INTERACTIVE EFFECTS OF ATMOSPHERIC CO2 ENRICHMENT AND SOIL N ON GROWTH AND ECTOMYCORRHIZAL COLONIZATION OF PONDEROSA PINE-SEEDLINGS

Interactive effects of elevated atmospheric CO2 and soil N fertility o n above- and belowground development of juvenile ponderosa pine (Pinus ponderosa Dougl, ex Laws.) were examined. Seedlings were grown from s eed in atmospheres containing 700 mu l l(-1), 525 mu l l(-1), or ambie nt CO2. Medium and high soil N treatments were created by adding suffi cient (NH4)(2)SO4 to the potting mix to increase total N by 100 mu g g (-1) and 200 mu g g(-1), respectively, while unamended mix, which had a total N concentration of approximately 300 mu g g(-1), served as the low N treatment. Three whole-seedling harvests at 4-month intervals p ermitted assessment of shoot and root growth and ectomycorrhizal forma tion resulting from inoculation with Pisolithus tinctorius (Pers.) Cok er and Couch. After 4 months, CO2 enrichment increased shoot volume an d dry weight of seedlings grown in high soil N, but this result was no t evident in the other N treatments and did not persist to the second harvest. Root weight, however, increased, and shoot/root ratio decreas ed as the CO2 concentration increased within all three N treatments at the first harvest. At the second harvest, root weights within the hig h and intermediate N treatments were lowest in seedlings grown in ambi ent CO2 and shoot/root ratios decreased as CO2 increased in these two N treatments as well. Although the ectomycorrhizal infection percentag e of seedlings grown in 700 mu l l(-1) CO2 was highest among the seedl ings grown in high N after 4 months, mycorrhizal colonization was vari able overall at the first and second harvests. After 1 yr, the 525 mu l l(-1) CO2 concentration stimulated above- and belowground growth mor e than the high CO2 atmosphere in both high and medium soil N. These s eedlings also had relatively extensive ectomycorrhizal formation, but colonization was again variable. Results presented here suggest the re sponse of juvenile ponderosa pine to CO2 enrichment is ephemeral, with the effects on roots more pronounced and persistent overall than thos e on shoots, and that the response is dependent on N availability.