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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]

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]