RESPONSE OF RED ALDER SEEDLINGS TO CO2 ENRICHMENT AND WATER-STRESS

Red alder (Alnus rubra Bong.) is a nitrogen-fixing pioneer tree specie s of the Pacific Northwest of North America. We investigated the respo nse of different seed sources of red alder to elevated atmospheric CO2 and to varied levels of water stress. Seeds were stratified, germinat ed and grown for up to 147 d under ambient (350 mu l l(-1)) or elevate d (700 mu l l(-1)) CO2. There were no significant interactions of seed source latitude with either treatment, although seedlings from more n ortherly sources were larger. Elevated CO2 and low moisture stress res ulted in larger plants with more leaf area; effects of the two factors appeared additive. Effects of both factors on biomass allocation, inc luding root:shoot ratios, were small or nonsignificant. Elevated CO2 d ecreased specific nitrogenase activity and generally increased photosy nthesis (A) and stomatal conductance (g). The ratio A:g, potential wat er use efficiency, also increased when plants were under water stress. Elevated CO2 appears to improve drought tolerance in red alder. Overa ll, these results indicate that red alder would benefit in total plant growth from increased ambient CO2 and could tolerate changes in preci pitation.