Physiological and climate controls on foliar mercury uptake by European tree species

[摘要] Despite the importance of vegetation uptake of atmospheric gaseous elementalmercury (Hg(0)) within the global Hg cycle, little knowledge exists on thephysiological, climatic, and geographic factors controlling stomatal uptakeof atmospheric Hg(0) by tree foliage. We investigate controls on foliarstomatal Hg(0) uptake by combining Hg measurements of 3569 foliage samplesacross Europe with data on tree species' traits and environmental conditions.To account for foliar Hg accumulation over time, we normalized foliar Hgconcentration over the foliar life period from the simulated start of thegrowing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake wastree functional group (deciduous versus coniferous trees). On average, wemeasured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduousleaves than in coniferous needles of the same age. Across tree species, forfoliage of beech and fir, and at two out of three forest plots with morethan 20 samples, we found a significant ( p <0.001 ) increase infoliar Hg values with respective leaf nitrogen concentrations. We thereforesuggest that foliar stomatal Hg uptake is controlled by tree functionaltraits with uptake rates increasing from low to high nutrient contentrepresenting low to high physiological activity. For pine and spruceneedles, we detected a significant linear decrease in daily foliar stomatalHg uptake with the proportion of time during which water vapor pressuredeficit (VPD) exceeded the species-specific threshold values of 1.2 and3 kPa, respectively. The proportion of time within the growing season duringwhich surface soil water content (ERA5-Land) in the region of forest plotswas low correlated negatively with foliar Hg uptake rates of beech and pine.These findings suggest that stomatal uptake of atmospheric Hg(0) isinhibited under high VPD conditions and/or low soil water content due to theregulation of stomatal conductance to reduce water loss under dryconditions. Other parameters associated with forest sampling sites (latitudeand altitude), sampled trees (average age and diameter at breast height), orregional satellite-observation-based transpiration product (Global Land Evaporation Amsterdam Model: GLEAM) did notsignificantly correlate with daily foliar Hg uptake rates. We conclude thattree physiological activity and stomatal response to VPD and soil watercontent should be implemented in a stomatal Hg model to assess future Hgcycling under different anthropogenic emission scenarios and global warming.

[学科分类] 大气科学