上海市公园绿地常见落叶树木叶片汞浓度时空特征及环境意义

植物叶片汞浓度与大气气态单质汞(GEM/Hg0)浓度的线性关系表明叶片汞浓度大小可用于指示植物生长区内GEM浓度的高低水平.通过分析上海市绿地公园(25座)中常见落叶树木樱花、水杉、法桐叶片汞浓度的时空变化特征,探究区域内GEM含量水平及分布特征.2017年5—10月对7座公园中这3种树木叶汞浓度进行连续监测,结果显示叶汞浓度与叶片生长时间呈显著线性正相关关系(P<0.01),表明叶片在生长期内不断吸收累积大气汞.而且在生长期内,3种树木叶汞浓度日累积速率(g·kg^-1·d^-1)具有相似的变化趋势,意味着不同树木叶汞的累积对外界环境的响应可能是一致的,除树种差异外.同年11月初,25座公园(包含上述7座)中樱花、水杉、法桐衰老叶片叶汞浓度为(54.2±12,31.8—76.7)μg·kg^-1、(42.0±9,23.5—67.9)μg·kg^-1、(36.1±11,21.4—60.3)μg·kg^-1(平均值,范围),有显著的种间差异(P<0.01),而在中心城区和郊区间无显著差异(P>0.05).空间插值分析结果初步表明衰老叶片叶汞浓度的空间梯度差异不大,且高值区域没有完全重合.这表明了利用衰老叶片叶汞浓度反映区域GEM浓度整体水平空间分布规律存在一定的不确定性,仍需进一步深入研究.

Temporal and spatial characteristics of mercury concentrations in leaves of common deciduous tree species in parks of Shanghai and its environmental implications

The linear relationships between the concentrations of gaseous element mercury(GEM or Hg0) and Hg concentrations in leaves suggests that leaves can be used as passive monitors of atmospheric Hg.In this study, total leaf Hg concentrations in three common deciduous tree species from 25 parks, including prunus serrulata(Cerasus sp.), metasequoia(Metasequoia glyptostroboides) and plane tree(Oriental plane), were investigated to study the uptake dynamics of GEM in leaves throughout the growing season, and improve an understanding of the spatial distribution of GEM in ambient air in Shanghai city area.Results showed that total leaf Hg concentrations in three trees were increased from May to October 2017(from emergence to senescence) and showed a strong positive correlation with leaf age(P<0.01), indicating that leaves accumulate Hg throughout the growing season.Although Hg concentrations in the senescence of leaves among three deciduous species showed a significant interspecies difference(P<0.01), the daily Hg accumulation rates(μg·kg^-1·d^-1) presented the similar seasonal variation during the growing season, suggesting that the uptake dynamics of Hg in leaves could be mostly related to environmental conditions, except for the interspecies difference.The senescing leaf Hg concentrations of prunus serrulata, metasequoia and plane tree were( 54.2 ±12, 31.8—76.7) μg·kg^-1,(42.0±9, 23.5—67.9)μg·kg^-1 and(36.1±11, 21.4—60.3) μg·kg^-1(average, range), in 25 parks of Shanghai, respectively.Furthermore, the senescing leaf Hg concentrations showed no significant differences between the downtown and the suburbs(P>0.05).The predicted results of Empirical Bayesian Kriging modeling for senescing leaf Hg concentrations in three deciduous species showed a relatively small gradient difference pattern.However, a different pattern was observed for spatial distribution of leaf Hg concentrations.These results indicated that there could be some uncertainty in using senescing leaf Hg concentrations to reflect the spatial distribution of GEM concentrations in the regional scale levels, which needs a further research.