Needle age and precipitation as drivers of Hg accumulation and deposition in coniferous forests from a southwestern European Atlantic region

Vegetation and climate are critical in the biogeochemical cycle of Hg in forest ecosystems. The study assesses the influence of needle age and precipitation on the accumulation of Hg in needle biomass and its deposition by litterfall in thirty-one pine plantations spread throughout two biogeographical regions in SW Europe. Well-developed branches of Pinus pinaster were sampled and pine needles were classified according to 4 age classes (y(0), y(1), y(2), y(3)). The concentration of total Hg (THg) was analyzed in the samples and Hg content in needle biomass and its deposition by litterfall were estimated. The concentration of total Hg (THg) increased with needle age ranging from 9.1 to 32.7 mu g Hg kg(-1) in the youngest and oldest needles, respectively. The rate of Hg uptake (Hg-R) three years after needle sprouting was 10.2 +/- 2.3 mu g Hg kg(-1) yr(-1) , but it decreased with needle age probably due to a diminution in photosynthetic activity as needles get older. The average total Hg stored in needle biomass (Hg-Wt) ranged from 5.6 to 87.8 mg Hg ha(-1) , with intermediate needle age classes (y(1) and y(2)) accounting for 70% of the total Hg stored in the whole needle biomass. The average deposition flux of Hg through needle litterfall (Hg-Lt) was 1.5 mu g Hg m(- 2) yr(-1), with the y(2) and y(3) needles contributing most to the total Hg flux. The spatial variation of THg, Hg-Wt and Hg-Lt decreased from coastal pine stands, characterized by an oceanic climate, to inland pine stands, a feature closely related to the dominant precipitation regime in the study area. Climatic conditions and needle age are the main factors affecting Hg accumulation in tree foliage, and should be considered for an accurate assessment of forest Hg pools at a regional scale and their potential con-sequences in the functioning of terrestrial ecosystems.

Automated summary

This study assessed the impact of needle age and precipitation on the accumulation and deposition of mercury (Hg) in forest ecosystems. The results showed that Hg concentration increased with needle age, and precipitation and needle age were identified as the main factors influencing Hg accumulation. Understanding the importance of these factors is crucial for accurately assessing regional-scale forest Hg pools and their potential consequences for ecosystem functioning.