The interactive effects of three global stressors on the growth morphology and chemistry of Acer saccharum and Pinus strobus seedlings

The interactive effects of springtime warming, UV-B, and nitrogen fertilization (equivalent to an additional 50 kg N ha-1 yr-1 ) on the growth and chemistry of sugar maple (Acer saccharum ) seedlings from two contrasting sites were assessed. The soils of the Oliver forest were deep luvisols overlying a strongly calcareous till, while the naturally acidic soils of Haliburton were derived from the Precambrian Shield. Warming significantly altered the carbon and nitrogen dynamics of the soils and significantly reduced the leaf area of individual sugar maple seedlings at both sites. Removal of ambient levels of UV-B led to increased foliar concentrations of some flavonoids and chlorogenic acid at both sites and appeared to be responsible for a significant reduction in herbivore activity. At Haliburton, nitrogen additions led to increased foliar concentrations of Mn and decreases in the concentration of measured phenolics and flavonoids, while at Oliver forest nitrogen led to increased concentrations of Mg and chlorogenic acid and decreases in some flavonoids in leaf tissues. This implies that there is a potential interaction between the acid sensitivity of these soils and the secondary chemistry of the plants growing in the soils. This was further tested in a greenhouse experiment in which seedlings of sugar maple and an additional species, white pine (Pinus strobus), were grown on soils collected from Oliver and Haliburton and exposed to a 30% increase in UV-B above current springtime levels. Seedlings grown on Haliburton soils had increased foliar concentration of acidic cations and decreased concentrations of phenolics and flavonoids and were subsequently more sensitive to increases in UV-B than those seedlings grown on Oliver soils. Overall, white pine appeared to be more tolerant of increased levels of UV-B, most likely due to anatomical and morphological differences from the deciduous sugar maple. This study highlighted the role of the underlying soils in determining the response of the understory seedlings to additional abiotic and biotic stressors.