Growth of autotrophic and root-hemiparasitic understory plants under elevated CO2 and increased N deposition

Effects of atmospheric CO2 enrichment (280, 420 and 560 mu mol CO2 mol(-1)) and increased N deposition (0, 30 and 90 kg ha(-1) a(-1)) on Oxalis acetosella, Homogyne alpina, and Melampyrum sylvaticum, growing in model ecosystems beneath spruce stands, were studied. Aboveground biomass in the less-shade-tolerant Homogyne and in the annual hemiparasite Melampyrum was strongly increased with increasing CO2, but not in the more shade-adapted Oxalis. In contrast, increased N deposition stimulated growth in Oxalis, but had no effect on Homogyne and Melampyrum. Due to spruce canopy closure Homogyne,le became light limited and its survivorship was strongly correlated with spruce canopy LAI in the second year of the experiment. Our results suggest, that elevated CO2 facilitates the expansion of Hamogyne into less favourable micro-habitats (deeper shade) and that increasing N deposition enables more vigorously growing species like Oxalis to increase in abundance. Growth of the hemiparasite Melampyrum was stimulated indirectly by increased heterotrophic carbon supply (carbon isotope data) from the host (Picea abies), and thus, this species may also increase in abundance with increasing CO2. However, possible indirect effects (canopy feedbacks) make predictions of long-term understory development difficult.