PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Growth, nitrogen uptake and allocation responses of six temperate tree species to elevated CO₂ .

ZERIHUN, AYALSEW¹, BASSIRIRAD, HORMOZ², ^{1 2}

ABSTRACT- Response of the natural vegetation to increased atmospheric CO₂ concentration depends on compensatory adjustments in N uptake, but the extent of such adjustments may vary among species. Even when N uptake is unaffected, elevated CO₂ may affect plant growth by altering N allocation among different tissues. Here we examined CO₂ effects on growth, ¹⁵NH₄⁺ and ¹⁵NO₃^- uptake and allocation in seedlings of Acer negundo, A. platanoides, Quercus bicolor, Q. macrocarpa, Pinus taeda and P. resinosa. In all species, elevated CO₂ increased whole plant biomass but the magnitude of this response differed among species. In both species of Pinus and Q. macrocarpa, biomass increased by 48-55% at elevated CO₂ whereas in the other species the increase was only 10-19%. Elevated CO₂ decreased specific uptake of ¹⁵NH₄⁺ and ¹⁵NO₃^- in the broad-leaved species but increased it in the conifers. Recovery of labeled-N ranged from less than 10% in P. resinosa to 80% in Q. bicolor and A. negundo. Most of this variation was explained by species differences in, and CO₂ effects on, fine root size. Shoot-root allocation of recently acquired ¹⁵N differed among species. In both Quercus species, up to 80% of the ¹⁵N was allocated to the roots. By contrast, A. negundo roots retained only 20% of the newly acquired ¹⁵N . In almost all species, the short- and long-term N allocation patterns were also strikingly different. Generally, compared to the shoots, the roots were the dominant sinks for newly taken up N while the reverse was true in the long-term.

KEY WORDS: elevated CO₂, N uptake and allocation, broad leaf species, conifers