The responses of shoot-root-rhizosphere continuum to simultaneous fertilizer addition, warming, ozone and herbivory in young Scots pine seedlings in a high latitude field experiment

It is not clear how climate change in combination with increasing soil nitrogen availability and herbivory affects boreal forests, the largest terrestrial biome in the world. In this study, Scots pine (Pinus sylvestris) seedlings were exposed to moderate warming (ca. 1 °C), 1.5 × ambient ozone (O3) concentration, fertilizer addition (120 kg N ha−1 yr−1) and shoot herbivory by pine sawfly (Acantholyda posticalis) alone and in combination. We measured fine root morphology, mycorrhizal colonization level, root fungal biomass (ergosterol), rhizosphere emission of biogenic volatile organic compounds (BVOCs), and microbial biomass (PLFAs) in the rhizosphere soil as well as seedling above- and below-ground growth. Warming and fertilization effects on fine root proportions or root fungal biomass occurred in combination with other factors, combination effects being usually negative on the studied variables, or then warming and fertilization in combination cancelled some other factors’ effects. O3 effects on needle growth, root fungal biomass and BVOCs were more often seen after the third exposure year, and sometimes only in combination with other studied factors. Of abiotic factors, fertilizer addition had reducing effect on rhizosphere BVOCs. Though increased nitrogen availability and warming increased both shoot and root dry masses, growth allocation to above- and below-ground parts was not equally increased in the combined exposures. Thus, we conclude that climate change factors together with increased nitrogen availability and herbivory are likely to affect the below-ground compartments negatively, more often than shoots, and ultimately change in growth allocation pattern which may affect overall seedling growth and survival in later years.