Important Accumulated Mercury Pool in a Remote Alpine Forest and Dynamic Accumulation Revealed by Tree Rings in China's Qilian Mountains

Abstract

Quantification of alpine forest mercury (Hg) pools is important for understanding the Hg assimilation, flux and even biogeochemical cycle in forests. We sampled soil, moss and different tissues of the dominant Qinghai spruce (Picea crassifolia Kom.) to investigate Hg concentration and pool and assess Hg accumulation dynamics in the Qilian Mountains, northwestern China. Mean Hg concentrations increase in the following order: trunk wood < branch < root < needle < bark < soil < litterfall < moss. Soil contained two orders of magnitude higher Hg pool than vegetation, and accounted 92.2% total Hg pool in alpine forest ecosystem. Moss contained a disproportionate 16.7% of the Hg pool although making up only 2.7% of total vegetation biomass. Aboveground tree tissues showed higher Hg pools in alpine forest than those in broad-leaved, coniferous and mixed forests in China and America. Dynamic accumulation indicated rise atmospheric Hg concentration and enhanced tree biomass contributed Hg assimilation in remote alpine forest, especially after 1960s. Our results underscore the relatively high levels of Hg in the aboveground tree tissues of alpine forests and reveal a notable increase in Hg accumulation, which is attributed to rising background Hg levels. We suggest that when assessing Hg dynamics in forest ecosystems, it is imperative to consider both the variability in atmospheric Hg exposure levels and the capacity for Hg accumulation in vegetation.

Keywords: mercury concentration, mercury pool, dynamic accumulation, tree ring, alpine forest