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High northern latitudes are climatic sensitive areas, and are also regions to which polycyclic aromatic hydrocarbons (PAHs) easily transport and accumulate with potential risk to natural ecosystems. However, the effect of PAHs on northern woody plant growth and defense under climate change is very little studied. Here, we conducted a unique experiment in greenhouses to investigate sex-related responses of the dioecious Populus tremula to pyrene (50 mg kg −1 ) and residue of pyrene in soils under ambient and elevated temperature (+1.8 °C on average) and CO 2 (740 ppm). Pyrene decreased stem biomass and leaf area by 9% and 6%, respectively under ambient conditions, and the reduction of leaf area was more severe under elevated temperature (38%), elevated CO 2 (37%), and combined T + CO 2 (42%). Other growth parameters were unchanged by pyrene. Pyrene did not affect the concentration of leaf total phenolics under ambient conditions, but increased it by 16%, 1%, and 20% compared to controls under elevated temperature, elevated CO 2 , and T + CO 2 , respectively. Pyrene had only minor sex-specific effects on plant growth and phenolics. The concentration of residual pyrene in pyrene-spiked soils was higher under elevated CO 2 than under ambient, elevated temperature, and combined T + CO 2 . The results suggest that both sexes of P. tremula have the capacity to regulate growth and metabolism to adjust to the stress of the tested pyrene contamination under elevated temperature and CO 2 , but potential risk of pyrene to plants still exists in the future changing climate. [ABSTRACT FROM AUTHOR]

The combined effects of climate change and chemical contaminants on plant performance are still not well understood. Especially, whether different sexes of dioecious plants respond differently to combined stresses is unknown. In order to study the sex-related responses of European aspen to soil nTiO 2 contamination (0, 50, 300 mg kg –1 ) under elevated temperature (+1.6 °C) and CO 2 (730 ppm), we conducted a study in greenhouses. Ti accumulated in roots exposed to nTiO 2 (1.1–3.3 and 2.7–21.1 mg kg –1 in 50 and 300 mg kg –1 treatments, respectively). Elevated CO 2 had no effects on Ti uptake, while elevated temperature increased it in the 300 mg kg –1 treatment. Males grew taller than females under ambient conditions, but females had greater height and biomass increment under elevated temperature. In all climate treatments, nTiO 2 increased leaf phenolics in females by 12–19% and 15–26% at 50 and 300 mg kg –1 , respectively. Leaf phenolics decreased under elevated temperature, but increased under elevated CO 2 in both sexes. Results suggest that females have better chemical defense against nTiO 2 than males under future climate conditions. In the longer run, this may cause changes in the competitive abilities of both sexes, which again may affect sex ratios and genetic variation in nature. [ABSTRACT FROM AUTHOR]

Future climatic changes may alter the balance between the sexes of dioecious species due to differential effects on resource allocation to growth and defense. Our purpose was to study the impacts of elevated temperature and CO 2 concentration on the relative changes in growth and phenolics accumulation in stem bark of the dioecious Populus tremula , a keystone species for boreal forest biodiversity and one that is browsed by many mammalian herbivores. In a greenhouse experiment, four female and four male genotypes of P. tremula were grown under single and combined treatments of elevated temperature (1.5 °C on average) and CO 2 concentration (720 ppm) for one growing season. Elevated temperature increased the height, diameter, leaf and stem biomass, and in addition decreased the concentration of phenolics including salicylates, flavonoids, phenolic acids, salireposide and lignan. Elevated CO 2 concentration, on the other hand, reduced the height growth (other growth parameters were unchanged) and increased the concentration of phenolics, especially salicylates and phenolic acids. In the combined treatment, P. tremula tended to grow more, but elevated temperature counteracted the effect of elevated CO 2 concentration on phenolics accumulation. Although statistically not significant, males tended to have greater growth and a lower level of phenolics than females. The smaller sexual differences were also not strongly affected by climatic factors. Under future elevated temperature and CO 2 concentration, both sexes of P. tremula will probably grow more and possibly accumulate lower levels of phenolics, but intersexual differences in growth and phenolics accumulation may be more pronounced after sexual maturation. [ABSTRACT FROM AUTHOR]