Road traffic contributes considerably to ground-level air pollution and is therefore likely to affect roadside ecosystems. Differences in growth and leaf traits among 13 hybrid aspen (Populus tremula×P. tremuloides) clones were studied in relation to distance from a motorway. The trees sampled were growing 15 and 30m from a motorway and at a background rural site in southern Finland. Litter decomposition was also measured at both the roadside and rural sites. Height and diameter growth rate and specific leaf area were lowest, and epicuticular wax amount highest in trees growing 15m from the motorway. Although no significant distance×clone interactions were detected, clone-based analyses indicated differences in genotypic responses to motorway proximity. Leaf N concentration did not differ with distance from the motorway for any of the clones. Leaf litter decomposition was only temporarily retarded in the roadside environment, suggesting minor effects on nutrient cycling.

Concentrations of Zn and Cd were measured in fruitbodies of ectomycorrhizal (ECM) fungi and leaves of co-occurring accumulator aspen. Samples were taken on three metal-polluted sites and one control site. Fungal bioconcentration factors (BCF = fruitbody concentration: soil concentration) were calculated on the basis of total metal concentrations in surface soil horizons (BCFtot) and NH4NO3-extractable metal concentrations in mineral soil (BCFlab). When plotted on log-log scale, values of BCF decreased linearly with increasing soil metal concentrations. BCFlab for both Zn and Cd described the data more closely than BCFtot. Fungal genera differed in ZnBCF but not in CdBCF. The information on differences between fungi with respect to their predominant occurrence in different soil horizons did not improve relations of BCF with soil metal concentrations. Aspen trees accumulated Zn and Cd to similar concentrations as the ECM fungi. Apparently, the fungi did not act as an effective barrier against aspen metal uptake by retaining the metals. Populus tremula and associated ectomycorrhizal fungi accumulate zinc and cadmium to similar concentrations.

In order to achieve efficient phytoextraction of heavy metals using trees, the metal allocation to aboveground tissues needs to be characterised. In his study, the distribution of heavy metals, macro- and micronutrients and the metal micro-localisation as a function of the leaf position and heavy metal treatment were analysed in poplars grown on soil with mixed metal contamination. Zinc was the most abundant contaminant in both soil and foliage and, together with cadmium, was preferentially accumulated in older foliage whereas excess copper and lead were not translocated. Changes in other element concentrations indicated an acceleration in aging as a consequence of the metal treatment. Excess zinc was irregularly accumulated inside leaf tissues, tended to saturate the veins and was more frequently stored in cell symplast than apoplast. Storage compartments including metabolically safe and sensitive subcellular sites resulted in sizable metal accumulation as well as stress reactions.

The phytoextraction potential of plants for removing heavy metals from polluted soils is determined by their capacity to store contaminants in aboveground organs and complex them safely. In this study, the metal compartmentation, elemental composition of zinc deposits and zinc complexation within leaves from poplars grown on soil with mixed metal contamination was analysed combining several histochemical and microanalytical approaches. Zinc was the only heavy metal detected and was stored in several organelles in the form of globoid deposits showing β-metachromasy. It was associated to oxygen anions and different cations, noteworthy phosphorous. The deposit structure, elemental composition and element ratios indicated that zinc was chelated by phytic acid ligands. Maturation processes in vacuolar vs. cytoplasmic deposits were suggested by differences in size and amounts of complexed zinc. Hence, zinc complexation by phytate contributed to metal detoxification and accumulation in foliage but could not prevent toxicity reactions therein.

To evaluate cadmium (Cd) remediation capacity of gray poplar (Populus × canescens Sm. referred to the hybrid of Populus tremula L. × Populus alba L.), the glasshouse experiment was conducted in hydroponics, and the effects of Cd (0, 10, 30, 50, and 70 μM) on plant growth as well as Cd uptake and translocation were investigated. The growth rate of all tissues in P. × canescens decreased slowly with an increase in Cd concentrations. Among different tissues, the root exhibited the highest level of bio-concentration factor (BCF), followed by leaves, bark, and wood. BCFs in bark and wood significantly decreased with an increase in Cd concentrations. The translocation factor in different tissues firstly increased and then declined with an increase in Cd concentrations, respectively. The translocation factor in different tissues decreased slowly with an increase in Cd concentrations. Cd accumulation rates significantly increased and reached about sevenfold the level after 70 μM than that observed after 0 μM (control) for 28 days. These results indicated that P. × canescens have good tolerance against Cd stress, varying in Cd accumulation and translocation. These properties need to be taken into account in selecting species for the phytoremediation of orefield.

Alkalisation of soil by dust pollution from a cement plant was assumed to be the principal cause of changes in heavy metal uptake and allocation between hybrid aspen (Populus tremula × Populus tremuloides Michx.) compartments. Emission of over 40 years of alkaline dust (pH 12.3–12.6) into the atmosphere had resulted in an increase of pH and an elevated concentration of total heavy metals in the upper layer of the soil (0–30 cm), which is considerable even 14 years after dust pollution has stopped. The accumulation and allocation of heavy metals in stem, shoot and leaves varied between themselves and between the trees from polluted and unpolluted plantations depending more on the mobility of elements and pH than element concentrations in the alkaline soil. High levels of heavy metals in the soil do not mean similar concentrations and ratios in plants growing in contaminated soil.