Responses of soil-grown Scots pine seedlings to experimental warming, moderate nitrogen addition and bark herbivory in a three-year field experiment.

Increased soil nitrogen (N), warming and bark herbivory all are expected to affect boreal forests in the future. We studied the effects of warming (0.5 °C and 4.0 °C above ambient air and soil temperature, respectively), moderate N addition (30 kg N ha-1 y-1) and bark herbivory by large pine weevil (Hylobius abietis L.) on soil-grown Scots pine (Pinus sylvestris L.) seedlings in a three-year (2014-2016) open-air field experiment. Seedling dry mass, root mass fraction (RMF), root morphology, mycorrhizal colonization, mycorrhizal morphotypes, root phenolics and microbial abundance in the rhizosphere area were studied. We observed that both moderate N addition and warming showed interactive effects, and generally improved seedling growth after the three consecutive growing seasons. However, soil dryness was increased due to combined warming and N addition treatment in 2016, and it seemed to limit the shoot growth stimulation as well as increase the dependence of the non-herbivory seedlings on the mycorrhizas. Moderate N addition generally reduced herbivory damage intensity and increased RMF. It also decreased total mycorrhizal colonization rate and increased SRL of the seedlings in 2016, but only in the absence of other factors. In 2016, herbivory affected soil exploration efficiency and mycorrhizal colonization without other factors, and had a tendency to increase root phenolics. There were only minor effects of N addition and herbivory on soil microbial abundances. We conclude that warming and N addition to soil may increase growth in young Scots pine if soil drought or herbivory do not start to limit it; and that in young Scots pine stands moderate bark herbivory are likely to affect roots more than shoots.