees was 1.3-5.9 times and 1.1-3.6 times that of conifers,respectively.Juniperus rigida dust retention and heavy metal adsorption were significantly the highest(p<0.05);Ulmus pumila(O,Si,Ca),Acer negundo(O,N),pyrus ussuriensis(N,Fe),Gleditsia japonica(Ca,Fe),Fraxinus mandschurica(Na,K),etc were the strongest adsorption capability for specific components of particulate matter.The difference in dust retention between the leaf surface layer and leaf wax layer was as follows: the dust retention amount was slightly higher than that of wax layer.Of them,the relative content of crystalline minerals,the adsorption amount of heavy metals,and the adsorption amounts of functional groups of C-O&O-H and Si-O-Si stretching and C-H bending of leaf surface layer were about1.5-3.1 times,0.7-2.0 times,1.3-1.5 times that of the wax layer,respectively.While the content of functional groups of C-H stretching,C=O stretching,COO-stretching&C-H bending on wax layer were 1.5-2.4 times that of the leaf surface layer.Correlation between leaf morphology and dust retention and dust retention components: large and thick leaves,with dense and small stomata,and less leaf wax,were beneficial to the adsorption of N,O,and K on the surface;small and thick leaves,with more leaf wax and large pores,were beneficial to the increase of dust retention,C element,and heavy metal adsorption,but decreasing the adsorption of functional groups of C-H stretching,COO-stretching&C-H bending,and C-H bending.3)During the heating period(mid-October to late April of the following year),evergreen trees play a major role in retaining dust.PM quantity and heavy metal(Pb,Cu,Ni,Cr,Cd)content of Juniperus rigida were 2.0-2.5 times,1.7-2.2 times that of Pinus tabuliformis var.mukdeais and Picea koraiensis.Of them,23.1%-38.9% were in the wax layer.There was a difference in the dust retention capability of evergreen conifer species in the heating season and non-heating season,as well as the contribution rate of dust retention of wax layer: The total dust retention(t PM)of spruce,black pine,and juniper in the heating period was 2.6-5.2 times of that in the non-heating period,and heavy metal content was 1.2-12.6 times of that in the non-heating period.The leaf wax layer played an important role in dust retention in both heating and nonheating periods,especially in non-heating period,the proportion of dust retention in the wax layer was as high as 53.6%-62.4%.The leaf dust retention ability of tree species was affected by multiple leaf morphological factors.The coupling relationship between leaf morphological characteristics and dust retention ability of tree species in heating and non-heating periods was basically consistent.Leaf morphology(LWC,PAOL,WC,etc)had a better explanatory effect on leaf PM adsorption capability.4)Based on the above urban forest survey data and the total and composition data of dust retention of tree species,we established the mathematical relationship between tree morphological characteristics and dust retention ability index,and formed a simulation analysis method of dust retention potential of urban forest based on tree species configuration adjustment.The result showed that adding conifers and reducing broadleaved trees can increase the dust retention capacity(t PM,s PM,w PM)by 61%-82.2%,the heavy metal adsorption capacity by41.6% in non-heating period.Furthe increasing Juniperus and decreasing Picea and Pinus could improve PM removal by 21.3%-78.4% and remove heavy metals by 1.8 times.Structural equation model analysis showed that tree size-related morphological characteristics and leaf dust retention ability were equally crucial for improving the dust retention ability of urban forests.In the future,the greening of vertical space and protection of large trees should be paid more attention.To sum up,the dust retention ability of Harbin urban forest was not only at the leaf surface but also deep into the wax layer.The PM quantity differences among species were significant,and PM components(infrared functional groups and heavy metals)differences should also be paid attention to more.A species-regulation-scenario stimulation showed that increasing evergreen conifer species could effectively enhance the dust retention ability of urban forests.Local evergreen species such as Juniperus rigida,Picea koraiensis,Pinus sylvestris,Picea meyeri etc.,were ideal tree species choices to improve the dust retention potential of urban forests.The results provide new ideas and data support for dust retention and promotion based on urban forest management in northern high-latitude areas. |
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