Impacts of elevated temperature and CO2 concentration on growth and phenolics in the sexually dimorphic Populus tremula (L.)

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 CO2 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 degrees C on average) and CO2 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 CO2 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 CO2 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 CO2 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.