| Environmental magnetism method has received extensive attention of researchers since its establishment. A number of studies have shown that there is a significantly positive correlation between magnetic parameters and heavy metal content. Magnetic parameters such as x, SIRM can be used as a substitute of heavy metal pollution index. Therefore magnetism method is gradually applied in monitoring urban environment pollution. The high tree species diversity, extensive distribution and long-term buildup (≥10a) contributed to the selection of Shanghai Green Belt as study objects. The magnetic characteristics of tree rings could reflect the changes of environment pollution in space and time scales because of long-term effect by urban industries, road traffics and other pollution. In this study,10 common tree species were selected around Shanghai Green Belt and magnetism tests were taken on their ring core samples. Single factor analysis was used to compare the difference of SIRM in tree rings between years, tree species and pollution levels. The main conclusions were as follows:(1) The magnetic characteristics of tree rings showed significant difference in time scale. Overall in the year 2004 to 2013, the SIRM in tree rings increased at first and decreased subsequently, and increased again. A maximum was observed before and after the year 2006, and then the value fell until the year of 2009 before a gradually increase. SIRM in tree rings showed the fastest increase between the year 2011 to 2013.(2) The difference of tree species could significantly influence the magnetic characteristics of tree rings (P<0.05). SIRM in tree rings changed along with the variation of tree species. Based on the comparison of mean value of SIRM during 2009 to 2013 and 2004 to 2013, tree rings can be divided into three categories. Populus spp., Sophora japonica, Elaeocarpus glabripetalus and Koelreuteria bipinnata var.integrifoliola have strong magnetic strength or ability of magnetic particles absorption. Cinnamomum camphora, Taxodium distichum var. imbricarium, Metasequoia glyptostroboides and Zelkova schneideriana are medium. Magnolia grandiflora and Sapindus mukorossi are weak.(3) The influence of pollution levels on tree ring magnetic characteristics was extremely obvious(P<0.05). In different pollution areas, rings from the same tree species showed significant differences on magnetic characteristics. Analysis and comparison of multiple tree species results indicated that SIRM in tree rings of industrial area was higher than that of single traffic pollution area, and pollution emissions less affected area. Also, the distance to industrial pollution source was an important factor to SIRM in tree rings. The further the distance, the smaller the SIRM.To sum up, the magnetic of tree rings (SIRM) showed significant difference in time scale. The difference of tree species and pollution levels could extremely affect the magnetic characteristics of tree rings. Tree rings ofPopulus spp., Sophora japonica, Elaeocarpus glabripetalus and Koelreuteria bipinnata var.integrifoliola have strong magnetic strength or ability of magnetic particles absorption. Therefore the magnetic characteristics of tree rings (SIRM) can be used to indicate the change and inversion of environment pollution in time and space scales. |
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