全球主要森林系统中植被汞分布特征及其影响因素

基于过去20a全球森林植被中汞循环过程的相关研究,本文通过荟萃分析法阐明全球主要森林类型中植被汞分布特征及其影响因素.结果表明,汞在植被中浓度排序特征(以中值计)依次为凋落物(38.9ng/g )>叶片(24.1ng/g)>树根(18.5ng/g)>树皮(13.2ng/g)>树枝(12.0ng/g)>树干(3.1ng/g).其中,凋落物与叶片汞浓度具有极显著相关性(rho=0.747,P<0.01,rho spearman相关系数,且与样本数量N有关,下同);叶片与树干具有显著相关性 (rho=0.265,P<0.05);而其余组织间无相关性(P>0.05).上述的相关性主要受控于植被叶片吸收大气零价汞,从叶片由上而下传输到树木木质部的过程.其次,常绿阔叶林的叶片汞浓度(中值:56.0ng/g,下同)高于针叶林(25.5ng/g)、阔叶落叶林(23.0ng/g)和红树林(18.6ng/g).这是由于在气孔导度、气孔数量、叶面积与叶寿命等因素综合影响下,常绿阔叶林叶片汞富集的能力最强.地区间植被汞浓度差异明显(如叶片汞浓度变化区间:0.90~225ng/g),其最主要原因是地区间植被类型与大气汞浓度差异引起的.本研究为进一步量化全球森林植被汞储库与认识全球森林系统汞循环提供了重要的基础数据与基本认识.

Distribution and influencing factors of vegetation mercury in major forest systems across the globe

Based on the studies on the global mercury cycling processes in forests over the last 20-year, this study aims to elucidate the vegetation mercury distribution and its influencing factors in major forest types globally through meta-analysis. Results showed that the descending order of mercury concentrations in vegetation (in terms of median value) was litterfall (38.9ng/g) > foliage (24.1ng/g) > root (18.5ng/g) > bark (13.2ng/g) > branch (12.0ng/g) > stem (3.1ng/g). Litterfall was highly significantly correlated with foliage Hg concentration (rho=0.747, p<0.01, rho: Spearman's correlation coefficient, depending on number of samples, hereafter), and a significant correlation (rho=0.265, P<0.05) between leaves and trunks, but no correlations among the rest of tissues (p>0.05). The above correlations are mainly controlled by the uptake of atmospheric elemental mercury by foliage and the upward transport from foliage to the xylem of trees. In addition, vegetation Hg concentrations in broadleaf evergreen forest leaves (median: 56.0ng/g, below) were higher than those in coniferous (25.5ng/g), broadleaf deciduous (23.0ng/g), and mangrove (18.6ng/g) forests. This is because the combined effects of stomatal conductance, stomatal number, leaf area and leaf longevity lead to the evergreen broadleaf forests with the greatest capacity for mercury enrichment. this study showed a significant inter-regional variation in vegetation Hg concentrations (e.g., up to range of 0.90~225ng/g). This can be attributed to the inter-regional differences in vegetation type and atmospheric mercury concentrations. This study provides important data and expanded our understanding to further quantify the global forest vegetation Hg pool and sink.